NCERT Class 9 Science Chapter 9 Atomic Foundations of Matter

By Parimol Roy / NCERT Class 9 Science Solutions

NCERT Class 9 Science Chapter 9 Atomic Foundations of Matter Solutions English Medium As Per New Syllabus. NCERT Class 9 Science Chapter 9 Atomic Foundations of Matter Notes to each chapter is provided in the list so that you can easily browse throughout different chapter NCERT Class 9 Science Chapter 9 Atomic Foundations of Matter Question Answer Download PDF and select needs one. CBSE Class 9 Science Textual Solutions.

NCERT Class 9 Science Chapter 9 Atomic Foundations of Matter

Also, you can read the CBSE book online in these sections Solutions by Expert Teachers as per (CBSE) Book guidelines. NCERT Class 9 Science Chapter 9 Atomic Foundations of Matter Solutions. These solutions are part of NCERT All Subject Solutions. Here we have given NCERT Class 9 Science Textbook Solutions English Medium Solutions for All Subject, You can practice these here.

Atomic Foundations of Matter

Chapter – 9

Pause And Ponder

1. A student burns 10 g of ethanol in an open beaker. After the reaction, no residue is left in the beaker. Does this mean the Law of Conservation of Mass is violated? Explain.

Ans: No, the Law of Conservation of Mass is not violated. When ethanol burns, it reacts with oxygen from the air to form carbon dioxide and water vapour. These products escape into the air because the reaction takes place in an open beaker. Therefore, although no residue is left in the beaker, the total mass of the reactants is equal to the total mass of the products.

2. When 20 g of hydrogen reacts completely with 160 g of oxygen, how much water is formed according to the Law of Conservation of Mass?

Ans: Given:

Mass of hydrogen = 20 g

Mass of oxygen = 160 g

According to the Law of Conservation of Mass,

Mass of water formed = Mass of hydrogen + Mass of oxygen

= 20 g + 160 g = 180 g

Ans: 180 g of water is formed.

Pause And Ponder

1. A compound consists of 40% sulfur and 60% oxygen by mass. In a sample of the same compound containing 20 g of sulfur, what mass of oxygen must be present to satisfy the Law of Constant Proportions?

Ans: Given:

Sulfur = 40%

Oxygen = 60%

Sulfur/Oxygen = 40 : 60 = 2 : 3

If sulfur = 20 g,

Oxygen = 2/3×20 = 30 g

2. Carbon monoxide (CO) contains carbon and oxygen in the mass ratio of 3:4. How much oxygen will combine with 9 g of carbon to form carbon monoxide?

Ans: Given:

Oxygen = 3 : 4

If Carbon = 9 g,

Oxygen = 4/3 × 9 = 12 g

3. The Law of Definite Proportions holds true for compounds but not for mixtures. Give reason.

Ans: Compounds always contain the same elements combined in a fixed ratio by mass, so they obey the Law of Definite Proportions. However, mixtures can contain components in any proportion, so they do not follow this law.

4. Students X and Y, both prepared an oxide of copper by combining copper and oxygen in the ratios of 4:1 and 8:2, respectively. Do their results justify the Law of Constant Proportions? Explain.

Ans: Yes. The ratios are:

Student X: 4 : 1

Student Y: 8 : 2 = 4 : 1

Since both students obtained the same mass ratio of copper to oxygen (4 : 1), their results justify the Law of Constant Proportions, which states that a compound always contains the same elements in the same fixed ratio by mass.

Pause And Ponder

1. Assertion (A): 2 g of hydrogen combines with 16 g of oxygen to form 18 g of water.

Reason (R): According to Dalton’s Atomic Theory, atoms combine in a simple whole number ratio by mass to form compounds.

Choose the correct option:

(i) Both A and R are true, and R is the correct explanation of A.

(ii) Both A and R are true, but R is not the correct explanation of A.

(iii) A is true, but R is false.

