Question

how does the size of atom generally vary in going from left to right in a period of a Periodic Table why does it vary this way

Answer 1

while going left to right an electron is added to the outermost shell but the no of shells remain constant thus the effective nuclear charge increases and thus the size of the orbit decreases

Answer 2

Atomic size gradually decreases from left to right across a period of elements. This is because, within a period or family of elements, all electrons are added to the same shell