Selina Concise Solutions for ICSE Class 9 Chemistry Chapter 7 Atomic Structure

Exercise 7

Question 1.
Answer: The latest research on atom has proved that most of the postulates of Dalton’s atomic theory contradict. But Dalton was right that atoms take part in chemical reactions. Comparison of Dalton’s atomic theory with Modem atomic theory.

In simple words: Dalton thought atoms were like solid, unbreakable balls that were all identical. Science now shows atoms are made of smaller parts, can be changed in nuclear reactions, and can have different weights (isotopes).

📝 Teacher's Note: Explain that scientific theories evolve as better technology (like electron microscopes and particle accelerators) allows us to see deeper into matter. Dalton's theory was a great start for the 1800s.

🎯 Exam Tip: Focus on the three key differences: divisibility, nuclear reactions, and isotopes. These are the "pillars" of the modern theory.

Question 2.
Answer:
(a) Inert elements: The elements which have complete outer most shell i.e. 2 or 8 electrons. They ordinarily do not enter into any reaction.
(b) These exist as monoatoms because molecules of these elements contain only one atom.
(c) Valence electrons: The number of electrons present in the outermost shell or valence shell is known as valence electrons.
In simple words: Inert elements are like people who are perfectly happy alone and don't want to make friends (react). Valence electrons are like the hands of an atom that reach out to grab other atoms.

📝 Teacher's Note: Use the "Octet Rule" analogy—atoms want to have 8 electrons in their outer shell to feel "satisfied" and stop reacting.

🎯 Exam Tip: Always remember that Helium is the exception to the octet rule; it is stable with only 2 electrons (duplet).

Question 3.
Answer: The three isotopes of hydrogen differ only due to their mass number which is respectively 1, 2 and 3 and named protium, deuterium and tritium.
The isotopes are represented as \( ^{1}_{1}H \), \( ^{2}_{1}H \), and \( ^{3}_{1}H \).
In simple words: All three are forms of Hydrogen, so they all have 1 proton. The only difference is the "extra baggage"—Deuterium has 1 neutron and Tritium has 2, making them heavier.

📝 Teacher's Note: This is the only element where isotopes have specific names. Usually, we just say Carbon-12 or Carbon-14.

🎯 Exam Tip: Draw the nuclei clearly showing the number of protons (always 1) and neutrons (0, 1, or 2).

Question 4.
Answer:

In simple words: Every element has a unique ID number called the Atomic Number. Valency is just the number of "arms" the atom uses to grab onto others.

📝 Teacher's Note: Encourage students to memorize the first 20 elements. It makes identifying elements from their atomic numbers much faster during exams.

🎯 Exam Tip: Valency is derived from the number of electrons an atom needs to gain or lose to reach 8. For atomic number 19 (Potassium), the configuration is 2, 8, 8, 1, so it loses 1 to have valency 1.

Question 5.
Answer:

Bohr-Bury Model Diagrams: In simple words:

📝 Teacher's Note: Remind students that Atomic Mass = Protons + Neutrons. This formula is the key to filling out these tables.

🎯 Exam Tip: When drawing Bohr models, always label the shells as K, L, M, and so on. Max electrons in shells follow the \( 2n^2 \) rule.

Question 6.
Answer: The significance of the number of protons found in the atoms of each different element is that it determines the element's unique identity and fixes its place in the periodic table (Atomic Number).
In simple words: Protons are like an atom's fingerprint. If you change the number of protons, you change the element itself.

📝 Teacher's Note: While electrons and neutrons can change (making ions or isotopes), protons define the element. It's the most stable part of an atom's identity.

🎯 Exam Tip: Atomic Number \( (Z) \) = Number of Protons. This is the fundamental property on which the Modern Periodic Table is based.

Question 7.
Answer:

(a) Y (2,7) forms Anion because it gains one electron to complete its octet.
(b) Z (2, 8, 2) forms Cation because it loses two electrons to achieve stability.
(c) X (2,4) has four electrons in the valence shell.
In simple words: Atoms gain or lose electrons to get a full outer shell. Losing negative electrons makes you a "positive" Cation. Gaining them makes you a "negative" Anion.

📝 Teacher's Note: Use the memory trick: **PAW** (Positive is Anode, but better for this context: **P**ositive is **C**ation). Think of **Cation** as "Cat-ion" and cats have "paws" (positive).

🎯 Exam Tip: Elements with 1, 2, or 3 valence electrons generally lose them (metals), while those with 5, 6, or 7 gain them (non-metals).

