Atoms that have fewer electrons in their valence shells tend to gain electrons to form charged ions.

Here's why:

* Valence Shell: The outermost shell of an atom is called the valence shell.

* Stability: Atoms are most stable when their valence shell is full.

* Octet Rule: Most atoms strive to have 8 electrons in their valence shell (except for hydrogen and helium, which are stable with 2).

* Gaining Electrons: Atoms with fewer electrons in their valence shell will readily gain electrons to complete their valence shell and achieve stability. This creates a negatively charged ion (anion).

Example:

* Chlorine has 7 electrons in its valence shell. To achieve an octet, it needs one more electron. It will gain an electron, forming a chloride ion (Cl⁻).

Let me know if you'd like more examples!