Here's why:

* Neutral Helium: Helium in its neutral state has 2 protons and 2 electrons.

* Gaining an Electron: To achieve a -1 charge, Helium must gain one electron. This gives it a total of 3 electrons and 2 protons, resulting in a net charge of -1.

However, it's important to note that Helium is an extremely unreactive element and is very unlikely to gain an electron. It's much more stable with its neutral 2-electron configuration.