You're right to notice that chlorine (Cl) and fluorine (F) readily gain an electron. This is because they are both in Group 17 (VIIA) of the periodic table, also known as the halogens.

Here's why halogens have this tendency:

* Electron Configuration: Halogens have 7 electrons in their outermost shell. They are just one electron short of achieving a stable, full outer shell (8 electrons).

* Electronegativity: Halogens are highly electronegative, meaning they have a strong attraction for electrons.

Therefore, other elements in Group 17 (halogens) will also easily take on one additional electron. These include:

* Bromine (Br)

* Iodine (I)

* Astatine (At)

Note: While Astatine is a halogen, it's radioactive and has very limited applications.

Let me know if you have any other questions about chemistry!