Smaller the atomic size get smaller as it goes across a period. This is because periods all have the same amount of valence electron shell numbers (like period 2 has 2 valance shells around the atom) but at the same exact time, the number of protons increase going down the period. This means there’s more/a stronger pull from the protons to the outer balance electrons making the valance electron layers pull inward more, but on the next layer of new balance electron shell is formed so the distance between the outer most balanced electron in the protons is wider than the previous level, but like before the gap decreases as a you go down the period. Hope this makes sense
