CLASS 9 • SCIENCE • CHAPTER 9
Atomic Foundations of Matter
Colourful Concept Notes and NCERT Book Solutions
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PART 1: SHORT CONCEPT NOTES
1. Laws of Chemical Combination
In an ordinary chemical reaction, matter is neither created nor destroyed. The total mass of the reactants equals the total mass of the products.
Total mass of reactants = Total mass of products
Mass before reaction = 12 + 32 = 44 g.
Mass after reaction = 44 g.
Therefore, mass is conserved.
A pure chemical compound always contains the same elements combined in a fixed ratio by mass, irrespective of its source or method of preparation.
In water, the mass ratio of hydrogen to oxygen is always 1 : 8.
Example 2: Carbon monoxide contains carbon and oxygen in a mass ratio of 3 : 4. For 9 g carbon, oxygen required = (4 ÷ 3) × 9 = 12 g.
2. Dalton’s Atomic Theory
John Dalton proposed his atomic theory in 1808. Its main postulates were:
- All matter is made of tiny particles called atoms.
- Atoms participate in chemical reactions.
- Atoms are not created or destroyed in ordinary chemical reactions.
- Atoms of different elements have different properties.
- Atoms combine in simple whole-number ratios to form compounds.
- A given compound has a fixed relative number and kind of atoms.
3. Atoms, Molecules and Chemical Bonds
- Atom: The smallest unit of an element that takes part in ordinary chemical reactions.
- Molecule: An electrically neutral group of two or more atoms bonded together that can exist independently.
- Chemical bond: The attractive force that holds atoms or ions together.
- Covalent bond: Formed when atoms share electron pairs.
- Ionic bond: Formed by electrostatic attraction between oppositely charged ions after electron transfer.
- Single bond: one shared pair of electrons, for example H—H and Cl—Cl.
- Double bond: two shared pairs, for example O=O.
- Triple bond: three shared pairs, for example N≡N.
| Formula | Name | Bonding |
|---|---|---|
| H₂ | Hydrogen | Single covalent bond |
| Cl₂ | Chlorine | Single covalent bond |
| O₂ | Oxygen | Double covalent bond |
| N₂ | Nitrogen | Triple covalent bond |
| H₂O | Water | Covalent bonds |
| NH₃ | Ammonia | Covalent bonds |
| CO₂ | Carbon dioxide | Two double bonds |
4. Ionic Bonding and Ions
Anion: A negatively charged ion formed when an atom gains electrons.
Sodium has one valence electron. It loses that electron to form Na⁺. Chlorine gains one electron to form Cl⁻. The opposite charges attract, forming ionic sodium chloride (NaCl).
| Ion name | Formula | Charge |
|---|---|---|
| Sodium | Na⁺ | +1 |
| Potassium | K⁺ | +1 |
| Calcium | Ca²⁺ | +2 |
| Magnesium | Mg²⁺ | +2 |
| Aluminium | Al³⁺ | +3 |
| Chloride | Cl⁻ | −1 |
| Oxide | O²⁻ | −2 |
| Sulfide | S²⁻ | −2 |
| Nitrate | NO₃⁻ | −1 |
| Hydroxide | OH⁻ | −1 |
| Sulfate | SO₄²⁻ | −2 |
| Carbonate | CO₃²⁻ | −2 |
| Ammonium | NH₄⁺ | +1 |
| Phosphate | PO₄³⁻ | −3 |
5. Writing Chemical Formulae
Write the positive ion first and the negative ion second. Balance their charges so that the total charge of the compound is zero. Use the simplest whole-number ratio.
Ca²⁺ and Cl⁻. One calcium ion needs two chloride ions.
Formula: CaCl₂
Example 2: Aluminium oxideAl³⁺ and O²⁻. The lowest charge-balanced ratio is 2 aluminium ions to 3 oxide ions.
Formula: Al₂O₃
Example 3: Magnesium hydroxideMg²⁺ and OH⁻. Two hydroxide ions are needed for one magnesium ion.
Formula: Mg(OH)₂
Example 4: Calcium carbonateCa²⁺ and CO₃²⁻. Their charges balance in a 1 : 1 ratio.
Formula: CaCO₃
- Do not write the charge in the final formula of a neutral compound.
- Do not write subscript 1.
- Use brackets when more than one polyatomic ion is needed, for example Al(OH)₃.
