Here's why:

* Alkali Metals: These are in Group 1 of the periodic table and have one valence electron. They are highly reactive and tend to lose this electron to achieve a stable octet.

* Halogens: These are in Group 17 of the periodic table and have seven valence electrons. They are also very reactive and readily gain one electron to achieve a stable octet.

When an alkali metal and a halogen react, the alkali metal atom loses its valence electron to form a positively charged ion (cation). The halogen atom gains this electron to form a negatively charged ion (anion). The opposite charges attract, forming an ionic bond.

Example: Sodium (Na) reacts with Chlorine (Cl) to form Sodium Chloride (NaCl).

* Na loses an electron to become Na+

* Cl gains an electron to become Cl-

* The oppositely charged ions then form an ionic bond.

Let me know if you'd like more examples or want to explore other types of chemical reactions!