(iv) A is false, but R is true.

Ans: (ii) Both A and R are true, but R is not the correct explanation of A.

Pause And Ponder

1. Nitrogen has five valence electrons. Draw the structure of the nitrogen molecule (N₂).

Ans: Each nitrogen atom has 5 valence electrons. To complete its octet, each nitrogen atom shares three electrons with the other nitrogen atom, forming a triple covalent bond.

Lewis Structure:

Each nitrogen atom has one lone pair of electrons and both atoms achieve a stable octet.

2. The atomic number of fluorine is 9. Explain the formation of the fluorine molecule (F₂).

Ans: Fluorine (atomic number 9) has the electronic configuration 2,7. Therefore, each fluorine atom has 7 valence electrons and needs one more electron to complete its octet.

Two fluorine atoms share one pair of electrons, forming a single covalent bond. As a result, both atoms achieve a stable octet.

Each fluorine atom has three lone pairs of electrons and one shared pair, making the F₂ molecule stable.

Pause And Ponder

1. Show the formation of the following molecules:

(i) Carbon dioxide (CO₂).

Ans: Carbon has 4 valence electrons and each oxygen atom has 6 valence electrons. Carbon shares two pairs of electrons with each oxygen atom, forming two double covalent bonds. Thus, all the atoms complete their octets.

Lewis Structure:

(ii) Hydrogen sulfide (H₂S).

Ans: Sulphur has 6 valence electrons and each hydrogen atom has 1 valence electron. Sulphur shares one electron with each hydrogen atom, forming two single covalent bonds. Each hydrogen attains a duplet, and sulphur completes its octet.

Lewis Structure:

(iii) Ammonia (NH₃).

Ans: Nitrogen has 5 valence electrons and each hydrogen atom has 1 valence electron. Nitrogen shares one electron with each of the three hydrogen atoms, forming three single covalent bonds. Nitrogen retains one lone pair of electrons and completes its octet.

Lewis Structure:

2. Neon (atomic number 10) neither transfers nor shares its valence electrons. Explain.

Ans: Neon has the electronic configuration 2,8. Its outermost shell contains 8 electrons, which is a complete octet. Therefore, neon is chemically stable and does not need to gain, lose, or share electrons. Hence, it neither transfers nor shares its valence electrons and remains inert.

Pause And Ponder

1. What kind of ion will oxygen (O) form?

Ans: Oxygen has 6 valence electrons. It gains 2 electrons to complete its octet and forms a negative ion (anion) called the oxide ion (O²⁻).

2. Fill in the blanks.

Among magnesium and chlorine, magnesium atom can give two electrons to become Mg². However, chlorine can take only one electron to become ______________ Now, ___________ ion of magnesium and _____________ ions of chlorine combine to give magnesium chloride.

Ans: Among magnesium and chlorine, magnesium atom can give two electrons to become Mg²⁺. However, chlorine can take only one electron to become Cl⁻. Now, one ion of magnesium and two ions of chlorine combine to give magnesium chloride (MgCl₂).

3. Show the formation of cations of potassium (K) and calcium (Ca) atoms, and the formation of their corresponding chlorides using diagrams.

Ans: (i) Formation of Potassium Chloride (KCl):

Electron-dot diagram:

(ii) Formation of Calcium Chloride (CaCl₂):

Electron-dot diagram:

4. Illustrate how sodium sulfide (Na,S) is formed.

Ans: Formation:

2Na → 2Na⁺ + 2e⁻

S + 2e⁻ → S²⁻

2Na⁺ + S²⁻ → Na₂S

Electron-dot diagram:

Pause And Ponder

16. Name the following:

(i) CO₂ ____________.

Ans: Carbon dioxide.

(ii) NO₂ ___________.

Ans: Nitrogen dioxide.

(iii) SF₆ ___________.

Ans: Sulphur hexafluoride.

(iv) PCl₃ ______________.

