Chapter 4 · Atoms and Elements
4.7Ions: Losing and Gaining Electrons
12 min · two checks
Predict
What is the charge of the ion main-group oxygen tends to form?
The idea
- Predict ion charges for main groups 1, 2, 16, and 17.
- Compute electrons from charge.
An ion is an atom, or a group of atoms, with a net charge because the electron count does not match the proton count. Metals on the left tend to lose electrons and become cations. Group 1 loses one electron (Na⁺). Group 2 loses two (Mg²⁺). Aluminum, Group 13, commonly loses three (Al³⁺). Nonmetals on the right tend to gain electrons and become anions. Group 17 gains one (Cl⁻). Group 16 gains two (O²⁻). The pattern is “reach the nearest noble-gas electron count,” which for these main groups means a full outer shell.
The charge is protons minus electrons, with the sign. Na⁺ has 11 protons and 10 electrons. O²⁻ has 8 protons and 10 electrons. Transition metals are less predictable from the group number alone; iron commonly forms Fe²⁺ or Fe³⁺, and you will be told which, or you will read it from a roman numeral in the name. Never change the proton count to make a charge. That would be a different element.
Keep these
- Cations: electrons lost, positive. Anions: electrons gained, negative.
- Group 1 → 1+, Group 2 → 2+, Group 17 → 1−, Group 16 → 2−.
- Electrons = protons − charge (using the signed charge carefully: Na⁺ has 10 electrons).
Worked path
Aluminum is element 13. How many electrons are in Al³⁺?
Observe
Z = 13, charge 3+.