NCERT Solutions for Class 9th Science Chapter 9 .4.1 A Molecules of elements — Pause and Ponder
Book page 1709 Updated on2026-09-08
Q8.
Nitrogen has five valence electrons. Draw the structure of the nitrogen molecule (N2).
Answer
Nitrogen (atomic number 7) has the configuration 2, 5. With five valence electrons it needs three more to reach an octet. Neither atom can take three from the other, so the two atoms share three pairs — a triple bond.
Each nitrogen atom contributes three electrons to the shared region and keeps one lone pair. Counting round either atom: 6 shared + 2 lone = 8 electrons, a complete octet.
Valence electrons on each N = 5
Electrons needed to complete the octet = 8 − 5 = 3
So three pairs are shared → a triple bond, written N ≡ N
Why it happens: a triple bond is the strongest of the common covalent bonds, and it is why nitrogen gas is so unreactive. Air is 78 % N₂ and yet it does not burn, rust or corrode anything — breaking three bonds at once needs a great deal of energy, which is why industrial ammonia manufacture requires high temperature, high pressure and a catalyst.
Q9.
The atomic number of fluorine is 9. Explain the formation of the fluorine molecule (F2).
Answer
Fluorine has atomic number 9, so its electronic configuration is 2, 7 — seven electrons in the valence (L) shell, one short of an octet.
A fluorine atom cannot gain that electron from another fluorine atom, because that atom needs one just as badly. The way out is for each atom to put one electron into a shared pair. The pair belongs to both nuclei at once, so both atoms now count eight valence electrons.
One shared pair between the two fluorine atoms; each atom also carries three lone pairs. Around either atom: 2 shared + 6 lone = 8 electrons.
F: 2, 7 → 7 valence electrons, needs 1 more
Electrons shared by each atom = 1 → one shared pair
Bond formed = single covalent bond, written F — F
The shared pair attracts both nuclei at the same time, and it is that double attraction that holds the two atoms together as one molecule, F₂.
Why it happens: the molecule forms because the total energy of the shared arrangement is lower than the energy of two separate fluorine atoms. Atoms combine for exactly this reason — a lower-energy arrangement is a more stable one.