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NCERT Solutions for Class 9th Science Chapter 9 .4.2 Bonding by electron transfer — Pause and Ponder

Book page 1749 Updated on2026-09-08

Q12.
What kind of ion will oxygen (O) form?
Answer

Oxygen forms the oxide anion, O²⁻ — a negatively charged ion carrying two units of negative charge.

Oxygen, Z = 8 → configuration 2, 6
Valence electrons = 6; electrons needed for the octet = 8 − 6 = 2
O + 2 e⁻ → O²⁻ (configuration 2, 8)
Protons = 8 (+8), electrons = 10 (−10) → net charge = −2

So oxygen is a divalent anion, valency 2. It gains rather than loses because with six valence electrons it is far closer to an octet by gaining two than by giving away six.

Why it happens: O²⁻ has the configuration 2, 8 — identical to neon. That is the stable arrangement every atom is heading for, and gaining two electrons is the cheapest route to it for oxygen.
Tip: this is why metal oxides come out the way they do. Na⁺ with O²⁻ gives Na₂O, Mg²⁺ with O²⁻ gives MgO, and Al³⁺ with O²⁻ gives Al₂O₃ — in each case the charges balance out to zero.
Q13.
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.
Answer

Completed sentence: “However, chlorine can take only one electron to become the chloride ion, Cl⁻. Now, one ion of magnesium and two ions of chlorine combine to give magnesium chloride.”

Mg (2, 8, 2) → Mg²⁺ (2, 8) + 2 e⁻ — two electrons are released
Cl (2, 8, 7) + 1 e⁻ → Cl⁻ (2, 8, 8) — each chlorine can take only one
So two chlorine atoms are needed to absorb the two electrons:
Mg²⁺ + 2 Cl⁻ → MgCl₂
Charge check: (+2) + 2(−1) = 0 ✓
Why it happens: electrons cannot be left over. Magnesium must dispose of exactly two, and each chlorine atom has room for exactly one, so the numbers must be 1 : 2. This is the whole idea behind criss-crossing charges to get a formula — it is nothing but electron book-keeping.
Q14.
Show the formation of cations of potassium (K) and calcium (Ca) atoms, and the formation of their corresponding chlorides using diagrams.
Answer

Potassium (Z = 19): 2, 8, 8, 1. One electron in the outermost shell — losing it leaves the stable 2, 8, 8 arrangement of argon.

K2, 8, 8, 1− 1 e⁻K⁺2, 8, 8
K (2, 8, 8, 1) loses its single valence electron to become K⁺ (2, 8, 8), a monovalent cation.

Calcium (Z = 20): 2, 8, 8, 2. Two electrons in the outermost shell — losing both leaves 2, 8, 8.

Ca2, 8, 8, 2− 2 e⁻Ca²⁺2, 8, 8
Ca (2, 8, 8, 2) loses two valence electrons to become Ca²⁺ (2, 8, 8), a divalent cation.

Potassium chloride, KCl. The one electron potassium gives away is taken by one chlorine atom (2, 8, 7 → 2, 8, 8).

K⁺2, 8, 8Cl⁻2, 8, 8KCl
K⁺ and Cl⁻, held together by electrostatic attraction. One cation to one anion, so the formula is KCl.

Calcium chloride, CaCl₂. Calcium must dispose of two electrons, and each chlorine can accept only one — so two chlorine atoms are needed.

Ca²⁺2, 8, 8Cl⁻2, 8, 8Cl⁻2, 8, 8CaCl₂
One Ca²⁺ with two Cl⁻ ions. Charge check: (+2) + 2(−1) = 0, so the formula is CaCl₂.
K → K⁺ + e⁻; Cl + e⁻ → Cl⁻; K⁺ + Cl⁻ → KCl
Ca → Ca²⁺ + 2 e⁻; 2 Cl + 2 e⁻ → 2 Cl⁻; Ca²⁺ + 2 Cl⁻ → CaCl₂
Why it happens: both metals are in the same situation as sodium — a nearly empty outermost shell sitting on top of a complete one. Emptying it is the quickest way to an octet. The chlorine atoms are in the opposite situation, one electron short, so the transfer suits both sides and the resulting opposite charges then hold the ions together.
Q15.
Illustrate how sodium sulfide (Na2S) is formed.
Answer

Sodium is 2, 8, 1 — one electron to give. Sulfur is 2, 8, 6 — two electrons short of an octet. Since each sodium can supply only one electron, two sodium atoms are needed for one sulfur atom.

Na2, 8, 1+S2, 8, 6+Na2, 8, 1transfer of 2 e⁻Na⁺2, 8S²⁻2, 8, 8Na⁺2, 8Na₂S
Two sodium atoms each transfer one electron to a single sulfur atom. Both Na⁺ ions end up with 2, 8 (like neon) and S²⁻ with 2, 8, 8 (like argon).
2 Na (2, 8, 1) → 2 Na⁺ (2, 8) + 2 e⁻
S (2, 8, 6) + 2 e⁻ → S²⁻ (2, 8, 8)
2 Na⁺ + S²⁻ → Na₂S
Charge check: 2(+1) + (−2) = 0 ✓

The two Na⁺ ions and the S²⁻ ion are then held together by the electrostatic force of attraction between opposite charges — an ionic bond. As with all ionic compounds, Na₂S does not exist as a separate molecule but as a 3-D crystal lattice; Na₂S is its formula unit, the simplest whole-number ratio of its ions.

Why it happens: the subscript 2 in Na₂S is not decoration — it is forced by the electron count. Sulfur needs two electrons; sodium can spare one each; so the ratio must be 2 : 1. The same reasoning gives Na₂O, K₂S and Li₂O.
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