Question 8.
Answer:
(a) \( X^{+1} \) represents an atom of X that has lost one electron.
(b) An atom that accepts electrons acts as an oxidising agent, while an atom that can donate electrons (like metals) acts as a reducing agent.
In simple words: Metals are generous—they give away electrons. This makes them reducing agents. Non-metals are greedy—they take electrons, making them oxidising agents.

📝 Teacher's Note: Use the acronym **OIL RIG**: Oxidation Is Loss, Reduction Is Gain (of electrons).

🎯 Exam Tip: A positive superscript like \( +1 \) or \( +2 \) always means electrons were **lost**, not added.

Question 9.
Answer:
(a) Mass number: The atomic mass number is defined as the sum of the number of protons and neutrons contained in the nucleus of an atom of that element. It is denoted by the symbol A.
(b) Ion: An atom or molecule that carries a positive or negative charge because of loss or gain of electrons.
(c) Cation: It is positively charged ion that is formed when an atom loses one or more electrons e.g. \( Na^+ \), \( Hg^{2+} \), \( Ca^{2+} \) etc.
(d) Atom: It is defined as the smallest unit of matter which takes part in a chemical reaction.
(e) Element: It is a substance which cannot be split up into two or more simpler substances by usual chemical methods of applying heat, light or electric energy. e.g. Hydrogen, Oxygen, Chlorine etc.
(f) Orbit: It is defined as a circular path around the nucleus in which electrons of the atom revolve.
In simple words: These are the basic building blocks of chemistry language. An atom is a single unit, an element is a pure substance made of one kind of atom, and an ion is an atom with a charge.

📝 Teacher's Note: Help students visualize the atom like a tiny solar system where the nucleus is the sun and orbits are the paths of the planets (electrons).

🎯 Exam Tip: Definitions are often asked verbatim. Ensure you mention "sum of protons and neutrons" for mass number and "takes part in chemical reaction" for atom.

Question 10.
Answer: Atomic number = 2, Mass number = 4
This element is Helium (He).
In simple words: If an atom has 2 protons (Atomic number 2), it's Helium. With a mass of 4, it also has 2 neutrons.

📝 Teacher's Note: Helium is the first noble gas. Its mass being twice its atomic number is common for many light elements.

🎯 Exam Tip: Always state the name of the element if the atomic number is given and the question asks for its identity.

Question 11.
Answer:
(a)
(i) Atom E contains 7 protons.
(ii) Atom E has an electronic configuration 2, 7.

(b)
Atom C stands for \( ^{7}_{3}Li \) (Lithium)
Atom D stands for \( ^{16}_{8}O \) (Oxygen)
Compound formula = \( Li_2O \)

(c) Metals are: A and C; Non-metals are: E, D, E.
In simple words: By looking at protons, we find the element. Lithium (metal) and Oxygen (non-metal) combine to form Lithium Oxide.

📝 Teacher's Note: Practice the criss-cross method for formulas: \( Li \) has valency 1, \( O \) has valency 2, so you need two \( Li \) for every one \( O \).

🎯 Exam Tip: Metals are found on the left side of the periodic table and have 1-3 valence electrons.

Question 12.
Answer: No. of electrons in M Shell = 2
Number of electrons in K and L shell will be 2, 8 respectively.
\( \implies \) Electronic configuration will be: 2, 8, 2.
\( \implies \) Atomic number = \( 2 + 8 + 2 = 12 \).
\( \implies \) Since, atomic number = Number of Protons.
\( \implies \) No of Protons = 12
In simple words: Shells must be filled in order (2 then 8). If the third shell (M) has 2, the atom must have a total of 12 electrons, meaning it's Magnesium.

📝 Teacher's Note: This logic only works for neutral atoms. For ions, the number of protons and electrons would differ.

🎯 Exam Tip: Always show the addition \( 2 + 8 + \dots \) to prove how you reached the final atomic number.

Question 13.
Answer:
(a) (i) Comparison of Magnesium Isotopes:

(a) (ii) Electronic configuration = 2, 8, 2
(b) Mass numbers are different for these two isotopes of magnesium because they contain a different number of neutrons (12 and 14 respectively).
In simple words: These are two versions of Magnesium. They act the same in chemicals because they have the same electrons, but one is slightly "chubbier" because it has more neutrons.

📝 Teacher's Note: Isotopes have identical chemical properties but different physical properties like density and boiling point because of the mass difference.

🎯 Exam Tip: When asked why isotopes have different mass, always specify that the **neutron** count is the only thing that changes.