Ans: Phosphorus trichloride.

17. Write the formula for the following:

(i) Sodium hydrogencarbonate ____________.

Ans: NaHCO₃

(ii) Sulfur dioxide ______________.

Ans: SO₂

(iii) Ferric chloride _____________.

Ans: FeCl₃

(iv) Cuprous oxide ___________.

Ans: Cu₂O

18. Write the formula for the compounds formed from the following pairs of ions:

(i) Fe³⁺ and OH⁻

Ans: Fe(OH)₃

(ii) K⁺ and CO₃²⁻

Ans: K₂CO₃

Pause And Ponder

1. What type of chemical bond is present in a solid compound that does not conduct electricity in the solid state but conducts electricity when dissolved in water?

Ans: The compound has an ionic (electrovalent) bond. In the solid state, the ions are fixed in position and cannot move, so it does not conduct electricity. When dissolved in water, the ions become free to move and conduct electricity.

2. Metal M, with two electrons in its valence shell (M shell), reacts with oxygen to form a compound that is slightly soluble in water. Predict its:

(i) Formula.

Ans: MO.

(ii) Type of bond.

Ans: Ionic (electrovalent) bond.

(iii) Electrical conductivity of its aqueous solution.

Ans: Its aqueous solution conducts electricity because it contains free ions, although the conductivity is low due to its slight solubility in water.

Pause And Ponder

21. Find the molecular mass of nitric acid (HNO₃). Atomic mass – H = 1 u; N = 14 u; O = 16 u.

Ans: Molecular mass of HNO₃ = (1 × 1) + (1 × 14) + (3 × 16).

= 1 + 14 + 48.

= 63 u.

22. Find the molecular mass of methane (CH₄). Atomic mass – C = 12 u; H = 1 u.

Ans: Molecular mass of CH₄ = (1 × 12) + (4 × 1).

= 12 + 4.

= 16 u.

Pause And Ponder

1. Find the formula unit mass of potassium chloride (KCl). Atomic mass – K = 39 u; Cl = 35.5 u.

Ans: Formula unit mass of KCl = 39 + 35.5.

= 74.5 u.

2. Find the formula unit mass of magnesium hydroxide, Mg(OH)₂. Atomic mass – Mg = 24 u; O = 16 u; H = 1 u.

Ans: Formula unit mass of Mg(OH)₂ = 24 + 2(16 + 1).

= 24 + 2 × 17.

= 24 + 34.

= 58 u.

Pause And Ponder

1. A particular element (A) has one electron in its third shell. There is another element (B) with six electrons in its second shell.

(i) How many electrons does A tend to give or take to become stable?

Ans: Element A tends to give (lose) 1 electron to attain a stable octet.

(ii) What kind of ion would it form?

Ans: By losing an electron, it forms a positively charged ion called a cation (A⁺ ).

(iii) How many electrons does B tend to give or take to become stable?

Ans: Element B has 6 valence electrons, so it tends to take (gain) 2 electrons to complete its octet.

(iv) What kind of ion would it form?

Ans: By gaining electrons, it forms a negatively charged ion called an anion (B²⁻).

(v) If A and B were to combine, what kind of bond would be formed?

Ans: An ionic bond would be formed due to the electrostatic force of attraction between the oppositely charged cation and anion created by the transfer of electrons.

(vi) What would be the formula for the compound thus formed?

Ans: Using the criss-cross method (A has a valency of 1, B has a valency of 2), the formula would be A₂B.

2. An element X has six electrons in its outer shell and forms a diatomic molecule.

(i) Why would that be so?

Ans: Element X needs two more electrons to complete its octet and become stable. Two atoms of X achieve this by sharing two pairs of electrons with each other, forming a stable diatomic molecule (X₂).

(ii) What kind of bond would it form?

Ans: It forms a double covalent bond because the two atoms are joined by two pairs of shared electrons.

(iii) Draw the structure of the molecule it would form.

