Chemistry · Chemical Bonding · NEET
In 1916 two scientists, Kossel and Lewis, worked separately and both explained bonding using electrons. Their main idea: noble gases (like Ne, Ar) are stable because their outer shell has 8 electrons (an octet). So other atoms bond to also get 8 electrons in their outer shell. They can do this by transferring electrons or by sharing electrons. This is the base of everything you study in Chemical Bonding, so NEET expects you to know it clearly.
They are two parts of the same theory. Lewis focused on SHARING electrons to make an octet (covalent bonds) and gave us Lewis dot symbols. Kossel focused on TRANSFER of electrons to make ions (electrovalent/ionic bonds), where a metal loses electrons and a non-metal gains them. Together they are called the electronic theory of chemical bonding. Simple split: Lewis = sharing, Kossel = transfer.
The octet rule says atoms combine to get 8 electrons in their outermost shell, because 8 is the stable noble-gas arrangement (ns2 np6). An atom reaches the octet by gaining, losing, or sharing valence electrons. Note: hydrogen and helium are happy with only 2 electrons (a duplet), not 8, because their nearest noble gas is helium.
A Lewis symbol shows only the valence electrons (outer-shell electrons) as dots around the element symbol. Example: nitrogen has 5 valence electrons, so you draw N with 5 dots. Only outer electrons are shown because only they take part in bonding. The inner electrons are protected and do not react.
Count the dots (valence electrons). The common valence is either that number of dots, OR 8 minus the number of dots — whichever is smaller. Example: oxygen has 6 dots, so valence = 8 - 6 = 2. Carbon has 4 dots, so valence = 4. This tells you how many bonds the atom usually forms.
When an atom loses or gains electrons to become an ion, the number of electrons transferred is its electrovalency. Sodium loses 1 electron to form Na+, so its electrovalency is 1. Calcium loses 2 electrons to form Ca2+, so its electrovalency is 2. In short: electrovalence = number of unit charges on the ion formed.
No, and NEET loves to test this. There are three main exceptions: (1) Incomplete octet — BeCl2 (4 electrons), BCl3, AlCl3 (6 electrons) have fewer than 8. (2) Expanded octet — PCl5 (10), SF6 (12) have more than 8, using d-orbitals. (3) Odd-electron molecules — NO, NO2 have an odd number of electrons so an octet is impossible. Remember these when a question asks which species does NOT have 8 electrons.
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
Which one of the following elements is unable to form MF6^3- ion?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Kossel and Lewis proposed it independently in 1916. It was the first logical electronic explanation of valence, based on the stability of noble gases.
Noble gases like Ne and Ar are very unreactive because their outer shell already has 8 electrons (ns2 np6). Other atoms copy this stable arrangement when they bond.
A duplet means 2 electrons in the outer shell. Hydrogen and helium follow the duplet rule instead of the octet because their nearest noble gas, helium, has only 2 electrons.
It is a good starting point and explains many molecules, but it cannot explain shapes, bond angles, or exceptions like odd-electron and expanded-octet molecules. Later theories (VSEPR, VBT, MOT) fix these gaps.
Usually as one-mark questions on octet-rule exceptions (electron-deficient, expanded, odd-electron molecules), Lewis structures, formal charge, and counting electrons around a central atom.