Question 14.
Answer:
Nucleons: Particles which constitute the nucleus are called nucleons. Protons and neutrons are the nucleons.
\( \implies \) At. weight of phosphorus = 31; Atomic number = 15
\( \implies \) Total number of nucleons = 31 (No. of P + No. of N)
\( \implies \) No. of neutrons = \( 31 - 15 = 16 \)
\( \implies \) Electronic configuration = 2, 8, 5
In simple words: Nucleons are the "roommates" living inside the nucleus. For Phosphorus, there are 31 roommates total: 15 protons and 16 neutrons.

📝 Teacher's Note: The term "Nucleon" is just a collective name for protons and neutrons. It's essentially the mass number.

🎯 Exam Tip: For Phosphorus (Atomic number 15), remember the shell filling: 2 in the first, 8 in the second, and the remaining 5 in the third.

Question 15.
Answer:
(a) Atoms of the same elements differing in the number of neutrons in their nuclei are known as isotopes.
The fundamental particle that differs is the Neutron.
Uses of isotopes:
1. Some isotopes are Radioactive; isotopes of cobalt are used for treating cancer and other diseases.
2. An isotope of uranium (\( ^{235}U \)) is used as a fuel in nuclear reactors.

(b) Diagrams for Hydrogen isotopes: Protium (\( ^{1}H \)), Deuterium (\( ^{2}H \)), Tritium (\( ^{3}H \)).
In simple words: Isotopes aren't just for science books—we use them to fight cancer and produce clean energy in power plants.

📝 Teacher's Note: Cobalt-60 is the specific isotope used in radiotherapy. Highlighting real-world uses makes the concept more relatable.

🎯 Exam Tip: When asked for the "fundamental particle that differs," the answer is always **Neutron**.

Question 16.
Answer: In chemical reactions, only electrons take part. The chemical properties depend upon the electronic configuration.
The isotopes of element \( ^{35}_{17}Cl \) and \( ^{37}_{17}Cl \) have the same atomic number and hence the same configuration. So they have the same chemical properties. They differ only in physical contents and weights because neutrons contribute to the mass of an atom.
\( ^{35}_{17}Cl \) and \( ^{37}_{17}Cl \) have a different number of neutrons: 18 and 20 respectively.
In simple words: Chemistry is all about electrons. Since isotopes have the same number of electrons, they react exactly the same way. Their only difference is their "weight" on a scale.

📝 Teacher's Note: This is a classic "Give Reason" question. The logic is: Same electrons \( \implies \) Same configuration \( \implies \) Same chemical behavior.

🎯 Exam Tip: If an exam asks why isotopes have the same chemical properties, your answer MUST mention "same electronic configuration."

Question 17.
Answer: The atomic masses of the isotopes of chlorine are 35 and 37. In nature, they occur in a 3:1 ratio (75% \( Cl^{35} \) and 25% \( Cl^{37} \)).
Fractional atomic weight calculation:
\( \text{Atomic weight} = \frac{3(35) + 1(37)}{4} = \frac{105 + 37}{4} = \frac{142}{4} = 35.5 \)
\( \implies \) Atomic weight of Chlorine = 35.5
In simple words: Because nature has more of the "35" version than the "37" version, the average weight isn't right in the middle (36), but closer to 35.

📝 Teacher's Note: This explains why many atomic masses on the periodic table are not whole numbers. They are weighted averages of all naturally occurring isotopes.

🎯 Exam Tip: The formula is: \( (\text{mass}_1 \times \%_1 + \text{mass}_2 \times \%_2) / 100 \). Practice this calculation as it's a favorite for word problems.

Question 18.
Answer:
(a) Atomic number: The number of protons present in the nucleus of an atom is the atomic number of that atom. It is represented by the symbol Z.
Atomic number (Z) = Number of protons (p)

(b)

(c) Electronic configuration of chlorine is 2, 8, 7.
In simple words: Even though the mass changes, the ID number (Atomic number) stays 17. That's why they are both called Chlorine.

📝 Teacher's Note: Note how the electron and proton counts are always equal for a neutral atom, regardless of which isotope it is.

🎯 Exam Tip: In the symbol \( ^{A}_{Z}X \), always remember that the bigger number \( (A) \) is on top and is the Mass Number.

