Predict the formula of sodium astatide by group logic.
Na+ with At- gives NaAt, on the pattern of NaCl, NaBr, NaI. Tracer chemistry confirms astatide co-precipitates with iodide — the group prediction holds at the scale of thousands of atoms.
What Is the Charge of Astatine?
Astatine commonly forms At- At+ in introductory ionic compounds.
Astatine, the rarest natural element (under 30 grams in Earth's entire crust at any moment), behaves as a halogen with -1 as its main state, plus +1 like iodine. All knowledge of it comes from tracer experiments on invisible quantities.
| Element | Astatine |
|---|---|
| Symbol | At |
| Atomic number | 85 |
| Common ionic charge | -1, +1 |
| Common ions | At-, At+ |
Visual reference
Astatine element image and atomic model
Images are used as visual references; charge rules still depend on the compound.
Why Astatine pulls one electron in
Astatine is [Xe]4f14 5d10 6s2 6p5 — one electron short, the halogen signature. But metallic character has crept so far down the group that astatide (At-) forms less eagerly than iodide, and calculations suggest bulk astatine (could it ever be assembled) might be a metal rather than an I2-like solid.
The Astatine mix-up to watch for
Writing confident astatine chemistry as if it were bench-tested. No one has ever seen astatine — any visible sample would vaporize itself with decay heat. Its -1/+1 states are established through carrier-free radiotracer work, a different epistemology than the rest of the table.
Worked example: At in a formula problem
At- and At+ in real compounds
NaAt
sodium astatine compound
Sodium ions can balance At- in simple ionic examples.
CaAt2
calcium astatine compound
Calcium ions can balance common negative Astatine ions.
Questions students ask about astatine
Why is astatine suddenly medically interesting?
At-211's alpha particles deposit lethal energy across just a few cell diameters — ideal for killing tagged cancer cells one by one. Its halogen -1 chemistry lets chemists bolt it onto antibodies the way they would iodine.
Where does natural astatine come from if it decays so fast?
Uranium and thorium decay chains continuously mint fresh atoms (half-lives of hours), maintaining a planet-wide steady state of about an ounce. Every atom you could point to is younger than a day.
Would astatine stain like iodine?
Extrapolation says darker and more metallic, but the honest answer is that no one will ever collect enough to see. Astatine is the element chemistry knows only statistically.
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.