What Are the Horizontal Rows of the Periodic Table Called?

On the periodic table, the seven horizontal rows are called periods. On the left-hand side of the periodic table, the row numbers are given as one through seven. Moving across a period from left to right, the atomic number of the elements increases.

Rows six and seven contain elements called transition elements, which are subdivided into the actinide and lanthanide series. While the elements numbered 58 through 71 are the lanthanide series, elements with the numbers 90 through 103 are the actinide series.

The vertical columns on this table are called groups. Groups are sometimes called families because the elements in them share certain physical and chemical properties. In the older system, these groups were given the Roman numerals I through VIII. In the new system, they are designated as groups 1 through 18.