Question

Why does the radius of an atom decrease as you go across (from left to right) a period on the periodic table?

Answer 1

The most reasonable anwer would be B

Answer 2

Answer is D) because you are adding positively charged protons, which pull in the outer electrons

The radius of an atom decreases because the atomic number, or number of protons is increasing. This not only pulls in the outer electrons due to the increasing positive charge, but also increases the mass of the atom, which has the effect of drawing electrons toward the nucleus.