Metal to non-metal across a period

Across a period, character generally changes from metallic on the left to non-metallic on the right. Metals with few outer electrons tend to lose them; non-metals with nearly full outer shells tend to gain or share electrons. Group 18 has full outer shells, including helium with two rather than eight.

Use Period 3 as the model. Sodium, magnesium and aluminium have 1, 2 and 3 outer electrons. They lose these electrons easily to form positive ions, so they are metals: shiny, good conductors, with high melting points. Silicon, with 4 outer electrons, is in between: it has a giant covalent structure and is a semiconductor (a metalloid). Phosphorus, sulfur and chlorine have 5, 6 and 7 outer electrons. They tend to gain electrons to form negative ions or share electrons in covalent bonds, so they are non-metals: dull, poor conductors, and mostly with low melting points. Argon has a full outer shell and is unreactive.