Lewis structure

How do chemists sketch where the electrons go in a molecule without doing any quantum mechanics? They draw a Lewis structure: a simple picture in which the bonding electrons between two atoms are shown as a line (a shared pair) and any leftover non-bonding electrons sit as dots (lone pairs) on the atoms. It is the back-of-the-envelope map of a molecule's electrons, invented by G. N. Lewis around 1916.

To build one, you count up all the valence electrons from every atom (adding electrons for an overall negative charge, removing them for a positive charge), arrange the atoms with the least electronegative one usually in the centre, join them with single bonds, then distribute the remaining electrons as lone pairs to give each atom (especially the outer ones) a full octet, and finally make multiple bonds if a central atom is short of electrons. Each line is two shared electrons; each pair of dots is a lone pair. Checking formal charges helps you pick the most reasonable arrangement when more than one is possible.

Lewis structures are the everyday workhorse of inorganic chemistry: they are the starting point for predicting shape with VSEPR, for spotting resonance, and for recognising the exceptions — electron-deficient species like BF3, hypervalent ones like SF6, and odd-electron radicals like NO that simply cannot satisfy a tidy octet. They are a bookkeeping device, not a literal photograph of the molecule; they say nothing directly about energies or true electron distribution, but they get the connectivity and electron count right, and that is enough to reason about an enormous amount of chemistry.