Chemistry · Chemical Bonding · NEET
Add the valence electrons of every atom. Valence electrons = the group number (for main-group atoms), so H=1, C=4, N=5, O=6, halogens=7. Then adjust for charge: for a negative ion ADD that many electrons, for a positive ion SUBTRACT that many. Example: CO3^2- = C(4) + 3xO(6) + 2 extra for the 2- charge = 4+18+2 = 24 electrons. This total is your electron 'budget' and it must never change while you draw.
Put the LEAST electronegative atom in the center (it likes to share with many atoms). Usually it is the single atom of its kind, or carbon. Hydrogen and fluorine are almost always on the outside because H can hold only 2 electrons and F is very electronegative. Example: in CO2, carbon is the center; in H2O, oxygen is the center.
First connect every atom with a single bond, then complete octets on the outer atoms, then give leftover electrons to the center. If the central atom still does NOT have 8 electrons, move a lone pair from a neighbouring atom to form a double or triple bond. Do this until the center reaches an octet. Example: in CO2 each oxygen shares a double bond so carbon reaches 8 electrons.
No. Hydrogen is happy with only 2 electrons (a duplet), because its first shell fills at 2. So H always makes exactly one bond and never carries lone pairs. Never try to put 8 electrons around hydrogen; that is a common mistake that loses easy NEET marks.
Three checks: (1) Total electrons drawn = total you counted at the start. (2) Every atom (except H) has an octet, or you have used the smallest number of formal charges. (3) The most stable structure has formal charges closest to zero, and any negative charge sits on the most electronegative atom. Formal charge = valence electrons - lone-pair electrons - (1/2 x bonding electrons).
When you draw O3, one O=O double bond and one O-O single bond both fit the octet rule, but you could place the double bond on either side. Since both are equally valid, the real molecule is an average of the two, called resonance. This is why both O-O bonds in ozone are actually the same length. NEET often tests this idea.
For the ozone (O3) Lewis structure with a central O doubly bonded to one terminal O (atom 2) and singly bonded to the other terminal O (atom 3), the correct formal charges on oxygen atoms numbered 2, 1 (central) and 3 respectively are:
Identify the correct statement.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Count the total number of valence electrons from all atoms, then adjust for the ion's charge. This total is fixed and controls the whole drawing.
Most main-group atoms want 8 electrons (the octet rule), but hydrogen wants only 2. Some atoms like S, P and Xe can hold more than 8 (expanded octet).
Lone pairs are pairs of valence electrons that are NOT shared in a bond. They sit on one atom as dots and strongly affect molecular shape (VSEPR).
Yes. Lewis structures are the base for formal charge, resonance, VSEPR shape and hybridisation questions, which appear almost every year in NEET Chemical Bonding.
Only add a double or triple bond if the central atom is short of an octet after single bonds and outer octets are complete. Move a lone pair from a neighbour to form the extra bond.
