Valency and writing chemical formulas

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

1. Alloys and their types

2. Why 2 and 3?

Chapter 1 writes chromium (III) oxide as

Cr₂O₃ C r_{2} O_{3}

. Why 2 and 3, and not CrO?

Pick the right answer

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

A. From the valencies of chromium (3) and oxygen (2)
B. They are random numbers to remember
C. From the masses of the atoms

Explanation

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

Cr₂O₃ C r_{2} O_{3}

3. Valency

Valency

The combining power of an element: the number of electrons its atom gains, loses or shares in a chemical reaction.

Valency	Elements	Atomic groups
1	H, Na, K, Cl	$OH O H$ , $NO₃ N O_{3}$ , $NH₄ N H_{4}$
2	O, Mg, Ca, Zn	$SO₄ S O_{4}$ , $CO₃ C O_{3}$
3	Al	$PO₄ P O_{4}$

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

SO₄ S O_{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 → $Al₂O₃ A l_{2} O_{3}$ .
Can 2 and 3 be divided by the same number? No, so $Al₂O₃ A l_{2} O_{3}$ is final.

Your turn: manganese (II) sulphate

Mn = 2 (from the name),

SO₄ S O_{4}

= 2. Swapping gives

Mn₂(SO₄)₂ M n_{2} (S O_{4})_{2}

. Now finish the last step.

Fill in the blanks

Simplified, manganese (II) sulphate is _____.

Explanation

2 and 2 both divide by 2: one Mn and one

SO₄ S O_{4}

, so

MnSO₄ M n S O_{4}

Need a hint?

Divide both numbers by 2.
When the group is taken once, you don't need brackets.

5. On your own

Fill in the blanks

Iron (III) chloride: _____ . Zinc oxide: _____ .

Explanation

Fe (3) and Cl (1):

FeCl₃ F e C l_{3}

. Zn (2) and O (2):

Zn₂O₂ Z n_{2} O_{2}

simplifies to

ZnO Z n O

Pick the right answer

Which is the formula of aluminum sulphate? (Al = 3,

SO₄ S O_{4}

= 2)

A. $Al₂(SO₄)₃ A l_{2} (S O_{4})_{3}$
B. $Al₂SO₄₃ A l_{2} S O_{43}$
C. $Al₃(SO₄)₂ A l_{3} (S O_{4})_{2}$
D. $AlSO₄ A l S O_{4}$

Explanation

Al = 3 and

SO₄ S O_{4}

= 2. Swapping gives 2 Al and 3

SO₄ S O_{4}

groups; 2 and 3 cannot be divided by the same number, and a group taken more than once goes in brackets:

Al₂(SO₄)₃ A l_{2} (S O_{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: $Al₂(SO₄)₃ A l_{2} (S O_{4})_{3}$ .

Symbols: Cr and O → Valencies: Cr = 3, O = 2
Valencies: Cr = 3, O = 2 → Swap: 2 Cr, 3 O
Swap: 2 Cr, 3 O → Simplify: 2 and 3 cannot be divided by the same number

The four steps on the hook's compound, chromium (III) oxide

Back to the hook: Cr (3) and O (2) swap to give

Cr₂O₃ C r_{2} O_{3}

. Nothing to simplify.

Can you explain why zinc oxide is ZnO and not

Zn₂O₂ Z n_{2} O_{2}

Concepts in this lesson