- Reduce subscripts to the simplest whole-number ratio, for example Mg₂O₂ becomes MgO.
| Formula | Name |
|---|---|
| CO | Carbon monoxide |
| CO₂ | Carbon dioxide |
| NO₂ | Nitrogen dioxide |
| SF₆ | Sulfur hexafluoride |
| PCl₃ | Phosphorus trichloride |
| N₂O₅ | Dinitrogen pentoxide |
| H₂S | Hydrogen sulfide |
6. Properties of Ionic and Covalent Compounds
| Property | Ionic compounds | Covalent compounds |
|---|---|---|
| Formation | Usually by electron transfer and attraction between ions | By sharing electrons |
| Melting and boiling points | Usually high | Usually low for simple molecular compounds |
| Electrical conductivity in solid state | Usually do not conduct because ions cannot move freely | Usually do not conduct |
| Conductivity in water | Conduct when soluble and their ions are free to move | Usually do not conduct; exceptions exist |
| Solubility | Many dissolve in water | Varies; many simple covalent compounds dissolve better in non-polar solvents |
7. Molecular Mass and Formula Unit Mass
Formula unit mass: The sum of atomic masses represented by the simplest formula unit of an ionic compound.
Atomic masses: H = 1 u, O = 16 u.
Mass = (2 × 1) + 16 = 18 u.
Example 2: Carbon dioxide, CO₂Mass = 12 + (2 × 16) = 44 u.
Example 3: Sodium oxide, Na₂OMass = (2 × 23) + 16 = 62 u.
Example 4: Calcium nitrate, Ca(NO₃)₂Mass = 40 + 2 × [14 + (3 × 16)] = 164 u.
8. Quick Revision
- Mass of reactants = mass of products in an ordinary chemical reaction.
- A pure compound has a fixed composition by mass.
- Atoms combine in simple whole-number ratios.
- Covalent bonds form by sharing electron pairs.
- Ionic bonds are attractions between oppositely charged ions.
- A cation is positive; an anion is negative.
- A chemical formula represents the composition or simplest ratio of a substance.
- Molecular mass is used for molecules; formula unit mass is used for ionic compounds.
PART 2: NCERT BOOK QUESTIONS WITH EASY SOLUTIONS
The questions below retain the wording and subparts of the chapter’s final “Revise, Reflect, Refine” and “The Journey Beyond” sections. Answers are given separately in simple language.
Revise, Reflect, Refine
Question 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?
(ii) What kind of ion would it form?
(iii) How many electrons does B tend to give or take to become stable?
(iv) What kind of ion would it form?
(v) If A and B were to combine, what kind of bond would be formed?
(vi) What would be the formula for the compound thus formed?
(i) A has one electron in its outermost shell and tends to lose 1 electron.
(ii) It forms a positive ion, A⁺.
(iii) B has six electrons in its outermost shell and tends to gain 2 electrons.
(iv) It forms a negative ion, B²⁻.
(v) They form an ionic bond by electron transfer.
(vi) Two A⁺ ions balance one B²⁻ ion, so the formula is A₂B.
Question 2
An element X has six electrons in its outer shell and forms a diatomic molecule.
(i) Why would that be so?
(ii) What kind of bond would it form?
(iii) Draw the structure of the molecule it would form.
(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.
(i) X needs two more electrons to complete its octet. Two X atoms can share two pairs of electrons.
(ii) It forms a double covalent bond.
(iii) The diatomic molecule is represented as X=X. Each atom also has two lone pairs of electrons.
(iv) Y has two electrons in its second shell, so it has four electrons in total and four valence electrons. In the usual first-18-element pattern, X can represent oxygen and Y can represent carbon. The compound is carbon dioxide, represented as O=C=O, with two double covalent bonds.
Question 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⁻
Correct options: (iii) and (iv).
(iii) Positive charge = 2 × 3+ = 6+; negative charge = 3 × 2− = 6−.
(iv) Positive charge = 3 × 2+ = 6+; negative charge = 2 × 2− = 4−. This is not 6−, so option (iv) does not meet the stated condition.
On checking the charges, only option (iii) gives a total positive charge of 6+ and a total negative charge of 6−. Option (i) gives 6+ and 3−, while option (ii) gives 6+ and 3−.
Question 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.
(ii) The molecule of a compound is always made up of two or more atoms of the same kind.
(iii) One molecule of nitrogen gas contains three nitrogen atoms.
(iv) Water is made of two hydrogen atoms, covalently bonded with one oxygen atom.
(i) False. Elements may exist as single atoms or molecules. Compounds contain atoms of two or more different elements chemically combined.
(ii) False. A compound contains atoms of different elements.
(iii) False. A nitrogen molecule, N₂, contains two nitrogen atoms.
(iv) Correct. A water molecule is H₂O, with two hydrogen atoms covalently bonded to one oxygen atom.
