Valency of Cobalt (Co)
Cobalt shows variable valency — most commonly 2 and 3 — because it is a d-block element whose outer electrons can join bonding in more than one way.
Looking for Cobalt'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 Cobalt page is one click away.
Counting the valence electrons
From the configuration [Ar] 3d^7 4s^2, Cobalt's outermost shell carries 2 electrons. For a d-block element the nearby inner "d" subshell can also lend electrons, which is where variable valency comes from.
Position check: Cobalt belongs to group 9 of the periodic table, period 4.
Why Cobalt has more than one valency
The common valencies are 2 and 3. Both the outer s electrons and the nearby (n−1)d electrons sit close in energy, so Cobalt can offer different numbers of them in different compounds — giving ions such as Co²⁺ and Co³⁺ and families of compounds like CoX / Co₃X₂.
Valency vs valence electrons
Valence electrons are what you count (2 for Cobalt); valency is what the atom will do to complete its outer shell (2 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 Cobalt from its group?
Cobalt sits in group 9; for this transition group the safe exam answer is its common valency, 2 and 3.
Why does Cobalt show variable valency?
Because both its outer shell and the nearby inner subshell can participate in bonding, Cobalt forms more than one stable ion — hence valencies 2 and 3.