Chemistry · Chemical Bonding · NEET
The real reason is energy. When two atoms come close and bond, the system reaches a lower energy than the two free atoms. NCERT says it clearly: 'bonding is nature's way of lowering the energy of the system to attain stability.' Anything at lower energy is more stable, so atoms combine to become more stable.
Noble gases (He, Ne, Ar...) almost never react. Their outer shell is already full — 8 electrons (or 2 for He). Kossel and Lewis noticed this and said: other atoms react to REACH the same full, stable arrangement. So an atom loses, gains, or shares electrons until its outer shell looks like the nearest noble gas. This target of 8 outer electrons is called the octet rule.
Because they already have a complete octet (full outer shell). They are already at low energy and stable. They have no reason to lose, gain, or share electrons, so they stay alone as single atoms. This is the proof that a full octet = stability, which is the whole idea behind bonding.
Forming a bond RELEASES energy. The bonded molecule sits at lower energy than the free atoms, and that extra energy is given out. To break the bond again, you must PUT energy back in (bond dissociation enthalpy). This is why molecules do not just fall apart on their own.
There are three ways, and they give the different bond types you study next. (1) Transfer electrons — one atom gives, one takes (ionic bond, e.g. NaCl). (2) Share electrons — both atoms share a pair (covalent bond, e.g. H2, Cl2). (3) One atom donates the whole shared pair (coordinate/dative bond). All three aim at the same goal: a stable octet and lower energy.
No — it is a useful guide, not a hard law. Some molecules break it. Incomplete octet: BeCl2 (4 e-), BCl3 and AlCl3 (6 e-). Expanded octet: PCl5 (10 e-), SF6 (12 e-). Odd-electron: NO, NO2. NEET loves testing these exceptions, so learn them by heart.
Amongst the following, the total number of species NOT having eight electrons around the central atom in its outermost shell is: NH3, AlCl3, BeCl2, CCl4, PCl5
Try the real previous-year questions from this chapter — each with the answer and a full solution.
To lower their energy and reach a stable, full outer shell (octet) like the nearest noble gas.
Kossel and Lewis, in 1916, based on the inertness (unreactivity) of noble gases.
The attractive force that holds atoms (or ions) together in a molecule or compound. It forms because it lowers the energy of the system.
Because the molecule has lower total energy. Lower energy always means more stable, so the atoms stay bonded instead of drifting apart.
Most do, but not all. Exceptions include incomplete octet (BeCl2, BCl3, AlCl3), expanded octet (PCl5, SF6) and odd-electron molecules (NO, NO2).
