Chapter 9: Atomic Foundations of Matter (Chemistry)
Step-by-step answers to all 15 "Revise, Reflect, Refine" questions of Chapter 9, Atomic Foundations of Matter (NCERT Class 9 Science, Exploration, 2026-27): ions, ionic and covalent bonds, writing chemical formulae, polyatomic ions, formula unit mass and the law of conservation of mass. All 15 questions are answered, with the key answer highlighted.
Atoms lose or gain electrons (ionic bond) or share them (covalent bond) to get a stable outer shell of 8 electrons (2 for the first shell). To write a formula, criss-cross the valencies (charges) and simplify; put brackets around a polyatomic ion taken more than once. Atomic masses used: H 1, C 12, N 14, O 16, Na 23, P 31 (u).
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?
Solution
A has the configuration 2, 8, 1 (sodium) and B has 2, 6 (oxygen).
(i) A tends to give (lose) 1 electron; then its outer shell becomes 2, 8.
(ii) A cation, A⁺ (positive ion).
(iii) B tends to take (gain) 2 electrons to complete its octet (2, 8).
(iv) An anion, B²⁻ (negative ion).
(v) An ionic bond, since electrons are transferred from A to B.
(vi) Two A⁺ ions balance one B²⁻ ion: A₂B (e.g. Na₂O).
(i) Gives 1 electron (ii) Cation A⁺ (iii) Takes 2 electrons (iv) Anion B²⁻ (v) Ionic bond (vi) A₂B
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.
Solution
(i) X needs 2 more electrons to complete its octet. Gaining or losing several electrons needs a lot of energy, so two X atoms instead share electrons: each atom contributes 2 electrons, and both atoms then have 8 electrons around them. Hence it forms a diatomic molecule X₂ (like oxygen, O₂).
(ii) A covalent bond; with two shared pairs it is a double covalent bond (X=X).
(iii) Structure of the X₂ molecule:
X₂ (e.g. O₂): two shared pairs, a double covalent bond, X=X. Dots (•) and crosses (×) are the outer electrons of the two atoms
(iv) Y (configuration 2, 2) has 2 outer electrons, so its valency is 2, the same as that of X. One X shares two electrons with one Y, which shares its two outer electrons: Y=X, formula YX. (For example, if X is oxygen and Y is beryllium, the compound is BeO.)
YX: X shares two electrons and Y shares its two outer electrons, giving two shared pairs, Y=X
(i) It needs 2 electrons, so two atoms share 2 pairs of electrons and both complete their octet. (ii) Covalent (double) bond (iii) X=X (iv) Y=X, formula YX.
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⁻
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.
Solution
Correct: (iv) only. Each water molecule (H₂O) has two H atoms covalently bonded to one O atom.
Corrections:
(i) Elements are made up of only one kind of atom (which may exist singly or as molecules such as O₂), while compounds are made up of atoms of two or more different elements combined in a fixed ratio.
(ii) The molecule of a compound is always made up of atoms of two or more different kinds (e.g. H₂O, CO₂).
(iii) One molecule of nitrogen gas (N₂) contains two nitrogen atoms.
(iv) is correct; (i), (ii) and (iii) are false (see corrections).
Which of the following, in Fig. 9.18, correctly represents the Cl⁻ ion (atomic number of chlorine = 17)?
Solution
Answer: (ii)
A chlorine atom has 17 electrons (2, 8, 7). The Cl⁻ ion has gained one electron, so it has 18 electrons: 2, 8, 8. Counting the shells in Fig. 9.18: (i) shows 2, 7, 8 (wrong order of filling), (ii) shows 2, 8, 8, (iii) shows 2, 8, 9 and (iv) shows 2, 8, 7 (the neutral Cl atom).
Determine the formula unit mass of the following substances. (i) Ammonium nitrate (NH₄NO₃), used as a nitrogen fertiliser. (ii) Phosphoric acid (H₃PO₄), used to make phosphate fertiliser and detergents. (iii) Sodium hydrogencarbonate (NaHCO₃), used to relieve acidity and help in digestion.
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
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.
Solution
Mass of reactants=5.3+6.0=11.3g
Mass of products=2.2+0.9+8.2=11.3g
The total mass of the reactants equals the total mass of the products, so mass is neither created nor destroyed in the reaction. The law of conservation of mass is valid.
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.
Solution
(i) Atomic number = protons = 11; mass number = 11 + 12 = 23.
(ii) It is a cation: it has 11 positive charges (protons) but only 10 negative charges (electrons), so it carries a net charge of +1.
(iii) 10 electrons: 2, 8.
(iv) Sodium ion, Na⁺.
(i) Z = 11, A = 23 (ii) Cation (one more proton than electrons, charge +1) (iii) 2, 8 (iv) Sodium ion, Na⁺
Two elements, A and B, have the following configurations: A: 2, 8, 5 and 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.
Solution
A is phosphorus (Z = 15) and B is chlorine (Z = 17); both are non-metals.
(i) B is more reactive. It needs only 1 electron to complete its octet, while A needs 3.
(ii) Covalent bonds. Both atoms need to gain electrons and neither tends to give them away, so electrons are not transferred; they are shared. A shares one of its electrons with each of three B atoms, and each B shares one electron with A. Then A has 5 + 3 = 8 electrons and each B has 7 + 1 = 8.
(iii) AB₃ (PCl₃).
(i) B (ii) Covalent: A shares 3 electrons, one with each of three B atoms (iii) AB₃ (like PCl₃)
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 the molten state, while in the 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.
Solution
Answer: (iii)
A is true: copper sulfate is an ionic compound and conducts electricity only when its ions can move. R is false; it is the other way round: in the solid state the Cu²⁺ and SO₄²⁻ ions are fixed in the crystal lattice, and in the molten state (or in solution) they become free to move and carry current.
