Question

What are group of elements in the same horizontal (side to side) row called?

Answer 1

Each row on the periodic table is also called a period.

Explanation:

In each period, the atomic numbers of the elements increase by `1` going from left to right, and contrary to groups (the columns), elements in the same period don't necessarily share many simlar chemical properties.

The number of total shells occupied by elements in a given period is usually that of the period number. That is to say, the valence electrons in period `3` will occupy orbitals of principal quantum number `n = 3`, and all elements in period `3` have electrons in the first, second, and third energy levels.

Notice how in the above image that elements in a given period number have that many occupied energy levels..

(This trend becomes more irregular once you reach the transition elements, but this topic is for a general usage, until you take higher-level classes where handling this sort of information becomes less stressful:)