Question

Chemical formula of aluminium sulphate is
1. Al₂SO₄
2. Al3(SO4)₂
3. Al2(SO4)3
4. Al(SO4)3

Answer 1

Correct Answer - Option 3 : Al2(SO4)3

Concept:

Valency of an element

• An atom can have a maximum of 8 electrons in its outermost orbit. 
• To complete octet, elements or ions combine with other elements and form compounds. 
• The number of electrons an atom (or ion) requires to complete 8 electrons in the outermost orbit is called valency. 
• This can happen, by losing or gaining electrons. 
• Metals have either 1, 2 or 3 electrons in outer orbit. To complete octet they lose electrons. 
• Non-metals have 5, 6, 7 electrons and they need to gain electrons. 
• The number of electrons an atom can loose or gain to complete its octet is called its valency.

Formation of compound

• If an element /ion  A has valency x and it can lose x electrons and another element / ion B can gain y electrons then the compound formed will be

A_yB_x

Explanation:

The electronic configuration of aluminum is 2,8, 3. 

It can lose 3 electrons. So it having a valency of 3. 

(SO₄) ^2- have a valency of 2. 

So, the compound formed will be Al2(SO4)3