Write the formula for aluminum selenide, then predict what it does in water.
Al3+ and Se2- cross to Al2Se3. Water hydrolyzes it to Al(OH)3 and H2Se — hydrogen selenide, a gas even more toxic than H2S. (A thought experiment, not a lab suggestion.)
What Is the Charge of Selenium?
Selenium commonly forms Se2- Se4+ Se6+ in introductory ionic compounds.
Selenium behaves like sulfur one row down: 2- as the selenide ion (Na2Se, CdSe quantum dots), +4 in selenites (SeO3 2-), and +6 in selenates (SeO4 2-). Your body runs about 25 selenoproteins with selenium in the -2 state.
| Element | Selenium |
|---|---|
| Symbol | Se |
| Atomic number | 34 |
| Common ionic charge | -2, +4, +6 |
| Common ions | Se2-, Se4+, Se6+ |
Visual reference
Selenium element image and atomic model
Images are used as visual references; charge rules still depend on the compound.
The electron count behind Selenium's charge
Selenium is [Ar]3d10 4s2 4p4 — six valence electrons, two short of krypton. Metals push it to -2; oxygen pulls it to +4 or +6. Its chemistry so closely shadows sulfur's that selenium slips into sulfur's spots in minerals and proteins alike.
The classic Selenium error
Assuming more selenium is better because it is 'an essential mineral.' The gap between the required dose (~55 µg/day) and toxicity (~400 µg/day) is one of the narrowest of any nutrient — the charge chemistry that makes it a great enzyme cofactor also makes excess reactive.
Worked example: Se in a formula problem
Se2- and Se4+ and Se6+ in real compounds
Na2Se
sodium selenium compound
Sodium ions can balance Se2- in simple ionic examples.
CaSe
calcium selenium compound
Calcium ions can balance common negative Selenium ions.
Questions students ask about selenium
What charge does selenium have in anti-dandruff shampoo?
Selenium sulfide, SeS2, pairs selenium and sulfur so evenly that oxidation states are nearly moot (formally Se +4). It works by slowing the scalp yeast's growth, not through ionic action.
Why do old photocopiers contain selenium?
Amorphous selenium at charge 0 is photoconductive — light makes it conduct. The drum holds electrostatic charge in the dark and releases it where light strikes, drawing the page in static electricity.
What is selenocysteine?
The '21st amino acid': cysteine with selenium (as -2 selenol) replacing sulfur. Its easier redox switching lets antioxidant enzymes like glutathione peroxidase run faster than sulfur versions could.
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.