The table below shows the formulae for common monatomic ions.
The numbers in the top row are group numbers from the periodic table.
| 1 | 2 | | | | | | | |
| | | | | | | O2– | | |
| Na+ | Mg2+ | Al3+ | | | S2– | Cl– | |
| K+ | Ca2+ | | | | | Br– | |
| | | | | | | I– | |
| | Ba2+ | | | | | | |
Elements in the same Group of the periodic table form monatomic ions having the same charge.
Metallic elements form monatomic
cations.
The size of the positive charge equals the last digit of the Group number.
The charge becomes more positive from left to right across a period.
Nonmetallic elements form monatomic
anions.
The size of the negative charge equals the Group number -
.
The charge becomes more negative from right to left across a period.
Note that Group
elements do NOT form ions.
.