Why do atoms tend to get larger going down a column on the periodic table?
Adding electrons and protons to an atom with a completely filled valence shell is another story. That will usually make the atom larger. Imagine the swarm again. This time, most of the bees are shoulder-to-shoulder around the queens. An additional bee won’t be able to penetrate the dense inner layer of bees attending the queen. This bee also may not be as strongly attracted to the queen, because the queen is effectively screened by her attendants. The additional bee will have to fly around the outer edges of the swarm. The swarm will be larger. For example, both sodium and potassium have a single valence electron. The potassium atom is much larger than the sodium atom, because potassium has a larger swarm of core electrons screening the valence electron from the nucleus. The valence electron roams the outer edges of the core. All of the smaller orbitals are occupied, and the valence electron must occupy a larger orbital (with higher n). The size of the atom is effectively determined by
