The oxidation number of iron, in its free state is

Step 1: Analyze the question We are asked for the oxidation number of iron (Fe) in its free state. This means iron as an uncombined element, not part of any compound or ion.

Step 2: The concept One of the basic rules for assigning oxidation numbers is:

> The oxidation number of any element in its free (uncombined) state is zero.

This applies whether the element is a single atom (Fe, Na, Mg, Cu) or a molecule made of identical atoms (O₂, H₂, Cl₂, N₂, S₈). No electrons are being gained, lost, or shared unequally, so the oxidation number is 0.

Step 3: Apply the rule

• Fe in its free state is just Fe(s), a metal made of neutral iron atoms.
• It has not lost or gained any electrons.
• So its oxidation number = 0.

Step 4: Check the other options

• +1: not a common iron oxidation state.
• +2: this is iron in Fe²⁺ compounds, such as FeSO₄ (iron(II) sulphate).
• +3: this is iron in Fe³⁺ compounds, such as Fe₂O₃ (iron(III) oxide, the main component of rust).

These apply to iron in combined form, not the free state.

Answer: A. 0

Exam tip: Whenever you see "free state," "elemental form," or "uncombined," think zero. This works for every element, including O₂, H₂, Cu, and Zn. Also remember that the oxidation numbers in a neutral compound add up to zero, while in an ion they add up to the charge on the ion.