# Question #58a3f

An atom, such as $M g$, is a neutral particle with electrons orbiting a positive nucleus. The noble gas notation for magnesium is $\left[N e\right] 3 {s}^{2}$.
An ion, such as $N {a}^{+}$ or $C {l}^{-}$ is atom that has gained or lost electrons. When an atom loses electrons, it becomes a positive ion, or cation. When an atom gains electrons, it becomes a negative ion, or anion.
The only atoms that do not form ions are noble gases as they have no ionic radius and all the electron shells are completely filled. They can, however form compounds, all except the gases helium ($H e$) and neon ($N e$).