Here's why:

* Oxygen (O), Sulfur (S), and Selenium (Se): These elements are all in Group 16 of the periodic table (also known as the chalcogens). They tend to gain two electrons to achieve a stable, noble gas electron configuration. This results in a -2 charge.

* Fluorine (F), Chlorine (Cl), and Bromine (Br): These elements are in Group 17 (also known as the halogens). They tend to gain one electron to achieve a stable, noble gas electron configuration, leading to a -1 charge.

Key Point: The tendency to form ions with specific charges is directly related to their position in the periodic table and their desire to achieve a stable electron configuration like the nearest noble gas.