Answer the following question given this paragraph: 

From left to right across the periodic table, each element has one more proton than the element to its left. Because atoms are always electrically neutral, for each added proton, one electron is also added. Electrons are added first to the lowest energy level possible until that level is full. Only then are electrons added to the next higher energy level.


Q: Because atoms are always electrically neutral, for each added proton, one of what is also added?


A:
electron