Why is magnesium a solid at room temperature but oxygen is a gas?

Answer 1

In a metal such as magnesium, the outermost electrons come loose from their atoms, leaving behind positive ions and are shared among all the atoms in a give chunk of the metal as a common electron pool. The positively charged ions are attracted to this electron pool, so the whole mass stays together. This phenomenon is called metallic bonding. In oxygen two atoms share a set of four atoms to form a two-atom molecule that is only weakly attracted to nearby molecules, so it can move freely of other particles.