Ans:

(iv) A certain other element Y has two electrons in its second shell. Draw the structure of the molecule that X would form with Y.

Ans: Y has two electrons in its second shell, so it is beryllium (Be) with electronic configuration 2,2. If X is chlorine (Cl), the molecule formed is BeCl₂. Beryllium shares one electron with each chlorine atom, forming two single covalent bonds.

Hence, the molecule formed is BeCl₂.

3. You want to design a new ionic compound, where the total positive charge is 6+ and the total negative charge is 6–. Which of the following combinations gives the correct number of ions?

(i) 2 Al³⁺ and 3 Cl–

(ii) 3 Mg²⁺ and 1 PO4³–

(iii) 2 Fe³⁺ and 3 O²–

(iv) 3 Ca²⁺ and 2 SO₄²–

Ans: (iii) 2 Fe³⁺ and 3 O²–

Reasoning: 2 atoms of Fe³⁺ give a total positive charge of +6 (2 × 3), and 3 atoms of O²⁻

give a total negative charge of −6 (3 × -2), perfectly balancing the charges.

4. Choose the correct statement(s) and correct the false statement(s).

(i) Elements are made up of molecules and compounds are made up of atoms.

Ans: (False).

Correction: Elements are made up of atoms of the same kind (which can form molecules of that element), and compounds are made up of molecules containing atoms of different elements combined together.

(ii) The molecule of a compound is always made up of two or more atoms of the same kind.

Ans: (False).

Correction: The molecule of a compound is made up of two or more atoms of different kinds.

(iii) One molecule of nitrogen gas contains three nitrogen atoms.

Ans: (False).

Correction: One molecule of nitrogen gas (N₂) contains two nitrogen atoms (it is diatomic).

(iv) Water is made of two hydrogen atoms, covalently bonded with one oxygen atom.

Ans: True.

5. Write the chemical formulae for the following compounds:

(i) Aluminium nitrate.

Ans: Al(NO₃)_3.

(ii) Calcium oxide.

Ans: CaO.

(iii) Ferric oxide.

Ans: F_{e 2}O₃

6. Write the formulae of the compounds formed from the following pairs of ions:

(i) Ca²⁺ and Br‒

Ans: CaBr₂

(ii) Al³⁺ and CO₃²–

Ans: Al₂(CO₃)₃

(iii) K⁺ and SO₄²–

Ans: K₂SO₄

(iv) NH₄⁺ and Cl–

Ans: NH₄Cl

7. Which of the following, in Fig. 9.18, correctly represents Cl– ion (Atomic number of chlorine = 17).

Ans: (ii).

Explanation: Chlorine has atomic number 17, so its electronic configuration is 2, 8, 7. A Cl⁻ ion gains one electron and its electronic configuration becomes 2, 8, 8. Therefore, the correct diagram is (ii), which shows 18 electrons with a complete outermost shell.

8. Determine the formula unit mass of the following substances:

(i) Ammonium nitrate (NH₄NO₃), used as a nitrogen fertiliser, which is essential for plant growth.

Ans: (14 × 1) + (1 × 4) + (14 × 1) + (16 × 3) = 14 + 4 + 14 + 48 = 80 u .

(ii) Phosphoric acid (H₃PO₄), used to make phosphate fertiliser and detergents.

Ans: (1 × 3) + (31 × 1) + (16 × 4) = 3 + 31 + 64 = 98 u .

(iii) Sodium hydrogencarbonate (NaHCO₃), used to relieve acidity and helps in digestion.

Ans: (23 × 1) + (1 × 1) + (12 × 1) + (16 × 3) = 23 + 1 + 12 + 48 = 84 u .

9. Write the formulae for the compounds formed by the reaction of:

(i) Magnesium and nitrogen.

Ans: Mg₃N₂(using Mg²+ and N³^–)

(ii) Lithium and nitrogen.