Question 5
Write the chemical formulae for the following compounds.
(i) Aluminium nitrate
(ii) Calcium oxide
(iii) Ferric oxide
(i) Aluminium nitrate: Al(NO₃)₃
(ii) Calcium oxide: CaO
(iii) Ferric oxide: Fe₂O₃
Question 6
Write the formulae of the compounds formed from the following pairs of ions.
(i) Ca²⁺ and Br⁻
(ii) Al³⁺ and CO₃²⁻
(iii) K⁺ and SO₄²⁻
(iv) NH₄⁺ and Cl⁻
(i) CaBr₂
(ii) Al₂(CO₃)₃
(iii) K₂SO₄
(iv) NH₄Cl
Question 7
Which of the following, in Fig. 9.18, correctly represents Cl⁻ ion (Atomic number of chlorine = 17).
Fig. 9.18
(i) (ii) (iii) (iv)
A chloride ion has 18 electrons because a chlorine atom gains one electron. Its electronic configuration is 2, 8, 8.
Select the diagram showing 18 electrons arranged as 2 in the first shell, 8 in the second shell and 8 in the third shell. 9.18.
Question 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.
(ii) Phosphoric acid (H₃PO₄), used to make phosphate fertiliser and detergents.
(iii) Sodium hydrogencarbonate (NaHCO₃), used to relieve acidity and helps in digestion.
(i) NH₄NO₃ = 2N + 4H + 3O
= (2 × 14) + (4 × 1) + (3 × 16) = 28 + 4 + 48 = 80 u.
(ii) H₃PO₄ = (3 × 1) + 31 + (4 × 16) = 3 + 31 + 64 = 98 u.
(iii) NaHCO₃ = 23 + 1 + 12 + (3 × 16) = 23 + 1 + 12 + 48 = 84 u.
Question 9
Write the formulae for the compounds formed by the reaction of:
(i) Magnesium and nitrogen
(ii) Lithium and nitrogen
(iii) Sodium and sulfur
(iv) Aluminium and oxygen
(i) Magnesium nitride: Mg₃N₂
(ii) Lithium nitride: Li₃N
(iii) Sodium sulfide: Na₂S
(iv) Aluminium oxide: Al₂O₃
Question 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.
Table 9.3
| 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 |
Question 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.
Total mass of reactants = 5.3 + 6.0 = 11.3 g.
Total mass of products = 2.2 + 0.9 + 8.2 = 11.3 g.
Mass of reactants = mass of products. Therefore, the Law of Conservation of Mass is obeyed.
Question 12
If a species has 11 protons, 12 neutrons and 10 electrons then
(i) what is its atomic number and mass number?
(ii) is it neutral, a cation or an anion? Explain.
(iii) write its electronic configuration.
(iv) name the species.
(i) Atomic number = number of protons = 11. Mass number = 11 + 12 = 23.
(ii) It is a cation with a +1 charge, because it has 11 protons but only 10 electrons.
(iii) Electronic configuration: 2, 8.
(iv) It is a sodium ion, ²³Na⁺.
The Journey Beyond
Question 13
Two elements, A and B, have the following configurations —
A: 2, 8, 5 B: 2, 8, 7
(i) Which element is more reactive?
(ii) Will A and B form ionic or covalent bonds when they combine? Explain using electron transfer or sharing.
(iii) Predict the formula of the compound they would form.
(i) Both are non-metals with incomplete outer shells. B has seven valence electrons and needs only one electron to complete its octet; its reactivity depends on the particular element and reaction. From the configurations, A corresponds to phosphorus and B to chlorine. Chlorine is generally more reactive than phosphorus in many common reactions.
(ii) They form a covalent compound by sharing electrons.
(iii) A has valency 3 and B has valency 1. The expected formula is AB₃, corresponding to PCl₃.
Question 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.
Correct option: (iii) A is true, but R is false.
Copper sulfate conducts when molten because its ions can move freely. In the solid state, the ions are fixed in the crystal lattice and cannot move freely. The reason statement has these conditions reversed.
Question 15
The species ²⁷Al, ⁸⁰Br⁻ and ²⁰¹Hg²⁺ have 13, 35 and 80 protons, respectively. How many electrons and neutrons do they have?
Number of neutrons = mass number − number of protons.
| Species | Protons | Electrons | Neutrons |
|---|---|---|---|
| ²⁷Al | 13 | 13 | 27 − 13 = 14 |
| ⁸⁰Br⁻ | 35 | 35 + 1 = 36 | 80 − 35 = 45 |
| ²⁰¹Hg²⁺ | 80 | 80 − 2 = 78 | 201 − 80 = 121 |