Samacheer Kalvi Class 9 Science Solutions Chapter 12 Periodic Classification of Elements

I. Choose The Correct Answer:

Question 1. If Dobereiner is related with 'law of triads', then Newlands is related with
(a) Modem periodic law
(b) Hund's rule
(c) Law of octaves
(d) Pauli's Exclusion principle
Answer: (c) Law of octaves
In simple words: Dobereiner grouped elements into sets of three called triads. Newlands grouped elements into sets of eight, similar to musical octaves. Both scientists tried to find patterns in elements.

🎯 Exam Tip: Remember the core idea behind each early classification attempt, like 'triads' for Dobereiner and 'octaves' for Newlands, to easily recall their contributions.

Question 2. Modern periodic law states that the physical and chemical properties of elements are the periodic functions of their ...........
(a) atomic numbers
(b) atomic masses
(c) similarities
(d) anomalies
Answer: (a) Atomic numbers
In simple words: The modern periodic table arranges elements by their atomic numbers, and this arrangement explains why elements show similar properties over and over again in a pattern. This is different from earlier laws that used atomic mass.

🎯 Exam Tip: Distinguish clearly between the modern periodic law (based on atomic number) and earlier laws (often based on atomic mass) as this is a common point of confusion.

Question 3. Elements in the modern periodic table are arranged in ..groups and .periods.
(a) 7, 18
(b) 18, 7
(c) 17, 8
(d) 8, 17
Answer: (b) 18, 7
In simple words: The modern periodic table has 18 vertical columns, called groups, and 7 horizontal rows, called periods. Elements in the same group often have similar chemical properties.

🎯 Exam Tip: Always remember that groups are vertical columns (18 of them) and periods are horizontal rows (7 of them) in the modern periodic table.

Fill In The Blanks:

Question 1. In Dobereiner's triads, the atomic weight of the middle element is the ________ of the atomic masses of 1 and 3rd elements.
Answer: average
In simple words: Dobereiner noticed that in a group of three similar elements, the middle element's weight was usually the average of the first and third elements' weights. This was a pattern he observed.

🎯 Exam Tip: When describing Dobereiner's triads, remember to mention both the grouping of three similar elements and the relationship between their atomic weights.

Question 2. Noble gases belong to ________ group of the periodic table.
Answer: 18th
In simple words: Noble gases are special elements found in the 18th group of the periodic table. They are called "noble" because they usually do not react with other elements.

🎯 Exam Tip: It's important to know that Group 18 contains the noble gases, which are known for their stable electron configurations and lack of reactivity.

Question 3. The basis of the classifications proposed by Dobereiner, Newlands and Mendeleev was .............
Answer: atomic mass
In simple words: Early scientists like Dobereiner, Newlands, and Mendeleev arranged elements mainly by their atomic mass. This was before the discovery of atomic numbers.

🎯 Exam Tip: Clearly differentiate that early classifications used atomic mass as their primary basis, while the modern periodic table uses atomic number.

Question 4. Example for liquid metal is ............
Answer: mercury
In simple words: Mercury is a unique metal because it is liquid at room temperature. Most other metals are solid.

🎯 Exam Tip: Mercury is the most common example of a liquid metal at standard temperature, making it a key exception to remember.

III. Match The Following:

Question. Match the following based on early classifications and modern periodic law.

🎯 Exam Tip: Memorize the key contributions of each scientist (Dobereiner, Newlands, Mendeleev, Moseley) and the common examples for different element types like alkali metals and alkaline earth metals.

IV. State Whether True Or False. If False, Correct The Statement:

Question 1. Newlands' periodic table is based on atomic masses of elements and modem periodic table is based on atomic number of elements.
Answer: True.
In simple words: This statement is correct. Newlands arranged elements by their atomic mass, but the current periodic table arranges them by atomic number. This change was a big step in understanding elements better.

🎯 Exam Tip: It is crucial to remember that Newlands and other early scientists used atomic mass, while the modern periodic table uses atomic number, which resolves many inconsistencies.

Question 2. Metals can gain electrons.
Answer: False. Correct statement: Metals tend to lose electrons.
In simple words: Metals usually lose electrons to become stable. Non-metals are the ones that typically gain electrons. Losing electrons makes metals positive.

🎯 Exam Tip: Remember that metals are electropositive, meaning they tend to lose electrons, while non-metals are electronegative, tending to gain electrons.

Question 3. Alloys bear the characteristics of both metals and nonmetals.
Answer: False. Correct statement: Metalloids bear the characteristics of both metals and nonmetals.
In simple words: Alloys are mixtures of metals, sometimes with non-metals, but metalloids are specific elements that naturally show properties of both metals and non-metals. Metalloids sit between metals and non-metals in the periodic table.

🎯 Exam Tip: Be careful not to confuse alloys (mixtures) with metalloids (elements with mixed properties). Metalloids have unique properties, making them useful in semiconductors.

Question 4. Lanthanides and actinides are kept at the bottom of the periodic table because they resemble each other but they do not resemble with any other group elements.
Answer: True.
In simple words: Lanthanides and actinides are placed separately at the bottom to keep the main periodic table from being too wide. They share similar traits among themselves but are very different from the main group elements.

🎯 Exam Tip: Understand that the separate placement of lanthanides and actinides is for structural clarity and reflects their distinct chemical behavior compared to the main blocks.

Question 5. Group 17 elements are named as Halogens.
Answer: True.
In simple words: The elements in Group 17, like fluorine, chlorine, bromine, and iodine, are collectively known as halogens. They are highly reactive non-metals.

🎯 Exam Tip: Know the common names for important groups, such as alkali metals (Group 1), alkaline earth metals (Group 2), halogens (Group 17), and noble gases (Group 18).

V. Assertion And Reason:

Question. Statement : Elements in a group generally possess similar properties but elements along a period have different properties. Reason: The difference in electronic configuration makes the element differ in their chemical properties along a period.
(a) Statement is true and reason explains the statement.
(b) Statement is false but the reason is correct.
Answer: (a) Statement is true and reason explains the statement.
In simple words: Elements in the same vertical group have similar chemical properties because they have similar outer electron arrangements. However, elements across a horizontal period have different properties because their electron configurations change gradually.

🎯 Exam Tip: In assertion-reason questions, first check if both statements are individually true. Then, check if the reason correctly explains the assertion. Focus on the relationship between electronic configuration and chemical properties.

VI. Answer The Following:

Question 1. State modern periodic law.
Answer: The modern periodic law states that: "The chemical and physical properties of elements are periodic functions of their atomic numbers." This means that when elements are arranged by increasing atomic number, their properties show a repeating pattern. This law helped to correct the shortcomings of earlier classifications.
In simple words: The modern periodic law says that when you arrange elements by how many protons they have (atomic number), their properties will repeat in a regular way.

🎯 Exam Tip: When stating the modern periodic law, ensure you explicitly mention "atomic numbers" as the basis, not atomic mass.

Question 2. What are groups and periods in the modern periodic table?
Answer:

• The horizontal rows in the periodic table are called periods. There are seven periods in total, and elements in the same period have the same number of electron shells.
• Vertical columns in the periodic table starting from top to bottom are called groups. There are eighteen groups, and elements in the same group tend to have similar chemical properties due to similar valence electron configurations.

🎯 Exam Tip: Clearly define periods as horizontal rows and groups as vertical columns, and briefly mention a key characteristic of each (e.g., electron shells for periods, similar properties for groups).

Question 3. What are the limitations of Mendeleev's periodic table?
Answer: Mendeleev's periodic table, despite its revolutionary nature, had several limitations:

• Elements with large differences in properties were sometimes included in the same group. For example, hard metals like copper (Cu) and silver (Ag) were placed alongside soft metals like sodium (Na) and potassium (K).
• No proper position could be given to the element hydrogen. Although a non-metal, hydrogen was placed with alkali metals.
• The increasing order of atomic mass was not strictly followed throughout the table. In some cases, elements with higher atomic mass were placed before those with lower atomic mass to fit their chemical properties.
• There was no specific place for isotopes in the periodic table, as isotopes have different atomic masses but the same chemical properties.

🎯 Exam Tip: Focus on 3-4 distinct limitations of Mendeleev's table, such as the position of hydrogen, the problem with isotopes, the order of atomic masses, and grouping of dissimilar elements.

Question 4. State any five features of the modern periodic table.
Answer: Here are five key features of the modern periodic table:

• All the elements are arranged strictly in the increasing order of their atomic number. This provides a more consistent arrangement than atomic mass.
• The horizontal rows are called periods, and there are seven periods in the periodic table. Elements in a period have the same number of electron shells.
• The elements are placed in periods based on the number of electron shells in their atoms, with each new shell starting a new period.
• Vertical columns in the periodic table, starting from top to bottom, are called groups. There are 18 groups in the periodic table, and elements within a group share similar chemical properties.
• Elements are grouped into various families based on their physical and chemical properties, such as alkali metals, halogens, and noble gases.

🎯 Exam Tip: When listing features, ensure you highlight the arrangement by atomic number and clearly differentiate between periods (horizontal rows, electron shells) and groups (vertical columns, similar properties).

Activity -1 Intext Activities

Question. Find the pair of elements having similar properties by applying Newlands' law of Octaves (Example: Mg & Ca): Set I: F, Mg, C, O, B Set II: Al, Si, S, Cl, Ca
Answer:
1. F-Cl
2. Mg-Ca
3. C-Si
4. O-S
5. B-Al
In simple words: Newlands' Law of Octaves states that every eighth element has similar properties to the first. Using this idea, we can pair up elements like Fluorine with Chlorine, or Magnesium with Calcium, as they would fall in similar positions in an 'octave' sequence.

🎯 Exam Tip: To apply Newlands' Law of Octaves, list elements in increasing atomic mass and then identify elements that are eight positions apart to find those with similar properties.

9th Science Guide Periodic Classification Of Elements Additional Important Questions And Answers

Choose The Correct Answer:

Question 1. Noble gases are placed in the ______ group in the modern periodic table.
(a) 13th
(b) 18th
(c)
(d) 2nd
Answer: (b) 18th
In simple words: Noble gases are in the 18th group of the periodic table. They are very stable elements that do not often react with other elements.

🎯 Exam Tip: Remember that noble gases are found in Group 18, characterized by their full outer electron shells and chemical inertness.

Question 2. Group 16 elements are collectively called as ___________.
(a) chalcogen family
(b) carbon family
(c) halogens
(d) nitrogen family
Answer: (a) Chalcogen family
In simple words: Elements in Group 16, which include oxygen, sulfur, and selenium, are known as chalcogens. These elements often form ores.

🎯 Exam Tip: Know the specific names for different groups, like Group 16 elements being called chalcogens, as this terminology is frequently tested.

Question 3. The maximum number of electrons that can be accommodated in s, p, d, and f subshells are ___________.
(a) 14, 10, 6, 2
(b) 6, 10, 2, 14
(c) 2, 6, 10, 14
(d) 6, 2, 14, 10
Answer: (c) 2, 6, 10, 14
In simple words: Different subshells can hold a maximum number of electrons: 's' can hold 2, 'p' can hold 6, 'd' can hold 10, and 'f' can hold 14. These numbers help us understand how electrons fill up around an atom.

🎯 Exam Tip: Clearly memorize the maximum electron capacity for each type of subshell (s, p, d, f) as this is fundamental for writing electron configurations.

Question 4. d-block elements are otherwise known as ___________.
(a) transition elements
(b) inner transition elements
(c) halogens
(d) alkali metals
Answer: (a) Transition elements
In simple words: The elements in the d-block of the periodic table are also known as transition elements. They are found in the middle of the periodic table and often have various oxidation states.

🎯 Exam Tip: Recall that d-block elements are the transition metals, found in Groups 3-12, known for their colorful compounds and ability to form multiple ions.

Question 5. Which block of the periodic table contains metals, non-metals and metalloids?
(a) s
(b) p
(c) d
(d) f
Answer: (b) p
In simple words: The p-block is special because it contains all three types of elements: metals, non-metals, and metalloids. The s-block and d-block mainly contain metals.

🎯 Exam Tip: Remember that the p-block is the most diverse, containing metals, non-metals, and metalloids, and its properties vary significantly from left to right.