Question 19.
Answer:
(a) Hydrogen
(b) Element of zero group i.e. He (Helium)
(c) Calcium (2, 8, 8, 2). There are 2 electrons in valence shell. Hence valency is 2.
(d) Chlorine atoms: \( ^{35}_{17}Cl \) and \( ^{37}_{17}Cl \)
(e) K shell.
In simple words: This is a quick identification guide. For (c), since it has 2 extra electrons it wants to get rid of, its valency is 2.

📝 Teacher's Note: Sub-part (e) refers to the shell closest to the nucleus, which always has the lowest energy level.

🎯 Exam Tip: Group 18 elements (Helium, Neon, Argon) are also called the "Zero Group" because their valency is zero.

Question 20.
Answer:
(a) Physical properties depend on the Atomic mass, and isotopes have different mass numbers because they have a different number of neutrons. Thus, isotopes have different physical properties.
(b) Argon does not react because it has a completely filled outer-most orbit. Its atomic number is 18, so its configuration is 2, 8, 8. With 8 electrons in the valence shell, it is stable.
(c) Actual Atomic Mass is usually greater than mass number (P + N) because mass number is just a whole number approximation. In reality, neutrons are slightly heavier than protons, and there are other sub-atomic particles involved.
(d) \( ^{35}_{17}Cl \) and \( ^{37}_{17}Cl \) are isotopes of chlorine which differ only in the number of neutrons. They are chemically alike because chemical properties depend on electronic configuration, which is identical for both.
In simple words: Mass = Physical properties (weight, boiling point). Electrons = Chemical properties (how it reacts). Argon is chemically "anti-social" because its electron shells are already full.

📝 Teacher's Note: Discuss the concept of "Inertia" in Argon—it's like a car in neutral; it has no desire to move (react).

🎯 Exam Tip: For the Argon question, always write out the configuration (2, 8, 8) to prove why it doesn't react.

Question 21.
Answer:
Element A: Atomic number = 7; Config = 2, 5; Valency = 8 - 5 = 3-
Element B: Config = 2, 8, 8; Valency = Zero
Element C: Electrons = 13; Config = 2, 8, 3; Valency = 3+
Element D: Protons = 18; Config = 2, 8, 8; Valency = Zero
Element E: Config = 2, 8, 8, 1; Valency = 1+

[ii] C and E are metals. [iii] A is a Non-metal. [iv] B and D are inert gases.
In simple words: Metals (C, E) have 1-3 outer electrons. Non-metals (A) have 5-7. Inert gases (B, D) have 8.

📝 Teacher's Note: This is a great exercise for identifying families of elements based solely on their electronic "address."

🎯 Exam Tip: If valence electrons are \( \le 3 \), valency = valence electrons. If \( \ge 4 \), valency = 8 - valence electrons.

Question 22.
Answer:
(a) C. Atomic nucleus
(b) A. 6
(c) C. 2, 8, 8, 1
In simple words: These are standard MCQ answers. The nucleus is the center of mass, Carbon-12 has 6 neutrons, and Potassium is 2, 8, 8, 1.

📝 Teacher's Note: For (b), remember: Mass (12) - Protons (6) = Neutrons (6).

🎯 Exam Tip: Don't rush MCQs. Calculate the neutron count even if it seems obvious.

Question 23.
Answer: Elements combine to attain the stable electronic configuration of the nearest noble gas (Duplet or Octet).

(a) Sodium Chloride:
Sodium (2, 8, 1) loses one electron to become \( Na^+ \).
\( Na \rightarrow Na^+ + e^- \)
Chlorine (2, 8, 7) gains that electron to become \( Cl^- \).
\( Cl + e^- \rightarrow Cl^- \)
They form an Ionic Bond.
(b) Hydrogen \( H_2 \):
Each Hydrogen atom contributes one electron to form a shared pair. This is a Covalent Bond.
In simple words: Ionic bonding is like "giving" a gift. Covalent bonding is like "sharing" a toy. Both ways, atoms end up happy and stable.

📝 Teacher's Note: Emphasize that ionic bonds happen between metals and non-metals, while covalent bonds usually happen between two non-metals.

🎯 Exam Tip: When drawing NaCl, show the charge on the ions (\( Na^+ \) and \( Cl^- \)) after the transfer is complete.

Question 24.
Answer:

In simple words: This summary table ties everything together—Atomic Number = Protons = Electrons, and Mass Number = Protons + Neutrons.

📝 Teacher's Note: The table has a few calculated values that differ slightly from standard textbook "perfect" isotope examples—always follow the mass number provided in the specific question.

🎯 Exam Tip: Double-check your subtraction! Neutrons = Mass Number \( - \) Atomic Number. This is the most common place for silly mistakes.