Valency of Thallium (Tl)
Thallium shows variable valency — most commonly 1 and 3 — because it is a p-block element whose outer electrons can join bonding in more than one way.
Looking for Thallium's valency without wading through a whole chapter? Here is the direct answer first, then the tables and derivations examiners want to see. Every number on this page comes from the same engine that powers the interactive table. Need the whole picture instead — history, characteristics, uses, exam questions? The full Thallium page is one click away.
Counting the valence electrons
From the configuration [Xe] 4f^14 5d^10 6s^2 6p^1, Thallium's outermost shell carries 3 electrons. These are the electrons shown as dots in electron-dot diagrams — the hands Thallium uses to hold other atoms.
Position check: Thallium belongs to group 13 of the periodic table, period 6. For main-group elements in groups 13–18, a quick rule is valency = group number − 10, which gives 3 here.
Why Thallium has more than one valency
The common valencies are 1 and 3. Both the outer s electrons and the nearby (n−1)d electrons sit close in energy, so Thallium can offer different numbers of them in different compounds — giving ions such as Tl¹⁺ and Tl³⁺ and families of compounds like Tl₁X₂ / Tl₃X₂.
Valency vs valence electrons
Valence electrons are what you count (3 for Thallium); valency is what the atom will do to complete its outer shell (1 and 3 here). Keep the two words straight and half the periodic-table chapter writes itself.
Students also ask
How do I find the valency of Thallium from its group?
Thallium sits in group 13; groups 13–17 follow valency = group − 10, giving 3 (though Thallium also shows 1 and 3).
Why does Thallium show variable valency?
Because both its outer shell and the nearby inner subshell can participate in bonding, Thallium forms more than one stable ion — hence valencies 1 and 3.