Selina Concise Solutions for ICSE Class 9 Chemistry Chapter 8 The Periodic Table

Exercise 8

Question 1.
Answer: (a) In 1913, Mosley made an important discovery of atomic number. Thus, he gave the modern periodic law that states that, The physical and the chemical properties of elements are the periodic functions of their atomic numbers.
(b) Eighteen groups and seven periods.

In simple words: The modern law says that if you arrange elements by their atomic number (the number of protons), their traits repeat in a predictable pattern. The table is laid out in 18 columns and 7 rows.

📝 Teacher's Note: Remind students that before Moseley, Mendeleev used atomic mass. Explain that the atomic number is a more fundamental property because it represents the number of protons and electrons in a neutral atom.

🎯 Exam Tip: When defining the Modern Periodic Law, ensure you use the phrase "periodic functions of their atomic numbers" as it is the key technical requirement.

Question 2.
Answer: Last elements of each period have outer most shell complete i.e. 2 or 8 electrons. Name is Inert-gases or Noble gases.
In simple words: The elements at the far right of the table have a full set of electrons, which makes them very stable and unlikely to react with other chemicals.

📝 Teacher's Note: Use the analogy of a "full house" to explain stability. Helium has 2 electrons (duplet), while others like Neon and Argon have 8 (octet) in their valence shell.

🎯 Exam Tip: If asked why noble gases are unreactive, mention their "stable electronic configuration" or "complete octet."

Question 3.
Answer:
(a) A group: Vertical columns in a periodic table which have same-number of valence electrons, and similar chemical properties are called a group.
(b) A period: In a periodic table elements are arranged in the order of increasing atomic numbers in horizontal rows called periods.
In simple words: Groups go up and down and contain "family members" with similar traits. Periods go side-to-side like sentences on a page.

📝 Teacher's Note: Help students remember: "G" for Group is vertical like a column, and "P" for Period is horizontal like a row.

🎯 Exam Tip: Remember that elements in the same group have the same chemical properties because they have the same number of valence electrons.

Question 4.
Answer: Atomic number can determine that which element will be the first and which is the last in a period of the periodic table.
In simple words: Knowing the atomic number tells us exactly where an element fits in a row, starting from the smallest number on the left.

📝 Teacher's Note: Explain that each period starts with an Alkali metal (1 valence electron) and ends with a Noble gas (full outer shell).

🎯 Exam Tip: The atomic number is unique for each element; no two elements have the same one.

Question 5.
Answer: (a) Group I A elements: Lithium, Sodium.
(b) Group 17 element: Fluorine, Chlorine
(c) Group 18 (zero group elements) : Helium; Neon
In simple words: These are examples of common elements found in specific groups of the periodic table.

📝 Teacher's Note: Encourage students to memorize the first three elements of Groups 1, 17, and 18, as they are frequently used in examples.

🎯 Exam Tip: Group 18 is also known as the "Zero Group" because its valency is zero.

Question 6.
Answer: (a) Group IA is known as Alkali Metals group.
(b) Group 17 is known as Halogens group. (c) Group 18 is known as Transition elements.
In simple words: These are the special names given to specific columns in the table based on how the elements behave.

📝 Teacher's Note: Note to students: While the source text lists Group 18 as Transition elements, Transition elements actually belong to groups 3 through 12. Group 18 is correctly known as Noble gases.

🎯 Exam Tip: Always associate Group 17 with "Halogens"—the salt-formers.

Question 7.
(a) Number of elements in 1st period of the modem periodic table - 2 (two)
(b) Number of elements in 3rd period of the modem periodic table - 8 (eight)
Answer: (a) 1st Period: 2 elements
(b) 3rd Period: 8 elements
In simple words: The first row of the table is very short with only 2 elements, while the third row has 8.

📝 Teacher's Note: Explain that the number of elements in a period corresponds to the maximum number of electrons that can fit in the shells (K shell holds 2, L and M hold 8 in short periods).

🎯 Exam Tip: The 1st period is called the "Very Short Period" and the 2nd and 3rd are "Short Periods."

Question 8.
Answer: (a) Increases
(b) Increases
In simple words: As you move from left to right across a row, the center of the atom gets stronger and the atom's ability to grab electrons also grows.

