On the periodic table, what are the rows called?

The horizontal rows on the periodic table of the elements are called periods. Every element in a period has the same number of atomic orbitals. For instance, hydrogen and helium are in the first period, so they both have electrons in one orbital.

The columns on the table divide the elements into groups with the same number of electrons in their outer shells. These electrons, called valence electrons, cause them to share chemical properties. For example, noble gases have full valence shells, so they are unreactive, and halogens, such as fluorine and chlorine, react vigorously with alkali metals, such as sodium and potassium.