A metal atom becomes a cation when the atom loses its electron and becomes a positively charged ion.
Cations are positively the most positive type of particle. When a single element or group of elements has a charge, it is known as an ion. When an ion is positively charged, it's called a cation.
When an element or compound is in cation form, it can be represented by either the element symbol or by a compound formula, with the amount of charge written as a superscript immediately after the symbol or formula
Cations form when an element or group of elements (compounds) lose one or more electrons. When an element loses an electron, it becomes less negative and more positive.
See Study.com's Cation: Definition & Examples to learn more.