Question 6. Which of the following non-metals can form alloys with metals?
(a) Boron
(b) Carbon
(c) Silicon
(d) All of the options
Answer: (d) All of the options
In simple words: Non-metals like boron, carbon, and silicon can all be mixed with metals to create alloys. Carbon, for example, is mixed with iron to make steel, which is a very strong alloy.

🎯 Exam Tip: While alloys are usually mixtures of metals, remember that some non-metals (like carbon) are crucial components in forming important alloys such as steel.

Question 7. Which of the following metals have low melting and boiling point?
(a) Sodium
(b) Potassium
(c) Iron
(d) Both (a) & (b)
Answer: (d) Both (a) & (b)
In simple words: Sodium and potassium are both alkali metals, and they have surprisingly low melting and boiling points compared to many other metals. You can even cut them with a knife.

🎯 Exam Tip: Recall that alkali metals (Group 1) generally have low melting and boiling points due to their weak metallic bonding, making them exceptions to the typical "high melting point" characteristic of metals.

Question 8. Which of the following metal (s) do not react with water?
(a) Gold
(b) Silver
(c) Copper
(d) All of the options
Answer: (d) All of the options
In simple words: Gold, silver, and copper are all metals that do not react with water. This property makes them useful for making jewelry and coins that last a long time without rusting.

🎯 Exam Tip: Remember that noble metals like gold and silver, along with copper, are known for their low reactivity, especially with water and air, which is why they retain their luster.

Question 9. If the electronic configuration of an element is \( 1s^2 2s^2 2p^6 3s^2 3p^1 \), then it will occupy ............ block of the periodic table.
(a) s
(b) p
(c) d
(d) f
Answer: (b) p
In simple words: To find the block, look at where the last electron is placed. In this electronic configuration, the last electron is in the \( 3p^1 \) subshell, which means the element belongs to the p-block.

🎯 Exam Tip: The block an element belongs to in the periodic table is determined by the subshell where its last valence electron is added (e.g., if the last electron is in a p subshell, it's a p-block element).

II. Fill In The Blanks:

Question 1. ............ group elements are called alkaline earth metals.
Answer: Second
In simple words: The elements found in the second group of the periodic table, like beryllium and magnesium, are known as alkaline earth metals. They are reactive but less so than alkali metals.

🎯 Exam Tip: Group 2 elements are consistently referred to as alkaline earth metals, a key piece of terminology to recall.

Question 2. The elements that follow lanthanum are called ............
Answer: Lanthanides
In simple words: After the element lanthanum, there is a special series of elements called lanthanides. These elements are usually placed in a separate row at the bottom of the periodic table.

🎯 Exam Tip: Remember the two series of inner transition elements: lanthanides (following lanthanum) and actinides (following actinium), both placed below the main body of the periodic table.

Question 3. f-block elements are also known as ............
Answer: inner transition elements
In simple words: Elements in the f-block are called inner transition elements. They are found in the two separate rows at the very bottom of the periodic table.

🎯 Exam Tip: The f-block elements are the lanthanides and actinides, also known as inner transition elements, due to their f-orbitals being filled.

Question 4. ............ block element is placed at the bottom of the periodic table.
Answer: f-
In simple words: The f-block elements are always shown at the bottom of the periodic table, separate from the main body. This is done to keep the table neatly arranged and easier to read.

🎯 Exam Tip: Understand that the f-block is placed separately at the bottom for aesthetic and organizational reasons, preventing the periodic table from becoming too wide.

Question 5. The mixture of metal with mercury is called ............
Answer: amalgam
In simple words: When mercury is mixed with another metal, the resulting mixture is called an amalgam. These mixtures have various uses, especially in dentistry.

🎯 Exam Tip: The term "amalgam" specifically refers to an alloy where one of the constituent metals is mercury.

Question 6. Elements are placed in periods based on the number of ............ in their atoms.
Answer: shells
In simple words: The period an element belongs to in the periodic table depends on the number of electron shells its atoms have. For instance, elements in Period 3 have three electron shells.

🎯 Exam Tip: Always remember that the period number corresponds to the number of electron shells (principal energy levels) an element's atoms possess.

III. Spot The Error:

Question 1. The elements of group 13 are called the carbon family.
Answer: The elements of group 13 are called the boron family. Alternatively, the elements of group 14 are called the carbon family.
In simple words: Group 13 elements are actually known as the boron family, because boron is the first element in that group. The carbon family is actually Group 14, where carbon is the first element.

