A variable oxidation number is the ability of a transition element to show more than one oxidation number, such as iron existing as Fe²⁺ and Fe³⁺.
A variable oxidation number is the ability of a transition element to show more than one oxidation number, such as iron existing as Fe²⁺ and Fe³⁺. Malay: nombor pengoksidaan berubah-ubah · Chinese: 可变氧化数.
This is one of the four special properties of the transition elements, and it is what most clearly sets them apart from the main-group metals. A Group 1 metal such as sodium has only one oxidation number, +1, because it has just one electron to lose; a Group 2 metal such as magnesium always shows +2. A transition element, by contrast, can lose different numbers of electrons in different reactions, so the same metal appears in compounds with two or more different charges, and each is written with a Roman numeral to show which one is meant.
Example. Iron shows +2 in iron(II) compounds, such as pale green FeSO₄, and +3 in iron(III) compounds, such as yellow-brown FeCl₃. Copper commonly shows +2 in blue copper(II) compounds, and manganese appears in several oxidation states across its compounds. The Roman numeral in the name, iron(II) or iron(III), is telling you exactly which oxidation number the metal has in that compound.
Confusion to avoid. Do not expect a fixed charge for a transition metal as you would for a Group 1 or Group 2 metal. Also, the oxidation number is not the same as the charge on a whole compound; it is the charge the metal ion itself carries, and you must read the Roman numeral to know it.
In the Periodic Table of Elements chapter the variable oxidation number is a headline feature of the transition elements, showing why the central block behaves so differently from the tidy, single-valued main groups on either side.
Want a teacher to make this click?
We teach SPM Chemistry one to one, so your child understands it and scores it.
from RM50/hr · One-hour paid trial · Same-day reply