NCERT Solutions for Class 9th Science Chapter 8 .8 Combining Capacity of an Atom: Valency — In-text Questions

Book page 1538 Updated on2026-09-08

Q1.

In NH₃ (ammonia) and MgCl₂ (magnesium chloride), what will be the combining capacities of nitrogen and magnesium respectively?

Answer

Nitrogen: 3. Magnesium: 2.

Rule: the combining capacity of an element is the number of hydrogen or chlorine atoms that combine with one of its atoms — because hydrogen and chlorine each have a combining capacity of one.

In NH₃: one N atom combines with 3 H atoms
⇒ combining capacity of nitrogen = 3

In MgCl₂: one Mg atom combines with 2 Cl atoms
⇒ combining capacity of magnesium = 2

Cross-check with the electronic configurations — the two methods must agree:

Element	Configuration	Valence electrons	To complete the octet	Valency
Nitrogen (Z = 7)	2, 5	5	needs 3 more	3 ✓
Magnesium (Z = 12)	2, 8, 2	2	loses its 2 outer electrons	2 ✓

Why it happens: the formula of a compound is not arbitrary — it is fixed by how many electrons each atom must gain, lose or share to reach a stable octet. Nitrogen is 3 electrons short of 8, so it shares with three hydrogens. Magnesium has 2 electrons more than a stable shell, so it hands one each to two chlorine atoms, each of which was 1 electron short.

Q2.

Can you predict what happens to the atoms that already have eight electrons in their outermost shell (except the elements with one shell only, where only two electrons are possible)? Will they still try to lose or gain electrons?

Answer

No. They stay as they are. Such atoms are already stable, so they are almost completely unreactive.

A complete octet is the arrangement every other atom is trying to reach by losing, gaining or sharing electrons. An atom that already has it has no reason to change.
Elements with a full outermost shell are the noble gases — neon (2, 8), argon (2, 8, 8) and so on. Helium is the exception noted in the question: it has only one shell, and 2 electrons fill it, so a duplet is complete for helium.
Their valency is 0. They do not normally form compounds, and they exist in nature as single, separate atoms rather than as molecules.

Why it happens: ask what the atom would gain by changing. Losing an electron from a full shell means breaking into a stable arrangement — it costs a great deal of energy. Gaining an electron means starting a brand new shell much further from the nucleus, where the attraction is weak. Either way the atom ends up less stable than it began, so neither happens.

Did you know? “Almost” unreactive, not absolutely. Under extreme conditions chemists have made compounds of xenon, such as XeF₄. The octet rule is a very good guide at your level, and the chapter notes that some compounds appear to break it — you will meet those in higher classes.

Q3.

Examine Table 8.4. Add one more column to it, and write down the common valency of each element.

Answer

Here is Table 8.4 with the extra column. Rule: if the valence shell has fewer than 4 electrons the atom loses them; if more than 4 it gains the electrons needed to reach 8; with exactly 4 it shares. Valency is that number of electrons lost, gained or shared.

Element	Symbol	Z	Configuration	Valence electrons	Lose / gain / share	Valency
Hydrogen	H	1	1	1	shares or loses 1	1
Helium	He	2	2	2	duplet complete	0
Lithium	Li	3	2, 1	1	loses 1	1
Beryllium	Be	4	2, 2	2	loses 2	2
Boron	B	5	2, 3	3	loses / shares 3	3
Carbon	C	6	2, 4	4	shares 4	4
Nitrogen	N	7	2, 5	5	gains 3	3
Oxygen	O	8	2, 6	6	gains 2	2
Fluorine	F	9	2, 7	7	gains 1	1
Neon	Ne	10	2, 8	8	octet complete	0
Sodium	Na	11	2, 8, 1	1	loses 1	1
Magnesium	Mg	12	2, 8, 2	2	loses 2	2
Aluminium	Al	13	2, 8, 3	3	loses 3	3
Silicon	Si	14	2, 8, 4	4	shares 4	4
Phosphorus	P	15	2, 8, 5	5	gains / shares 3	3
Sulfur	S	16	2, 8, 6	6	gains 2	2
Chlorine	Cl	17	2, 8, 7	7	gains 1	1
Argon	Ar	18	2, 8, 8	8	octet complete	0

Why it happens: read the valency column downwards and a pattern appears — 1, 0, 1, 2, 3, 4, 3, 2, 1, 0, then 1, 2, 3, 4, 3, 2, 1, 0 again. It rises to 4 and falls back, and it repeats after neon. That repetition is the seed of the periodic table: elements with the same valency (Li and Na; Be and Mg; F and Cl) behave alike because they have the same number of valence electrons.

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