# Oxidation state in compounds

> Finding the metal's oxidation state from a formula


## Objectives

- Find the oxidation state of an element in a compound (sum = 0) or in an atomic group (sum = its charge).
- Use O = −2, Na and K = +1, Cl in chlorides = −1 and the $\ce{SO4}$ group = −2.
- Name a compound with a Roman numeral, e.g. iron (II), iron (III).

## 1. Properties of iron




## 2. No charge written

Predict: in $\ce{FeO}$, oxygen counts as −2. What does iron count as?

- 0
- +1
- +2 ✓
- −2

A neutral compound has a total charge of zero. With oxygen at −2, iron in $\ce{FeO}$ counts as +2, because +2 + (−2) = 0.

[ar-EG] الـ compound المتعادل مجموع الـ charges فيه صفر. ولما الـ oxygen يبقى −2، الـ iron في الـ $\ce{FeO}$ يتحسب +2، عشان +2 + (−2) = 0.
## 3. Oxidation state

Oxidation state The charge an atom has, or seems to have, in a compound or ion. It is written with a sign, like +2 or −1. The two easy cases Simple ions are easy: read the charge. In a compound, you calculate it.


## 4. Worked: iron in Fe₂O₃

Rules you need in Chapter 1 Step 1: Call iron x Two Fe atoms give 2x. Three O atoms give 3 × (−2). Step 2: Make the sum zero Step 3: Name it Iron is +3, so $\ce{Fe2O3}$ is iron (III) oxide. The Roman numeral is the metal's oxidation state.


## 5. Balance chromium in CrCl₃

Cl in a chloride counts as −1. Move the slider until the sum for $\ce{CrCl3}$ is zero. oxidation state of Cr 3 Cl atoms sum: x + 3(−1) Balanced The sum is zero. Cr is +3, so $\ce{CrCl3}$ is chromium (III) chloride.


## 6. Your turn

What is the name of $\ce{Fe2(SO4)3}$?

- iron (II) sulphate
- iron (VI) sulphate
- iron (III) sulphate ✓

Three $\ce{SO4}$ groups give −6; two Fe atoms share +6, so each is +3.

[ar-EG] تلات $\ce{SO4}$ groups بيدّوا −6، والـ 2 Fe atoms بيتقسموا الـ +6، فالواحدة منهم +3.
## 7. Summary

Element alone = 0; simple ion = its charge. In a compound the oxidation states add up to 0: O = −2, Na and K = +1, Cl in chlorides = −1, $\ce{SO4}$ = −2. The Roman numeral in a name is the metal's oxidation state: iron (II) = +2, iron (III) = +3. Next in Chapter 1: the book's table lists each metal's oxidation states with its compounds. Can you explain why iron is +3 in $\ce{FeCl3}$ but +2 in $\ce{FeCl2}$? Cl in chlorides counts as −1. In $\ce{FeCl3}$ three Cl give −3, so iron is +3; in $\ce{FeCl2}$ two Cl give −2, so iron is +2. The sum is 0 each time.



Chapter: Transition Elements (https://yaani.ai/en/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/en/lesson/chem-s3-en.lesson-pt.oxidation-number