🎯 Exam Tip: For "spot the error" questions, not only identify the incorrect part but also provide the accurate correction, sometimes offering alternative correct statements if applicable.

Question 2. There are 5 periods and 17 groups in the modern periodic table.
Answer: There are 7 periods and 18 groups in the modern periodic table.
In simple words: The periodic table has 7 horizontal rows called periods and 18 vertical columns called groups. The statement had the wrong numbers for both.

🎯 Exam Tip: Ensure you know the exact number of periods (7) and groups (18) in the modern periodic table, as this is a fundamental fact.

IV. Pick The Odd One Out And Write The Reason:

Question 1. Copper, brass, gold ornament, bronze.
Answer: Copper. Copper is metal whereas the other three are alloys.
In simple words: Copper is a pure metal, but brass, gold ornament (which is an alloy, not pure gold), and bronze are all mixtures of different metals, also known as alloys.

🎯 Exam Tip: Differentiate between pure metals and alloys; alloys are mixtures designed to have improved properties compared to their constituent pure metals.

Question 2. Bromine, carbon, hydrogen, aluminium.
Answer: Aluminium. Aluminium is metal whereas the other three are non-metals.
In simple words: Bromine, carbon, and hydrogen are all non-metals. Aluminium, on the other hand, is a metal. That's why aluminium is the odd one out in this list.

🎯 Exam Tip: Be able to classify common elements as metals, non-metals, or metalloids to identify the odd one out in such questions.

V. To Match:

Question. Match the following elements or concepts to their descriptions or groups.

🎯 Exam Tip: Knowing the family names associated with specific group numbers (e.g., Group 1: Alkali metals, Group 17: Halogens) is essential for quickly identifying elements and their properties.

VI. Answer In Brief:

Question 1. What are the s-block elements?
Answer: S-block elements are those whose last valence electron is filled in the s subshell. These elements include Group 1 (alkali metals) and Group 2 (alkaline earth metals) of the periodic table. They are typically soft, reactive metals with low melting points.
In simple words: S-block elements are the ones where the outermost electron goes into an 's' orbital. They are found on the far left of the periodic table, in Groups 1 and 2.

🎯 Exam Tip: Define s-block elements by the filling of the s subshell and remember that they comprise Group 1 and Group 2, characterized by their metallic and reactive nature.

Question 2. What are the p-block elements?
Answer: P-block elements are those where the last electron in their outermost shell enters a p subshell. These elements are located in Groups 13 to 18 of the periodic table. The p-block is diverse, containing metals, non-metals, and metalloids, and their properties vary significantly.
In simple words: P-block elements are where the last electron fills a 'p' orbital. They are on the right side of the periodic table, from Group 13 to 18, and include many different kinds of elements.

🎯 Exam Tip: When describing p-block elements, highlight that their last electron enters a p subshell and that this block is unique for containing metals, non-metals, and metalloids.

Question 3. What are alloys?
Answer: Alloys are mixtures formed by combining two or more metals, or sometimes a metal with a non-metal, in a molten state. They are created to enhance specific properties like strength, hardness, or corrosion resistance that the individual pure metals might lack. Steel, for example, is an alloy of iron and carbon.
In simple words: Alloys are special mixtures made by melting different metals together, and sometimes non-metals too. These mixtures are stronger or better than the original metals.

🎯 Exam Tip: Clearly state that alloys are mixtures, typically of metals, and that their primary purpose is to improve the properties of the individual components.

Question 4. State Mendeleev's Law of Periodicity.
Answer: Mendeleev's Law of Periodicity states that the physical and chemical properties of elements repeat in a pattern when they are arranged by their atomic masses. This arrangement helped reveal underlying patterns in elemental behavior.
In simple words: This law says that if you arrange elements by their weight, their properties will show a regular pattern.

🎯 Exam Tip: Mentioning "periodic functions" and "atomic masses" are key phrases for full marks when defining Mendeleev's Law.

