Chapter 3: Can You Recall? (30)

1. What are the types of molecules of elements and compounds?

→ Molecules of elements are homoatomic (i.e. they are made up of only one type of atom), whereas molecules of compounds are heteroatomic (i.e. they are made up of more than one type of atoms).
These molecules can be monoatomic (in case of elements), diatomic, triatomic, tetratomic and so on.

	Elements	Compounds
Monoatomic	Na	---
Diatomic	O₂	CO
Triatomic	N₃	CO₂
Tetratomic	P₄	NH₃
Polyatomic	S₈	Ca(OH)₂

2 Marks

2. What is meant by valency of elements?

→ The capacity of an atom to combine is called its valency. It is the number of chemical bonds formed by one atom of an element with other atoms.
1 Mark

3. What is the requirement for writing molecular formulae of different compounds? How are the molecular formulae of the compounds written?

→ In order to write the molecular formulae of different compounds, we need to know the symbols of the radicals and their valencies.
The steps of writing the molecular formula of a compound are as follows:

Write the symbols of radicals with basic (positive) radical on the left and acidic (negative) radical on the right.
Write the valencies below the respective radicals.
Cross multiply the valencies and cancel out the common factors to get them in the simplest form.
Indicate the number of radicals by using appropriate subscripts in the molecular formula.

3 Marks