A bond, as you have seen, is a pair of electrons shared by two atoms. Two hydrogen atoms in H–H are exactly alike, so they share their pair exactly evenly. When the two atoms are different, one of them can pull the pair closer to itself.
Picture a tug of war with the shared pair as the rope. Each atom's nucleus is positive, and it pulls on the negative electrons. How strongly an atom pulls on a shared pair is called its electronegativity. The atom that pulls harder keeps the pair closer to it, and the electrons spend more of their time at its end of the bond.
Who pulls hardest
Fluorine, oxygen, nitrogen and chlorine are strong pullers. Fluorine, oxygen and nitrogen keep their outer electrons in the same shell as carbon, and they have more protons in their nucleus (9, 8 and 7, against carbon's 6), so they grip a shared pair more tightly. Carbon and hydrogen pull with almost the same strength, so a C–H bond shares its pair almost evenly.
| Atom | Pull |
|---|---|
| fluorine, F | 3.98 |
| oxygen, O | 3.44 |
| chlorine, Cl | 3.16 |
| nitrogen, N | 3.04 |
| carbon, C | 2.55 |
| hydrogen, H | 2.20 |
What matters in a bond is the gap between the two numbers. Oxygen and hydrogen are 1.24 apart, a big gap, so an O–H bond shares very unevenly. Carbon and hydrogen are only 0.35 apart, so a C–H bond shares almost evenly. A C–C bond joins two identical atoms, so it shares exactly evenly, just like H–H.