Question 5. State Newlands' Law of Octaves.
Answer: Newlands' Law of Octaves states that when elements are arranged by increasing atomic weight, every eighth element shows properties similar to the first one, just like how the eighth musical note resembles the first. This pattern, though groundbreaking, was observed to be consistent only up to calcium.
In simple words: Newlands found that if you line up elements by their weight, every eighth element acts like the first one in the list, much like musical notes.

🎯 Exam Tip: Comparing elements to musical octaves is a critical point when describing Newlands' Law.

Question 6. An element has atomic number 12.
a) Write the electric configuration.
b)To which group and period do it belong.
Answer:
a) The electronic configuration for an element with atomic number 12 is K=2, L=8, M=2. This arrangement shows electrons filling distinct energy shells.
b) This element belongs to Group No. 2 and Period No. 3 of the periodic table. The number of valence electrons often indicates the group, and the number of electron shells indicates the period.
In simple words:
a) For an atom with 12 electrons, they are arranged as 2 in the first shell, 8 in the second, and 2 in the third.
b) This element is found in Group 2 and Period 3 of the periodic table.

🎯 Exam Tip: For electron configuration questions, remember that the number of valence electrons (outermost shell electrons) often determines the group, and the total number of electron shells determines the period.

Question 8. Relate the names of the following scientists with the statements given below. (Mendeleev, Newlands, Dobereiner).
a) Arranged elements into groups containing three elements each.
b) Arranged elements in a group of seven with increasing atomic masses and eighth elements with similar properties kept below the first like eight note in an act of music.
c) Arranged elements in the increasing order of atomic mass
Answer:
a) Dobereiner: He grouped elements into 'triads', where each group had three elements with similar properties and the middle element's atomic mass was roughly the average of the other two.
b) Newlands: He arranged elements by their increasing atomic masses, observing that every eighth element had similar properties, an idea he compared to musical octaves. This pattern helped pave the way for future classifications.
c) Mendeleev: He arranged elements in order of increasing atomic mass, creating the first widely accepted periodic table that also predicted undiscovered elements.
In simple words:
a) Dobereiner grouped elements into sets of three.
b) Newlands noticed that every eighth element had similar properties, like notes in music.
c) Mendeleev sorted elements mainly by their atomic weight.

🎯 Exam Tip: Clearly associate each scientist with their unique contribution to the periodic classification, focusing on their specific rules (triads, octaves, atomic mass arrangement).

Question 9. Complete the following triads by inserting the missing elements.
a) Cl, .........., I
b) Li, .........., K
c) Ca, .........., Ba
Answer:
a) Cl, **Br**, I (Bromine completes this halogen triad)
b) Li, **Na**, K (Sodium completes this alkali metal triad)
c) Ca, **Sr**, Ba (Strontium completes this alkaline earth metal triad)
Dobereiner's triads highlighted a key relationship where the middle element's properties and atomic mass often lay between those of the other two.
In simple words:
a) The missing element is Bromine (Br).
b) The missing element is Sodium (Na).
c) The missing element is Strontium (Sr).

🎯 Exam Tip: Remember common triads from the periodic table, such as halogens, alkali metals, and alkaline earth metals. The middle element's atomic weight is approximately the average of the other two.

Question 10. How are elements grouped into various families in the periodic table?
Answer: Elements are grouped into different families in the periodic table based on their similar physical and chemical properties. Elements within a family often react in similar ways due to having a similar number of valence electrons.
In simple words: Elements are put into families because they act alike and have similar traits.

🎯 Exam Tip: Emphasize that both physical and chemical properties are key factors in classifying elements into families within the periodic table.

VII. To Interpret:

Question 1. Group 1 (except hydrogen) elements are called alkali metals.
Answer: Group 1 elements, excluding hydrogen, are known as alkali metals. These metals react strongly with water to produce solutions that are highly alkaline, or basic. These basic solutions can turn red vegetable dye blue, confirming their alkaline nature and giving them their name.
In simple words: Group 1 elements, except hydrogen, are called alkali metals because they form very basic (alkaline) solutions when they react with water.

🎯 Exam Tip: To explain why they're called alkali metals, always mention their vigorous reaction with water and the production of alkaline solutions.

Question 2. d-block elements are called transition elements.
Answer: D-block elements are found in the central part of the periodic table and are called transition elements. Their properties lie between those of s-block and p-block elements, displaying a 'transition' in characteristics across the period, such as variable oxidation states and the formation of colored compounds.
In simple words: D-block elements are in the middle of the periodic table. They are called transition elements because their properties are a mix between the elements on the left (s-block) and right (p-block).

🎯 Exam Tip: When defining transition elements, mention their location (centre/d-block) and their intermediate properties.

Question 3. Group 3-12 elements in the modern periodic table are called d block elements.
Answer: Elements in groups 3 to 12 of the modern periodic table are known as d-block elements. This is because their outermost valence electrons are found in the d subshells. These elements are characterized by variable valency and often form colored compounds.
In simple words: Groups 3 to 12 elements are called d-block elements because their last electrons fill up the d-subshells.

🎯 Exam Tip: Remember that d-block classification is based on the filling of d-orbitals, and they typically include elements from groups 3 to 12.

VIII. Complete the Following Table:

🎯 Exam Tip: Remember the maximum electrons per shell: 2 for K, 8 for L, and 18 for M. Noble gases (He, Ne, Ar) naturally have full outer shells, making them stable.

IX. Assertion and Reason:

Question 1. Assertion (A): Group 2 elements in the modern periodic table are called alkaline earth metals. Reason (R): The oxides of group 2 elements produce alkaline solutions when they are dissolved in water.
(a) A is right R is wrong
(b) R explains A
(c) R does not explain A
(d) R is right A is wrong
Answer: (b) R explains A
Assertion (A) is true because Group 2 elements are indeed called alkaline earth metals. Reason (R) is also true and correctly explains A, as their oxides dissolve in water to form basic (alkaline) solutions. These metals are commonly found in the earth's crust.
In simple words: Both the statement (Group 2 elements are alkaline earth metals) and the reason (their oxides make alkaline solutions in water) are true, and the reason correctly explains why they are called that.

🎯 Exam Tip: For assertion-reason questions, first check if both statements are individually true. If so, then determine if the reason correctly explains the assertion.

Question 2. (A): Noble gases are chemically inert in nature. Reason (R) : Noble gases have stable electronic structures.
(a) Both A & R are right
(b) Both A & R are wrong
(c) A is right R is wrong
(d) A is wrong R is right
Answer: (a) Both A & R are right
Assertion (A) is correct: Noble gases are chemically inert, meaning they do not easily react with other elements. Reason (R) is also correct and explains A: Noble gases have a complete outermost electron shell, which makes their electronic structure very stable and thus unreactive. This stability is usually due to having a full octet of electrons.
In simple words: Both the statement (noble gases don't react much) and the reason (they have full outer electron shells) are true and correctly linked.

🎯 Exam Tip: The inertness of noble gases is directly due to their stable electron configuration, typically an octet (8 valence electrons) or a duet (2 for Helium), which makes them unreactive.

Question 3. Assertion (A): Non-metals are electronegative. Reason (R): Non-metal lose electrons to form cation.
(a) Both A & R are right
(b) Both A & R are wrong
(c) A is right R is wrong
(d) A is the wrong R is right
Answer: (c) A is right R is wrong
Assertion (A) is true: Non-metals are electronegative, meaning they tend to attract electrons and gain them. Reason (R) is false: Non-metals usually gain electrons to form anions (negative ions), not lose electrons to form cations (positive ions).
In simple words: The statement that non-metals are electronegative is true. But the reason that non-metals lose electrons is false; they actually gain electrons.

🎯 Exam Tip: Understand the difference between electronegativity (tendency to attract electrons) and electropositivity (tendency to lose electrons). Non-metals are electronegative and primarily form anions (negative ions) by gaining electrons.

X. Long Answer Type Questions:

Question 1. List the advantages of alloys.
Answer: Alloys offer several advantages over pure metals:
* Alloys resist corrosion much better than pure metals, meaning they rust or wear away very slowly. This enhanced resistance makes them useful in harsh environments.
* They are generally harder and stronger than their pure metal components. For instance, gold is mixed with copper to make it harder and more durable for jewelry, as pure gold is too soft for everyday use.
In simple words: Alloys don't rust easily and are stronger and harder than pure metals. For example, gold jewelry is often mixed with copper to make it tougher.

🎯 Exam Tip: When discussing alloy advantages, focus on enhanced properties like corrosion resistance, hardness, and strength, and provide a relevant example for each.

Question 2. Write the advantages of the Modern Periodic Table.
Answer: The Modern Periodic Table offers several significant advantages:
* It is based on the atomic number, which is a more fundamental property of an element than atomic mass.
* The table clearly shows the relationship between an element's position and its electronic configuration.
* The completion of each period is logical; as atomic number increases, electron shells gradually fill up until a stable noble gas configuration is achieved.
* It is simple to understand, remember, and recreate due to its clear structural organization.
* Each group is treated as a single independent group, effectively doing away with the older concept of subgroups.
* It provides a justified and consistent place for all isotopes of an element, as isotopes have the same atomic number.
* The table accurately places all transition elements in the middle block, reflecting their intermediate properties between s-block and p-block elements.
* The table clearly separates metals from non-metals, with non-metals being located in the upper right corners.
* The positions of certain elements which were earlier misfit (interchanged) in Mendeleev's periodic table are now justified because it is based on an atomic number. (Enriching sentence)
* Justification has been offered for placing lanthanides and actinides at the bottom of the periodic table, preventing them from making the main table too wide.
In simple words: The modern periodic table is better because it uses atomic number, clearly shows electron arrangements, is easy to use, groups similar elements well, correctly places isotopes, and separates metals from non-metals. It provides a more logical and comprehensive view of elements.

🎯 Exam Tip: When listing advantages, ensure you cover key points like the basis (atomic number), electron configuration correlation, isotope placement, and clear metal/non-metal separation, along with its simplicity.

Question 3. Explain the position of hydrogen in the periodic table.
Answer: Hydrogen holds a unique and somewhat debated position in the periodic table because its properties are quite distinct and can resemble both alkali metals (Group 1) and halogens (Group 17). It is the lightest, smallest, and first element, with an electron configuration of \( 1s^1 \).
* Like alkali metals, hydrogen can lose its single electron to form a positive hydrogen ion \( (H^+) \).
* Similar to halogens, it can also gain one electron to form a negative hydride ion \( (H^-) \).
* However, unlike alkali metals which are solid at room temperature, hydrogen is a gas. This gaseous state is a significant difference.
Due to these dual and unique characteristics, its placement remains a subject of discussion among scientists, emphasizing its one-of-a-kind nature.
In simple words: Hydrogen is special because it can act like alkali metals (by losing an electron) and also like halogens (by gaining one). But it's a gas, unlike solid alkali metals. So, its exact place in the table is still debated because it's so unique.

🎯 Exam Tip: Highlight hydrogen's ability to both lose and gain electrons, drawing parallels to Group 1 and Group 17 elements, and explicitly mention its gaseous state as a key differentiating factor.

Question 4. List the features of Mendeleev's Periodic Table.
Answer: Mendeleev's Periodic Table had several key features that marked a significant advancement in chemistry:
1. It organized elements into eight vertical columns called 'groups' and seven horizontal rows known as 'periods'.
2. Each group was further divided into two subgroups, 'A' and 'B'. All elements within a specific group were found to have similar chemical properties.
3. For the first time, elements were systematically classified, ensuring that elements with similar characteristics were placed together. This methodical approach helped in understanding chemical relationships.
4. Mendeleev was able to correct the atomic masses of certain elements that had been wrongly determined. For example, the atomic mass of beryllium was reassessed from 14 to 9, allowing it to be placed correctly in the table.
5. He famously left empty spaces for elements that had not yet been discovered and accurately predicted their properties. For instance, he named these hypothetical elements 'Eka-Aluminium' and 'Eka-Silicon'. The later discovery of Gallium and Germanium, whose properties matched Mendeleev's predictions, demonstrated the strength of his periodic law. This predictive power was a major breakthrough.
Here is a comparison of Mendeleev's predictions for Eka-Silicon (which later became Germanium) and its actual properties:

🎯 Exam Tip: When describing Mendeleev's contributions, always include his correction of atomic masses and the prediction of undiscovered elements (like Eka-Aluminium and Eka-Silicon) as these are his most significant achievements.