1) What is the number of first and last vertical column?​ 2) What are horizontal rows known as? How many are they in numbers?
3) How many elements are there in the second and third horizontal rows each?
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Horizontal rows are called periods

Explanation:

1.Modern periodic table has eighteen (18) vertical columns. These are called groups

2.The elements are arranged in seven horizontal rows, in order of increasing atomic number from left to right and top to bottom. The rows are called periods, and they are numbered from 1 to 7

3.The second and third periods have eight elements each.