Question

why atomic radii decreases gradually from left to right across a period

Answer 1

As you go from left to right across the periodic table, you are adding one more proton every time hence increasing the nuclear charge of the atom. An increased nuclear charge means the nucleus attracts the electrons more to it hence the size of the atom decreases as it "tightens". hence the atomic radius decreases every time.

Answer 2

does that mean all electrons wil get attracted more, or only the ones that get added as we move right

Answer 3

i mean all electrons in an atom wil get MORE attracted toward nucleus or only the outermost electrons...