Ans: Li₃N (using Li + and N³⁻)

(iii) Sodium and sulfur.

Ans: Na₂S (using Na +and S²⁻)

(iv) Aluminium and oxygen.

Ans: Al₂O₃(using Al³+and O²⁻)

10. Complete the Table 9.3 by writing the formulae of the compounds formed by the cations on the left and the anions at the top (LiNO₃ is given as an example):

	NO^–₃	SO^2-₄	PO^3-₄
NH⁺₄
Li⁺	LiNO₃
Al³⁺
Cu²⁺

Ans:

Cations ↓ / Anions →	NO₃⁻	SO₄²⁻	PO₄³⁻
NH₄⁺	NH₄NO₃	(NH₄)₂SO₄	(NH₄)₃PO₄
Li⁺	LiNO₃	Li₂SO₄	Li₃PO₄
Al³⁺	Al(NO₃)₃	Al₂(SO₄)₃	AlPO₄
Cu²⁺	Cu(NO₃)₂	CuSO₄	Cu₃(PO₄)₂

11. 5.3 g of sodium carbonate and 6.0 g of acetic acid react to produce 2.2 g of carbon dioxide, 0.9 g of water, and 8.2 g of sodium acetate. Verify whether the law of conservation of mass is valid.

Ans: Total mass of Reactants: 5.3 g + 6.0 g = 11.3 g

Total mass of Products: 2.2 g + 0.9 g + 8.2 g = 11.3 g

Since the total mass of the reactants is exactly equal to the total mass of the products, mass was neither created nor destroyed. Yes, the Law of Conservation of Mass is valid.

12. If a species has 11 protons, 12 neutrons and 10 electrons then:

(i) What is its atomic number and mass number?

Ans: Atomic number = 11 (number of protons).

Mass number = 23 (11 protons + 12 neutrons).

(ii) Is it neutral, a cation or an anion? Explain.

Ans: It is a cation. A neutral atom would have 11 electrons to balance the 11 positive protons. Because this species has only 10 electrons, it has an overall positive charge (+1) due to the loss of one electron.

(iii) Write its electronic configuration.

Ans: 2, 8 (Because it has 10 electrons).

(iv) Name the species.

Ans: Sodium cation (Na+).

13. Two elements, A and B, have the following configurations —

A: 2, 8, 5

B: 2, 8, 7.

(i) Which element is more reactive?

Ans: Element B is more reactive. It has 7 valence electrons and only needs to gain 1 electron to complete its octet, whereas Element A needs to gain or share 3 electrons.

(ii) Will A and B form ionic or covalent bonds when they combine? Explain using electron transfer or sharing.

Ans: They will form covalent bonds. Since both elements are short of electrons to complete their octets (A needs 3, B needs 1), neither will transfer electrons to the other. Instead, they will share their valence electrons to achieve stability.

(iii) Predict the formula of the compound they would form.

Ans: Element A requires 3 electrons (valency 3), and element B requires 1 electron (valency 1). By criss-crossing their valencies, the formula of the covalent compound is AB₃.

14. Assertion (A): Copper sulfate conducts electricity in the molten state but not in the solid state.

Reason (R): Copper and sulfate ions are fixed in the lattice in molten state, while in solid state they can move freely.

Choose the correct option:

(i) Both A and R are true, and R is the correct explanation of A.

(ii) Both A and R are true, but R is not the correct explanation of A.

(iii) A is true, but R is false.

(iv) A is false, but R is true.

Ans: (iii) A is true, but R is false.

Explanation: The assertion is true, but the reason is completely reversed. Ionic compounds do not conduct electricity in the solid state because their ions are held in fixed positions by strong forces. They conduct electricity in molten or aqueous states because the ions become free to move.

15. The species ²⁷Al, ⁸⁰Br– and ²⁰¹Hg²⁺ have 13, 35 and 80 protons, respectively. How many electrons and neutrons do they have?

Ans: