Write the formula for the compound of aluminum and fluorine.
Al3+ needs three F- to balance: AlF3. Used industrially in aluminum smelting.
What Is the Charge of Fluorine?
Fluorine commonly forms F- in introductory ionic compounds.
Fluorine is -1 in every single compound it forms — no exceptions, no asterisks. Whether in NaF (toothpaste fluoride), CaF2 (the mineral fluorite), or SF6, fluorine always takes one electron and becomes fluoride, F-.
| Element | Fluorine |
|---|---|
| Symbol | F |
| Atomic number | 9 |
| Common ionic charge | -1 |
| Common ions | F- |
Visual reference
Fluorine element image and atomic model
Images are used as visual references; charge rules still depend on the compound.
Why Fluorine pulls one electron in
Fluorine is the most electronegative element (3.98 on the Pauling scale). Its 2s2 2p5 configuration needs exactly one electron to reach neon's, and no element can out-pull it. That is why fluorine is the one element whose oxidation state you never have to calculate.
The Fluorine mix-up to watch for
Assuming fluorine works like chlorine. Chlorine reaches +1, +5, even +7 in oxyanions like ClO4-, so students expect fluorine to do the same. It cannot — there is no 'perfluorate,' because nothing can force fluorine positive.
Worked example: F in a formula problem
F- in real compounds
NaF
sodium fluorine compound
Sodium ions can balance F- in simple ionic examples.
CaF2
calcium fluorine compound
Calcium ions can balance common negative Fluorine ions.
Questions students ask about fluorine
Is the fluoride in drinking water and toothpaste the same F- ion?
Yes — whether delivered as NaF or SnF2, the active species is the same fluoride ion, which swaps into tooth enamel to form acid-resistant fluorapatite.
What happens in OF2 — is fluorine positive there?
No, this is the one case where oxygen goes positive (+2) instead. Fluorine stays -1 even when bonded to oxygen; it is the immovable reference point of oxidation-state rules.
Why is F- so much less reactive than F2?
F2 is desperate to gain electrons and reacts with almost everything. F- has already gained its electron — it is satisfied, stable, and safe enough to put in toothpaste.
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.