Wayne Tregidga, of Buller High School, asks :-
Why is a chlorine atom smaller than a sodium atom?
Owen Curnow, a chemist at the University of Canterbury, responded.
Having decided on a definition that treats all atoms equally, we find that, despite having more protons and electrons, a chlorine atom is smaller than a sodium atom (and an argon atom is even smaller).
Perhaps we can think of it this way: You're in a room with a table of delicious food, the more food, the closer you want to get and you don't care so much about how many other people are in the room (and safety regulations don't permit more than eight people in the room at one time). So it is with atoms, the more protons in the middle, the closer the electrons want to get (they don't much notice the other electrons in other parts of the room) and the atom gets smaller.
Of course, this can't go on forever, eventually the electrons do notice each other and physical principles only permit a certain number of electrons (such as eight) in this orbit around the protons. At this point, additional electrons go into a new orbit and suddenly the atom gets very much bigger. This occurs, for example, on going from argon (element 18) to potassium (element 19). By analogy, this would be like putting additional people out in the hallway.