Question

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

Answer 1

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.