Answer:
a. Periods form horizontal rows; groups form vertical columns
Explanation:
On the periodic table of elements, it is true that periods form horizontal rows and groups form vertical columns.
In the periodic table, elements having the same number of electrons in the outermost shell of their atoms are placed over one another in vertical columns.
- Each of the resulting vertical column is called a group or family.
- The resulting horizontal row is know as period or row.