📝 Teacher's Note: Explain that as protons are added to the nucleus, the "pull" on electrons gets stronger, which is why nuclear charge increases.

🎯 Exam Tip: Ionization energy and electron affinity also increase from left to right across a period.

Question 9.
Answer: The valency of elements increases from 1 to 4 upto the element carbon(C), and then falls to 1 upto the element fluorine (F) and Neon (Ne) Zero.
In simple words: Valency (bonding power) goes up like a hill and then comes back down as you move across a row.

📝 Teacher's Note: Use the octet rule to show that atoms with 1-4 outer electrons lose/share them, while those with 5-7 gain them to reach 8.

🎯 Exam Tip: Remember the pattern for Period 2: 1, 2, 3, 4, 3, 2, 1, 0.

Question 10.
Answer: (a) periods
(b) increases
(c) decreases
In simple words: These are the standard trends seen as you travel through the rows of the periodic table.

📝 Teacher's Note: Across a period, elements change from metals on the left to non-metals on the right, so metallic character actually decreases.

🎯 Exam Tip: Always double-check trends; atomic size *decreases* across a period due to stronger nuclear pull.

Question 11.
Answer: In a period, the size of an atom decreases from left to right. This is because the nuclear charge increases from left to right in the same period, thereby bringing the outermost shell closer to the nucleus.
Thus, in a particular period, the alkali atoms have the largest size and the halogen atoms are the smallest.
In simple words: Atoms shrink as you go across a row because the center of the atom gets more positive and pulls the electrons in tighter.

📝 Teacher's Note: This is a key trend. Even though more electrons are added, they are all in the same shell, and the increasing nuclear pull wins out, making the atom smaller.

🎯 Exam Tip: Always mention "increasing nuclear charge" as the reason for the decrease in atomic size across a period.

Question 12.
(a) (i) Hand P are noble gases
(ii) G and 0 are halogens
(iii) A and I are alkali metals
(iv) D and L have valency of 4.
(b) The formula of the resulting compound will be \( \text{Li}_2\text{O} \). Because, A stands for Lithium and F stands for O and valency of Lithium is +1 and valency of O is -2. That is, \( \text{A}_2\text{F} \).
(c) The atomic number of G is 9. Therefore, its electronic arrangement is 2 and 7. That is, (2, 7).
Answer:
(a) (i) Noble Gases: H and P
(ii) Halogens: G and O (Note: source says 0, likely O)
(iii) Alkali Metals: A and I
(iv) Elements with valency 4: D and L.
(b) Resulting compound: \( \text{Li}_2\text{O} \). Lithium (A) has valency +1 and Oxygen (F) has valency -2. Using the criss-cross method, we get \( \text{A}_2\text{F} \).
(c) For G (Fluorine), atomic number 9: \( (2, 7) \).
In simple words: This solution uses code names for elements to test your knowledge of their groups and how they combine to form compounds.

📝 Teacher's Note: The "criss-cross method" is essential for formula writing. Remind students that the total charge of a neutral compound must be zero.

🎯 Exam Tip: When writing electronic configurations, ensure the first shell only holds up to 2 electrons.

Question 13.
Answer: This is because; Na and Al have capacity to donate the electron due to which there valency is positive. Whereas Cl and K can gain or lose one electron due to which their valency is -1 and + 1 respectively. Only this is the difference between these two.
In simple words: Some atoms like to give away electrons (positive valency), while others prefer to take them (negative valency).

📝 Teacher's Note: Explain that metals usually lose electrons and non-metals usually gain them. Note: In the source text, Potassium (K) is listed alongside Chlorine as gaining/losing, though K is an alkali metal that usually only loses 1.

🎯 Exam Tip: Cations have positive valency and Anions have negative valency.

Question 14.
(a) The greatest metallic character can be expected at the bottom of the group.
(b) The largest atomic size can be expected at the lower part of the group.
Answer: (a) Metallic character: Highest at the bottom.
(b) Atomic size: Largest at the bottom.
In simple words: As you go down a column, elements become more "metallic" and their atoms get much bigger.

📝 Teacher's Note: Atomic size increases down a group because new electron shells are being added for each period.

