Why does atomic number increase as atomic radius decreases? This is for chemistry class.
Ahh a wonderful question.
Remember that as atomic numbers increase, the number of protons increase as well in the nucleus. This concentration of positive charge in the middle of an atom makes all the electrons come closer to the nucleus. Note that opposite charges attract (electrons are negative and protons are positive). Thus, the radius of the atom decreases because the electron cloud is less spread out.
As we go down the table, the number of shells increase. Think of a chocolate truffle. Instead of just 1 harder chocolate shell, as you go down the table, you could have 3 or more shells. thickness of these shells and the addition of orbitals that come with it makes the radius even larger.