Chemical Bonding

Many atoms become more stable when their outer shell becomes complete. Hydrogen aims for a duplet of 2 electrons, while many other atoms tend toward an octet of 8 valence electrons.

In ionic or electrovalent bonding, electrons are transferred from a metal atom to a non-metal atom. The metal forms a positive ion and the non-metal forms a negative ion. Opposite charges attract and form an ionic compound.

Sodium has one valence electron and chlorine has seven. Sodium loses one electron to become Na+, and chlorine gains one electron to become Cl-. The attraction between Na+ and Cl- forms sodium chloride.

In covalent bonding, atoms share pairs of electrons. This usually happens between non-metal atoms. A shared pair forms a single covalent bond; two shared pairs form a double bond; three shared pairs form a triple bond.

Ionic compounds usually have high melting points because strong forces hold the ions together. They conduct electricity when molten or dissolved in water because ions can move. Solid ionic compounds do not conduct because ions are fixed in place.

Simple covalent compounds usually have lower melting and boiling points than ionic compounds. They generally do not conduct electricity because they do not contain free ions. Many are gases, liquids or soft solids.