NCERT Solutions for Class 9th Science Chapter 9 .4.1 B Molecules of compounds — Pause and Ponder
Book page 1719 Updated on2026-09-08
Q10.
Show the formation of the following molecules: (i) Carbon dioxide (CO2) (ii) Hydrogen sulfide (H2S) (iii) Ammonia (NH3)
Answer
In each case, first write the configuration, then count how many electrons each atom is short of, then match the shortages.
(i) Carbon dioxide, CO₂
C (Z = 6): 2, 4 → 4 valence electrons, needs 4
O (Z = 8): 2, 6 → 6 valence electrons, needs 2
One carbon (4) is matched by two oxygens (2 + 2) → formula CO₂
Carbon shares two electrons with each oxygen, forming two double bonds: O = C = O. Carbon now has 8 valence electrons, and so does each oxygen.
(ii) Hydrogen sulfide, H₂S
S (Z = 16): 2, 8, 6 → needs 2 electrons
H (Z = 1): 1 → needs 1 electron to complete its duplet
Two hydrogen atoms are needed for one sulfur → formula H₂S
Two single covalent bonds, H — S — H. Sulfur keeps two lone pairs; each hydrogen has its duplet of 2 electrons.
(iii) Ammonia, NH₃
N (Z = 7): 2, 5 → needs 3 electrons
H: needs 1 each → three hydrogen atoms are needed
Formula NH₃
Three single covalent bonds around nitrogen, with one lone pair left over on the nitrogen atom. Nitrogen: 6 shared + 2 lone = 8 electrons.
Why it happens: the number of bonds an atom forms is simply the number of electrons it is short of. Carbon is short of 4 and forms 4 bonds; nitrogen is short of 3 and forms 3; oxygen and sulfur are short of 2 and form 2; hydrogen is short of 1 and forms 1. That single rule generates all three formulae above.
Tip: notice that NH₃ keeps a lone pair. That leftover pair is the reason ammonia solution is basic and smells sharp — you will meet it again in higher classes.
Q11.
Neon (atomic number 10) neither transfers nor shares its valence electrons. Explain.
Answer
Because neon's valence shell is already full.
Neon, Z = 10 → electronic configuration 2, 8
Electrons in the valence (L) shell = 8 — a complete octet
Atoms lose, gain or share electrons for one reason only — to reach an octet. Neon has already reached it, so it has nothing to gain from bonding:
It will not lose an electron — that would break up a complete octet and leave it less stable, and pulling an electron out of a full shell needs a great deal of energy.
It will not gain an electron — there is no room in the L shell, so the extra electron would have to start a new M shell far from the nucleus, which is not a stable arrangement.
It will not share — sharing only pays when it completes a shell, and neon's shell is complete.
Neon therefore exists as single, separate atoms, not as Ne₂ molecules, and forms no ordinary compounds. Helium behaves the same way with a complete duplet (2).
Why it happens: when atoms combine, the total energy of the system falls — that energy drop is the reward that drives bonding. For a noble gas there is no reward: any bond would raise the energy. This is why Group 18 elements are called inert or noble gases and are used where chemical inactivity matters, such as in neon signs and in argon-filled bulbs.