Q.Define - Valency.

Kossel and Lewis's 1916 approach to chemical bonding starts from a single observation: the noble gases are chemically inert because they already have a completely filled valence shell. Every other element, they proposed, bonds by losing, gaining, or SHARING electrons until it too reaches this stable, noble-gas-like electron count -- a principle crystallised as the octet rule: atoms transfer or share electrons so that every atom involved ends up with eight electrons in its valence shell (a duet, two electrons, for hydrogen and helium).

Lewis dot structures are the notation built on this idea. An element's valence electrons are drawn as dots around its symbol -- the first four singly, one to each side, and the fifth electron onward paired up with an existing dot. Carbon (four valence electrons) gets four single dots; nitrogen (five) gets four singles plus one pair; oxygen (six) gets two singles and two pairs; helium, the sole exception to the octet rule, gets just one pair (a duet).

Drawing a molecule's Lewis structure follows five steps: (1) sketch the skeletal frame, with the most electronegative atom central and hydrogen/fluorine (which form only one bond each) always terminal; (2) count total valence electrons, adding the charge magnitude for an anion or subtracting it for a cation; (3) draw single bonds first, each using two electrons; (4) distribute the remaining electrons as lone pairs, starting with the most electronegative atom, to give every atom an octet; (5) check every atom's octet, and if one is short, convert a neighbour's lone pair into an extra bond (forming a double or triple bond). …