Lesson 2: Sharing is bonding

A closer look at a bond

Section 1 of 4

In lesson 1 you met the idea that a bond is a shared pair of electrons. Now let's zoom in on one bond and give it its proper name.

For the atoms in this course, an atom is most stable when its outer shell is full. For hydrogen a full shell is 2 electrons; for carbon, nitrogen and oxygen it is 8.

A hydrogen atom has just 1 electron. When two hydrogen atoms meet, each brings its electron and they share the pair. Both atoms count both electrons, so each now has a full shell of 2. That shared pair holds the two atoms together. Chemists call it a covalent bond, and draw it as a line: H–⁠H.

Two hydrogen atoms forming a covalent bond Three steps from left to right. First, two separate hydrogen atoms, each a nucleus with one electron on a ring. Second, the two rings overlap and both electrons sit in the overlap, shared by both atoms. Third, the same molecule drawn as H, a line, H. + + − − + + + − − H H two H atoms a shared pair one line
Two hydrogen atoms each bring one electron. They share the pair, so each counts 2. The shared pair is a covalent bond, drawn as one line.

Every line in a chemical drawing means the same thing: one shared pair, two electrons. When you count lines, you are counting shared pairs.

Quick check

In a hydrogen molecule, H–⁠H, what does the line stand for?