🎯 Exam Tip: Francium is the most metallic element because it is at the bottom of Group 1.

Question 15.
Answer: No. of valence electrons remain same as we go down in a group that too on the left side of a period.
In simple words: All elements in a family (column) have the same number of "fingers" (outer electrons) to grab other atoms with.

📝 Teacher's Note: This is the reason why elements in the same group have similar chemical properties—their bonding behavior is almost identical.

🎯 Exam Tip: The group number (for groups 1, 2, 13-17) indicates the number of valence electrons.

Question 16.
Answer: (a) Magnesium is the ‘x’ element which belongs to the third period and group II of the periodic table and has atomic number 2. Therefore, the number of valence electrons are 2 (two).
(b) Valency is + 2.
(c) Alkaline earth metals.
(d) The elements is Magnesium
In simple words: Magnesium is a metal found in the second column that likes to give away two electrons to become stable.

📝 Teacher's Note: Note to students: Magnesium's atomic number is actually 12. Its valence electron count is 2.

🎯 Exam Tip: Group 2 elements are always called "Alkaline Earth Metals."

Question 17.
Answer:  (a) VII A
(b) Third period
(c) Seven
(d) Valency of T = -1
(e) Non - metal
In simple words: This element is a gas that needs just one more electron to be "happy," making it a very reactive non-metal.

📝 Teacher's Note: Element 'T' is Chlorine. Having 7 valence electrons makes it extremely reactive as it only needs 1 more for a full octet.

🎯 Exam Tip: Elements in Group 17 always have a valency of -1.

Question 18.
Answer: (a) Helium
(b) Silicon
(c) 4, 3
(d) Argon
(e) Noble gases
(f) Carbon tetrachloride (\( \text{CCl}_4 \))
(g) Silicon, Phosphorus
(h) Sodium chloride (NaCl)
(i) Li and Mg; Be and Al; B and Si
(j) Sodium
(k) Typical elements of Period 2 belonging to group 14 and 15 are carbon and nitrogen. Typical elements of’ Period 3 belonging to group 14 to 15 are silicon and phosphorus.
(l) Beryllium
In simple words: This list provides short facts and examples for various categories of elements and their relationships.

📝 Teacher's Note: "Diagonal relationship" describes how certain elements in the 2nd period share traits with elements one group over in the 3rd period.

🎯 Exam Tip: Carbon tetrachloride is a common example of a covalent compound formed by Group 14 elements.

Question 19.
Answer:

In simple words: This table matches family names with their defining characteristics, like unreactive Noble gases or highly reactive Alkali metals.

📝 Teacher's Note: Use this matching table as a summary guide for students to learn the different blocks and families of the Periodic Table.

🎯 Exam Tip: Transition elements occupy the middle block of the periodic table.

Question 20.
Answer:

In simple words: This table groups elements together based on their electron "addresses." Elements with similar configurations live in the same group.

📝 Teacher's Note: Remind students that the "last number" in the electronic configuration (valence electrons) is what determines which group an element belongs to.

🎯 Exam Tip: Group 1 elements (Alkali metals) always end with "1" in their configuration.

Question 21.
Answer: The relative atomic mass of a light element up to calcium is approximately 20 its atomic number. (Note: Source likely means *twice* its atomic number).
In simple words: For many common small atoms, the weight is usually about double the atomic number.

📝 Teacher's Note: This is a useful "rule of thumb" for students. For example, Oxygen (8) has a mass of 16, Carbon (6) has a mass of 12.

🎯 Exam Tip: This rule works best for the first 20 elements; heavier elements often have more neutrons, making their mass more than double.

Question 22.
Answer: Atomic number of P = 19
Electronic configuration = 2, 8, 8, 1
Group number of the element = 1 A
Period number of the element = 4
P is a metal.
In simple words: Potassium has 19 protons. Since it has only one electron in its outer shell, it's a reactive metal found in the 4th row.

📝 Teacher's Note: Element 'P' here stands for a placeholder, but atomic number 19 is specifically Potassium (K).

🎯 Exam Tip: The number of shells tells you the Period number (4 shells = Period 4), and the number of valence electrons tells you the Group number (1 electron = Group 1).