Question

1. what is a period?

mark all that apply
vertical columns
horizontal rows
tells main energy level
tells the number of valence electrons if a main group element

Explanation:

In the periodic table, a period is defined as a horizontal row. Also, the period number corresponds to the main energy level (principal quantum number, \(n\)) of the electrons in atoms of elements in that period. Vertical columns are groups. The number of valence electrons for main - group elements is determined by the group number, not the period number. So "Horizontal rows" and "Tells main energy level" are correct.

Answer:

B. Horizontal rows, C. Tells main energy level