Write formulas for thallium(I) sulfate and thallium(III) oxide.
Tl+ with SO4 2- takes two: Tl2SO4. Tl3+ with O2- crosses to Tl2O3. Same element, different numerals, different arithmetic — read the Roman numeral first.
What Is the Charge of Thallium?
Thallium commonly forms Tl+ Tl3+ in introductory ionic compounds.
Thallium prefers +1 — Tl2O, TlCl, Tl2SO4 (the old rat poison) — with +3 as the less stable runner-up. It is boron's group turned upside down: by period 6, the lighter members' +3 habit has fully surrendered to +1.
| Element | Thallium |
|---|---|
| Symbol | Tl |
| Atomic number | 81 |
| Common ionic charge | +1, +3 |
| Common ions | Tl+, Tl3+ |
Visual reference
Thallium element image and atomic model
Images are used as visual references; charge rules still depend on the compound.
How Thallium's outer electrons decide its charge
Thallium is [Xe]4f14 5d10 6s2 6p1. Relativistic effects bind the 6s pair so tightly that usually only the single 6p electron leaves — the textbook 'inert pair effect' at maximum strength. Tl3+ exists but is a strong oxidizer, always looking to fall back to +1.
The Thallium mistake worth a warning
Applying the aluminum/gallium +3 reflex to thallium. Group 13 slides from +3-only (Al) through +3-mostly (Ga, In) to +1-mostly (Tl). If a formula problem says thallium without a numeral, +1 is the safer default — the exact reverse of the top of the group.
Worked example: Tl in a formula problem
Tl+ and Tl3+ in real compounds
TlCl
Thallium chloride
Tl+ balances chloride ions.
Tl2O
Thallium oxide
Thallium can form oxides where total positive and negative charge balances.
Questions students ask about thallium
Why was Tl+ such an effective (and sinister) poison?
Its size and charge mimic K+ so well that nerve and muscle pumps import it freely, where it then jams sulfur-containing enzymes. Same disguise-as-nutrient strategy as cadmium, one column over.
Does thallium have legitimate modern uses?
Tl-201 (as Tl+) served in cardiac stress-test imaging — exploiting the very K+ mimicry that makes it toxic — and thallium compounds appear in specialized infrared optics and superconductor research.
What is the inert pair effect in one sentence?
In the heaviest p-block metals, the outer s2 pair becomes too tightly bound (partly for relativistic reasons) to join bonding, so elements sit two units below their group's maximum charge — thallium +1, lead +2, bismuth +3.
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.