Chemical bonds

Atoms bond to reach a full outer shell. There are two main ways. In an ionic bond one atom hands electrons to another: sodium gives its one valence electron to chlorine, leaving Na⁺ and Cl⁻, and the opposite charges hold them together. Ionic bonds form between a metal and a nonmetal, and ionic compounds are usually hard, high-melting crystals like salt.

In a covalent bond two atoms share a pair of electrons. This happens between nonmetals. Hydrogen makes one bond, oxygen two, nitrogen three and carbon four: that's why water is H₂O, ammonia NH₃ and methane CH₄. Atoms can share two pairs (a double bond, as in O₂ and CO₂) or three (a triple bond, as in N₂). More shared pairs make a shorter, stronger bond.

When the two atoms in a bond have different electronegativities, the electrons are shared unevenly and the bond is polar: the oxygen end of an O–H bond is slightly negative (δ−) and the hydrogen slightly positive (δ+). A molecule with polar bonds is polar overall only if they don't cancel. Water is bent, so it's polar; carbon dioxide is straight, O=C=O, so its two polar bonds point opposite ways and cancel.

To write an ionic formula, balance the charges. Magnesium forms Mg²⁺ and chlorine Cl⁻, so you need two chlorides for each magnesium: MgCl₂. Aluminum forms Al³⁺ and oxygen O²⁻; the smallest whole numbers that balance are two of each to three: Al₂O₃ (2 × +3 = +6, 3 × −2 = −6).

Metals have a third kind of bonding: their valence electrons drift freely through the whole lump of metal, which is why metals conduct electricity and heat.