U1-2-7 - Valency and Formulae

Question 1

Valency

Answer 1

The no. of bonds an atom can make

Question 2

What is the valency of elements in groups 1–4?

Answer 2

1, 2, 3, 4 – same as the group number

(e.g. Ca is in group 2, valency = 2)

Question 3

What is the valency of elements in groups 5–8?

Answer 3

3, 2, 1, 0 – same as the no. of electrons needed to get a full outer shell

(e.g. Cl is in group 7, valency = 1)

Question 4

What does the (III) mean in iron(III) oxide?

Answer 4

Valency of iron is 3

(III = roman numeral 3)

Question 5

The valencies of two elements are shown.

Fe N

2 3

What do you do to get the formula for a compound of Fe and N?

Answer 5

We must cross over the valencies:

Fe₃N₂

Question 6

The valencies of two elements were crossed over, giving the following formula:

Mn₂O₄

What must be done next?

Answer 6

We must simplify the formula (to the simplest whole number ratio, so 2 : 4 = 1 : 2)

MnO₂

Question 7

The charge number of an ion is equal to the element’s …

Answer 7

valency.

(e.g. O has valency 2, and charge 2–)