What are the horizontal rows in the periodic table?

The horizontal rows in the periodic table are called “Periods”. There are 7 such rows in the periodic table. All elements having the same number of electron shells in their atoms are placed in each row of the periodic table. This leads to a gradual variation

There are two elements in the first row of the periodic table. There are 8 elements each in the second and the third rows, 18 elements each in the fourth and the fifth row, 32 elements in the sixth row, and the 7^th row is considered an incomplete row, because newly discovered elements are added to it. The sixth and seventh rows of the periodic table contain the elements that are shown in two rows beneath the periodic table, namely, the Lanthanide and the Actinide series.

The rows in the periodic table show a gradual variation of characteristics. This is a part of the phenomenon called the “Periodicity of properties in the periodic table.” The rows contain elements of increasing atomic numbers (and, consequently, atomic weights) from left to right. Properties like atomic size, metallic character, electron affinity, electronegativity and ionization potential show gradual variation in the rows of the periodic table.

The row number of an element in the periodic table indicates the number of electron shells present in the atoms of that element. This helps in determination of electronic configuration of an element from its position in the periodic table. For example, the first row elements, Hydrogen (H) and Helium (He), contain 1 shell in their atoms, the second row elements, Lithium (Li) to Neon (Ne), contain 2 electron shells in their atoms respectively.