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