Question

How do atoms become ions?

Answer 1

An atom becomes an Ion (a) if it gains one or more electron(s) or (b) if it loses one or more electron(s). When it gains electrons it becomes negatively charged and is called an anion. When it loses electron(s) it becomes positively charged and is called a cation.

https://sites.google.com/site/internationalgcsechemistry/year-9-topics/ionic-compounds/ionic-bonding

Calcium atom with electron arrangement K (2),L(8),M(8),N(2) loses two electrons from its outermost shell ( N shell) and forms positive ions called Calcium , `Ca^ (2+)` ion.

Oxygen atom with electron arrangement K (2),L(6), gains two electrons from its outermost shell ( L shell) and forms negative ions called oxide , `O^ (2-)` ion.