Question

Why do metals tend to lose electrons and nonmetals tend to gain electrons when forming ions?

Answer 1

Ionic compounds are molecules that form through the gain and loss of electrons. A metal atom that loses an electron takes on a positive electric charge; a non-metal that gains an electron becomes negatively charged. Because opposite charges attract, the two atoms stick together, forming a strong, stable chemical bond.

Complete answer:
Elements can gain or lose electrons in order to attain their nearest noble gas configuration. Formation of ions for completion of octet helps them gain stability. In a reaction between metals and nonmetals, metals generally lose electrons to complete their octet and non-metals gain electrons to complete their octet.
Metal atoms lose electrons from their outer shell when they form ions: the ions are positive, because they have more protons than electrons. The ions formed have full outer shells. The ions have the electronic structure of a noble gas (group zero element), with a full outer shell.
Nonmetals are further to the right on the periodic table, and have high ionization energies and high electron affinities, so they gain electrons relatively easily, and lose them with difficulty. They also have a larger number of valence electrons, and are already close to having a complete octet of eight electrons.

Note:
Metals are good conductors of heat and electricity, and are malleable and ductile. Most of the metals are solids at room temperature, with a characteristic silvery shine (except for mercury, which is a liquid). Nonmetals are (usually) poor conductors of heat and electricity, and are not malleable or ductile; many of the elemental nonmetals are gases at room temperature, while others are liquids and others are solids.