# Valency and writing chemical formulas

> From a name to a formula, ready for Chapter 1's compounds


## Objectives

- Define valency and read it from a name with a Roman numeral, e.g. chromium (III).
- Write a formula by swapping the valencies, then simplifying.
- Use brackets for an atomic group taken more than once, e.g. $Al_2(SO_4)_3$.

## 1. Alloys and their types




## 2. Why 2 and 3?

Predict: where do the 2 and the 3 come from?

- They are random numbers to remember
- From the masses of the atoms
- From the valencies of chromium (3) and oxygen (2) ✓

The 2 and the 3 come from the valencies: chromium (III) has valency 3 and oxygen has valency 2. Swapping them gives 2 Cr and 3 O, so $Cr_2O_3$.

[ar-EG] الـ 2 والـ 3 جايين من الـ valencies: الـ chromium (III) الـ valency بتاعته 3 والـ oxygen الـ valency بتاعته 2. لما تبدّلهم يبقى 2 Cr و 3 O، يعني $Cr_2O_3$.
## 3. Valency

Valency The combining power of an element: the number of electrons its atom gains, loses or shares in a chemical reaction. Common valencies Some elements have more than one valency. The Roman numeral in the name tells you which: iron (III) has valency 3. An atomic group, like $SO_4$, acts as one unit with its own valency.


## 4. Worked, then your turn

Worked: aluminum oxide Write the symbols: Al and O. Write the valencies: Al = 3, O = 2. Swap them: each valency becomes the other element's number of atoms → $Al_2O_3$. Can 2 and 3 be divided by the same number? No, so $Al_2O_3$ is final. Your turn: manganese (II) sulphate Mn = 2 (from the name), $SO_4$ = 2. Swapping gives $Mn_2(SO_4)_2$. Now finish the last step. Simplified, manganese (II) sulphate is ___.


## 5. On your own

Which is the formula of aluminum sulphate? (Al = 3, $SO_4$ = 2)

- $Al_2SO_{43}$
- $Al_3(SO_4)_2$
- $AlSO_4$
- $Al_2(SO_4)_3$ ✓

Al = 3 and $SO_4$ = 2. Swapping gives 2 Al and 3 $SO_4$ groups; 2 and 3 cannot be divided by the same number, and a group taken more than once goes in brackets: $Al_2(SO_4)_3$.

[ar-EG] Al = 3 و $SO_4$ = 2. التبديل بيدّي 2 Al و 3 groups من $SO_4$؛ والـ 2 والـ 3 مينفعش يتقسموا على نفس الرقم، والـ group اللي بتتاخد أكتر من مرة بتتحط بين أقواس: $Al_2(SO_4)_3$.
## 6. Summary

Valency is the combining power of an element; the Roman numeral in a name gives it, e.g. chromium (III). Write symbols → write valencies → swap → simplify. An atomic group taken more than once goes in brackets: $Al_2(SO_4)_3$. The four steps on the hook's compound, chromium (III) oxide Back to the hook: Cr (3) and O (2) swap to give $Cr_2O_3$. Nothing to simplify. Key words: **valency**, **atomic group**, **swap**, **simplify**. Can you explain why zinc oxide is ZnO and not $Zn_2O_2$?



Chapter: Transition Elements (https://yaani.ai/ar/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/ar/lesson/chem-s3-en.lesson-chem.valency-formulas