Lons form when atoms gain or lose valence electrons. When an atom gains electrons, it becomes negatively charged, and when an atom loses
electrons it becomes positively charged. Why?

In neutral atom number of electrons (negativity charged particle) and number of proton ( positively charged particle) are equal.

When one or more electrons are lost so positive charge particle increases and cation is formed.

For example in Na 11 electrons and 11 protons are present when it loses one electrons protons are 11 and 10 electron are remaining so one positive charge is present.

Morever when an element gain electrons one negative charge increases and anion is formed. For example in Cl 17 proton and 17 electrons are present when it gain one electron 18 electrons and 17 protons are now present one negative is more so anion negative ion is formed.

Explanation: