What are group of elements in the same horizontal (side to side) row called?
Each row on the periodic table is also called a period.
In each period, the atomic numbers of the elements increase by
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
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:)