Covalent bonding in simple molecules

Atoms can bond by sharing outer-shell electrons. A covalent bond is a shared pair of electrons between two atoms. The shared pair counts towards the outer electronic configuration of both bonded atoms.

Sharing gives each atom a stable outer electronic configuration like that of a noble gas. Hydrogen attains two electrons in its outer shell; carbon, nitrogen, oxygen and chlorine each attain eight.

Reading dot-and-cross diagrams

A dot-and-cross diagram shows outer-shell electrons. Dots and crosses distinguish electrons supplied by different atoms; they do not represent different kinds of electron. A dot and a cross between two atoms form a shared pair. Electrons shown around an atom but not between bonded atoms are non-bonding electrons.

Before bonding, hydrogen has one outer-shell electron, carbon has four, nitrogen has five, oxygen has six and chlorine has seven. In the diagrams below, all hydrogen electrons are shown as crosses when several hydrogen atoms bond to the same central atom.

One shared pair: $$\ce{H2}$$, $$\ce{Cl2}$$ and $$\ce{HCl}$$

In hydrogen, $$\ce{H2}$$, each hydrogen atom supplies one electron. The shared pair gives each hydrogen two outer-shell electrons:

In chlorine, $$\ce{Cl2}$$, each chlorine atom supplies one electron to the shared pair. Each also has three non-bonding pairs. Counting the shared pair gives each chlorine eight outer-shell electrons:

In hydrogen chloride, $$\ce{HCl}$$, hydrogen and chlorine share one pair. Hydrogen counts two electrons in the pair; chlorine counts the pair along with its six non-bonding electrons, giving it eight:

Several shared pairs: $$\ce{H2O}$$, $$\ce{NH3}$$ and $$\ce{CH4}$$

In water, $$\ce{H2O}$$, oxygen shares one pair with each of two hydrogen atoms. Oxygen also has two non-bonding pairs. It counts eight outer-shell electrons in total, while each hydrogen counts two:

In ammonia, $$\ce{NH3}$$, nitrogen shares one pair with each of three hydrogen atoms. Nitrogen also has one non-bonding pair. The three shared pairs and the non-bonding pair give nitrogen eight outer-shell electrons; each hydrogen counts two:

In methane, $$\ce{CH4}$$, carbon shares one pair with each of four hydrogen atoms. Carbon counts the four shared pairs as eight outer-shell electrons, and each hydrogen counts two: