1.3 Valence

The atoms of a molecule share a specific type of bond, called a covalent bond. There's some nifty quantum physics behind how it works, but I am going to oversimplify it for now and go into more detail later. Each element has a valence, which is how many bonds its atoms can form. Hydrogen's valence is 1; an atom of hydrogen can only form one covalent bond. We can say it is monovalent. Carbon has a valence of 4; it is tetravalent, and 4 turns out to be the perfect number for making molecules, and I'll explain why.

Atoms seek to fill their valences at all times. Carbon and hydrogen and the others are almost never found as isolated atoms, because isolated atoms are very reactive and will resort to extreme measures to satisfy their craving for bonds. If you've heard of free radicals, these are pieces of molecules that have one or more atoms missing a bond, and they tend to wreak havoc on their neighbors. So we almost always find molecules with every one of their atoms' valences satisfied.

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