# How do you write the ion notation for an atom with 20 protons and 18 electrons?

$C {a}^{2 +}$
If it has got 20 protons, 20 positively charged, massive nuclear particles, the element is calcium by defintion, i.e. $Z$, the atomic number $=$ $20$.
The neutral metal has 20 electrons; the metal can ionize to form caclium ion, $C {a}^{2 +}$.