Why in a ‘period’ the size of an atom decreases if one moves from left to right?
In a period from left to right the size of an atom gradually decreases because of following reasons.
(a) Increase in the effective nuclear charge: Due to increase in nuclear charge, the electrons in the shells are attracted at greater force by the nucleus. Due to this the distance between nuclei and outermost shell decreases. And that is why size of the atom also decreases.
(b) Additions of electrons in the same shell: Due to addition of electrons in the same shell electrostatic forces between the electrons and nucleus increases. And it is also responsible for the decrease in size of the atom.