Question

how and why does the atomic size varies as you go from left to right along A period down a group

Answer 1

pls award with braily

as u go from left to right along A period the atomic size decreases because the electronegativity of the atom increases and the charge on nuclei becomes more and electron attract to the positive charge for the size of the atom decreases .

Answer 2

1.along a period, the number of protons in the nucleus increases. therefore the nuclear charge pulls the electron towards it, decreasing the size of the atom from left to right along A period.
  
2.down the group, increase in the no. of shells, increases the atomic size