An atom's electrons are not all the same distance from the nucleus nor are they held to the atom with equal strength. The electrons that are the farthest away (on the outermost energy level) or most loosely held are called valence electrons. Many properties of the atom are determined by the number of valence electrons. Atoms react with each other as a result of how many valence electrons are in each atom. Atoms are most stable, or less reactive, when they have filled the outer shell of electrons. The number of valence electrons is the key to how an atom reacts with other atoms. This is true because the valence electrons are involved in bonding.

Look at the atom diagrams below. How many valence electrons are in each atom? Enter the number of valence electrons in the boxes below.

Interactive exercise. Assistance may be required.

When an atom forms a chemical bond, one of two things usually happens. Either the number of valence electrons increases to a total of eight, or all the valence electrons are given up. When atoms end up with eight or zero valence electrons, the atoms become more stable, or less reactive, than they were before.