Question

Show why in the period the size of an atom decreases if one moves from left to right

Answer 1

Explanation:

while moving from left to right in the periodic table, we notice 2 things--

1-- the nuclear charge of the atom increases

2-- an electron gets added to the outermost shell

Now,

as nuclear charge increases, the nucleus of the atom is able to pull the electrons and shells towards the center of the atom thereby decreasing its radius. Now if you'd like a deeper explanation,

nucleus is made up of protons and neutrons. Protons have a +ve charge while neutrons are neutral in charge i.e they have no charge.

so the nucleus is overall +vely charged as the protons contribute the +ve charge. Electrons on the other hand, are -vely charged. This is the basic reason why the electron experiences an attraction from the nucleus.

now, think for a moment and realize that the more the +ve charge the more the attraction of electron toward nucleus, the lesser the distance between them and so the lesser the radius.

as we move from left to right across a period, the atomic no. increases i.e effective nuclear charge increases ( remember atomic no. is the no. of protons in the nucleus of the respective element) so size of atom decreases.

now you may have a doubt whether the addition of electrons have no role to play here.

actually they do... but addition of the new electron is into the same shell, there is no addition of a new shell. thus this has very less influence on increasing the radius.. because the increase in the nuclear charge is more effectively seen.

hope this helps...