关于电子排布的一道AP化学题Consider an atom that contains an electron wit

关于电子排布的一道AP化学题
Consider an atom that contains an electron with the quantum numbers 3,1,0,-1/2,All that can be said about this electron is that it may exist in any element with an atomic number of 13 or greater.
这句话是书上的讲解,我不太明白为什么是13+而不是21+（我自己推的是21）