Describe transition elements as having ions with variable oxidation numbers, including iron(II) and iron(III)

The Periodic Table – Transition Elements

What are Transition Elements?

Transition elements are the metals found in the centre of the periodic table (groups 3–12). They are special because they can lose different numbers of electrons when they form ions, giving them many possible oxidation states.

Why Do They Have Variable Oxidation Numbers?

Think of a transition metal like a superhero who can change costumes. Each costume (oxidation state) shows a different number of electrons removed. The d‑orbitals in these elements can hold electrons that are easily lost or shared, so the metal can exist as \$M^{+}\$, \$M^{2+}\$, \$M^{3+}\$, etc.

Iron – A Classic Example ⚙️

Iron is one of the most famous transition metals. It can form two common ions:

  • \$Fe^{2+}\$ – called ferrous (iron(II))

  • \$Fe^{3+}\$ – called ferric (iron(III))

In everyday life, you can see both forms:

  1. When iron rusts, it starts as \$Fe^{2+}\$ and then oxidises to \$Fe^{3+}\$, forming iron(III) oxide.

  2. In blood, iron(II) is used to carry oxygen in haemoglobin, while iron(III) is found in ferritin, the storage protein.

Other Transition Elements with Variable Oxidation States

ElementCommon Oxidation States
Copper (Cu)\$Cu^{+}\$, \$Cu^{2+}\$
Nickel (Ni)\$Ni^{2+}\$, \$Ni^{3+}\$
Zinc (Zn)\$Zn^{2+}\$ (usually only one)
Chromium (Cr)\$Cr^{3+}\$, \$Cr^{6+}\$

How to Predict Oxidation States?

Use these simple rules:

  1. Look at the element’s group number – the last two digits of the group give the maximum oxidation state.

  2. Check the element’s position in the d‑block – the number of d‑electrons can be lost.

  3. Remember that the most stable states are often the ones that give a full d‑subshell (e.g., \$d^{10}\$).

Example: Chromium is in group 6. It can lose up to 6 electrons, but the most stable states are \$Cr^{3+}\$ (d\$^3\$) and \$Cr^{6+}\$ (d\$^0\$).

Fun Fact 🎉

Did you know that the colour of many transition metal compounds comes from d‑d electronic transitions? That’s why copper(II) salts look blue and iron(III) salts look orange‑brown!

Now you’re ready to spot transition elements and understand why they can wear many “costumes” (oxidation states) in chemistry.