Valence is like an atom's social life — it describes how many "friendships" (bonds) an atom can form with other atoms. A hydrogen atom has a valence of 1, meaning it can form one bond. Oxygen has a valence of 2, so it bonds with two hydrogen atoms to make water (H₂O). Carbon is the social butterfly of the periodic table with a valence of 4, allowing it to form four bonds simultaneously — which is why carbon can build an astonishing variety of molecules, from simple methane (CH₄) to the incredibly complex molecules of life like proteins and DNA.
The concept of valence emerged in the mid-1800s when chemists like Edward Frankland noticed that atoms combined in fixed ratios. Frankland observed that nitrogen always bonded with three hydrogen atoms (NH₃), while carbon always bonded with four (CH₄). He proposed that each element had a definite "combining power." Today we understand that valence is determined by the number of electrons in an atom's outermost shell (valence electrons) and how many more it needs to achieve a stable, filled shell — typically 8 electrons, following the octet rule.
Valence explains molecular shapes and the diversity of chemical compounds. Carbon's valence of 4 allows it to form long chains, branching structures, rings, and double or triple bonds, giving rise to millions of organic compounds. Silicon also has a valence of 4, which is why it forms the backbone of countless minerals (silicates make up 90% of Earth's crust). Some elements display variable valence — iron can have a valence of 2 or 3, sulfur can be 2, 4, or 6 — which is why they form so many different compounds. Valence is the key to understanding why atoms combine the way they do.