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

Hint: An ion is a charged species. The net charge can be calculated by the difference of positive and negatives charges present in that species. It is denoted as a superscript at the top right corner.

Complete step by step answer:
An element is defined by its atomic number. Atomic number is equal to the number of protons. Therefore, the atomic number of the given ion is \[20\].
We know that a proton has a unit positive charge and an electron has a unit negative charge.
Charge on ion = Positive charge – Negative charge
\[ = + 20 - 18\]
\[ = + 2\]
Now, the atomic number \[20\] belongs to calcium, \[{\text{Ca}}\].
The following conventions for notation are followed:
Atomic number is mentioned as a subscript at the bottom left corner.
Charge is mentioned as a superscript at the top right corner.
The numerical value of charge is written first and then the nature of charge. Few examples: for \[ + 1\] charge, \[ + \]; for \[ - 2\]charge, \[2 - \]; for \[ + 3\]charge, \[3 + \].

Hence, the required ion notation is \[_{{\text{20}}}{\text{C}}{{\text{a}}^{{\text{2 + }}}}\].

Additional Information:
The number of neutrons in an atom is equal to the number of protons. Therefore, number of neutrons in calcium atom\[ = 20\]
Mass number of an atom is equal to the sum of the number of protons and the number of neutrons. Therefore, mass number of calcium\[ = 20 + 20 = 40\].
Mass number is represented as a superscript at the top left or top right corner.
A neutral calcium atom can be represented as \[{}_{20}^{40}{\text{Ca}}\] or \[_{{\text{20}}}{\text{C}}{{\text{a}}^{40}}\] and the given ion can also be represented as \[{}_{20}^{40}{\text{C}}{{\text{a}}^{2 + }}\].

Note: We should not relate the number of electrons to the atomic number. The number of electrons can vary for different ions of the same element.