Question

why the size of atom does not decrease regularly in a period​

Answer 1

Explanation:

It is because of the protons and electrons present in the atom of the elements. ... The size of the elements of one period increases due to increase in the number of protons

Answer 2

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Because you have two competing effects. Yes the nuclear charge and therefore electrostatic charge increases between the nucleus and electrons so decreasing the radius. However there are also more neutrons partially shielding this increased charge and increasing the radius.