Transferring electrons

Some atoms don't share: they give or take electrons.

• Atoms with fewer than 4 valence electrons usually give them away: lithium, beryllium, sodium, magnesium (metals).
• Atoms with more than 4 usually take more (or share).

Sodium gives one. Sodium (2, 8, 1) loses its single outer electron. It now has 11 protons but only 10 electrons, so it has a charge of +1: the sodium cation, Na⁺. A positive ion is a cation.

Chlorine takes one. Chlorine (2, 8, 7) gains one electron to make 8. Now it has one extra electron: the chloride anion, Cl⁻. A negative ion is an anion. Both are called ions.

Opposite charges attract. The pull between Na⁺ and Cl⁻ is an ionic bond: Na⁺ + Cl⁻ → NaCl, common salt. In short: sodium gives, chlorine takes; that's transfer, so it's ionic. Hydrogen can't give away its only electron, so it shares: that's covalent.

Ionic compounds don't make separate molecules. They build 3-D crystals. In salt, each Na⁺ has six Cl⁻ around it (up, down, left, right, front, back), and each Cl⁻ has six Na⁺. Other ions: sulfur gains two electrons to become S²⁻; oxygen becomes O²⁻.

Naming: the cation (usually a metal) comes first, then the anion (usually a non-metal); a simple anion ends in -ide. Mg²⁺ needs two Cl⁻ to balance it: MgCl₂, magnesium chloride. Others: sodium chloride, calcium oxide, magnesium sulfide.

Polyatomic ions are groups of atoms that act as one ion:

Some metals can form two ions. A trick: -ous is the smaller charge, -ic the bigger one.

• Iron: ferrous Fe²⁺, ferric Fe³⁺.
• Copper: cuprous Cu⁺, cupric Cu²⁺.

A useful small goal: learn the 15 ions in Table 9.1 this week.