Aluminum oxide — what is the formula?
Al3+ and O2-. Lowest common multiple of 3 and 2 is 6: two Al3+ (+6) balance three O2- (-6). Formula: Al2O3, the main component of rubies and sandpaper alike.
What Is the Charge of Oxygen?
Oxygen commonly forms O2- in introductory ionic compounds.
Oxygen is 2- in nearly every compound: the oxide ion O2- in MgO, and the -2 oxidation state in water, rust, and silicates. The two exceptions worth memorizing are peroxides like H2O2 (each O is -1) and its bond with fluorine in OF2 (+2).
| Element | Oxygen |
|---|---|
| Symbol | O |
| Atomic number | 8 |
| Common ionic charge | -2 |
| Common ions | O2- |
Visual reference
Oxygen element image and atomic model
Images are used as visual references; charge rules still depend on the compound.
The electron count behind Oxygen's charge
Oxygen's 2s2 2p4 configuration is two electrons short of neon, and its electronegativity (3.44) is second only to fluorine. It takes two electrons from almost any partner. Only when oxygen bonds to itself (peroxides) or to fluorine does the -2 rule bend.
The classic Oxygen error
Applying -2 blindly to peroxides. In H2O2, hydrogen's two +1 charges must balance two oxygens, so each oxygen is -1, not -2. Sodium peroxide Na2O2 trips students the same way.
Worked example: O in a formula problem
O2- in real compounds
Na2O
sodium oxygen compound
Sodium ions can balance O2- in simple ionic examples.
CaO
calcium oxygen compound
Calcium ions can balance common negative Oxygen ions.
Questions students ask about oxygen
Why is oxygen 2- and not 1- or 3-?
Adding two electrons exactly fills the 2p subshell to neon's configuration. One electron leaves the shell incomplete; a third would have to start a new shell with no payoff.
What is oxygen's charge in O2 gas?
Zero. An element bonded only to itself always has oxidation state 0 — charges appear only when different elements share or transfer electrons unevenly.
What is a superoxide?
The O2- ion (one negative charge spread across two oxygens), found in KO2. Each oxygen averages -1/2 — a curiosity that shows why 'oxygen is always -2' needs its footnotes.
How do I use this charge in formulas?
Combine positive and negative ions so the total charge is zero — or let the ionic compound calculator do the crossing for you. The full chart lives on the periodic table with charges.
Where this data comes from
Charges and element facts on this page are checked against the primary references below. Found an error? Email hello@periodictablewithcharges.com — corrections ship in the next content update. See how this site is edited.