Write the formula for aluminum sulfate, used to clarify drinking water.
Al3+ and SO4 2-. Cross to the lowest common multiple of 6: two Al3+ (+6) and three sulfates (-6) give Al2(SO4)3.
What Is the Charge of Aluminum?
Aluminum commonly forms Al3+ in introductory ionic compounds.
Aluminum forms a 3+ ion, Al3+, in all of its common compounds: Al2O3 (the oxide skin that stops cans from corroding), AlCl3, and the alum in water treatment. No Roman numerals needed — aluminum has one setting.
| Element | Aluminum |
|---|---|
| Symbol | Al |
| Atomic number | 13 |
| Common ionic charge | +3 |
| Common ions | Al3+ |
Visual reference
Aluminum element image and atomic model
Images are used as visual references; charge rules still depend on the compound.
How Aluminum's outer electrons decide its charge
Aluminum is [Ne]3s2 3p1. All three outer electrons go, exposing the neon core; a fourth would have to come out of that core, which is off the table energetically. Unlike true transition metals, aluminum has no d electrons to juggle, so there is no second common charge.
The Aluminum mistake worth a warning
Giving aluminum a Roman numeral — 'aluminum(III) oxide' — on naming exercises. Roman numerals are reserved for metals with multiple charges (iron, copper, lead). Aluminum is fixed at 3+, so it is just 'aluminum oxide.'
Worked example: Al in a formula problem
Al3+ in real compounds
AlCl3
Aluminum chloride
Al3+ balances chloride ions.
Al2O3
Aluminum oxide
Aluminum can form oxides where total positive and negative charge balances.
Questions students ask about aluminum
Why is Al3+ so small, and why does it matter?
Radius just 54 pm — a lot of charge in a tiny package. That high charge density is why Al3+ grabs six water molecules in solution and why aluminum solutions are mildly acidic.
Can aluminum be 1+ like some textbooks' 'AlCl' suggests?
Al(I) species exist only at high temperature in the gas phase and disproportionate immediately at room temperature. For every school and lab purpose, aluminum is 3+.
What is the charge of aluminum in foil?
Zero — foil is elemental metal. The instant the surface meets air it forms a nanometers-thin Al2O3 layer (aluminum 3+ there), which seals and protects the neutral metal underneath.
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.