Here's why:

* Group 1 metals (alkali metals) have a +1 charge when they form ions (e.g., Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺).

* Oxygen has a -2 charge when it forms an oxide ion (O²⁻).

To balance the charges and form a neutral compound, you need two group 1 metal ions (+1 each) for every one oxide ion (-2). This results in the general formula M₂O.

Examples:

* Lithium oxide: Li₂O

* Sodium oxide: Na₂O

* Potassium oxide: K₂O

* Rubidium oxide: Rb₂O

* Cesium oxide: Cs₂O