Elements are made up of molecules and compounds are made up of atoms. Class 9

Elements are made up of molecules and compounds are made up of atoms. Class 9

Question 1.

Choose the correct statement(s) and correct the false statement(s). Class 9

(i) Elements are made up of molecules and compounds are made up of atoms.

(ii) The molecule of a compound is always made up of two or more atoms of the same kind.

(iii) One molecule of nitrogen gas contains three nitrogen atoms.

(iv) Water is made of two hydrogen atoms, covalently bonded with one oxygen atom.

Answer:

(i) FALSE. Correct statement: Elements are made up of atoms and compounds are made up of molecules (which contain atoms of two or more different elements). For example, iron is an element made up of iron atoms; water is a compound made up of molecules each containing 2 hydrogen atoms and 1 oxygen atom.

(ii) FALSE. Correct statement: The molecule of a compound is made up of two or more atoms of different kinds (different elements). For example, HCl is made of hydrogen and chlorine atoms (different kinds). It is the molecule of an element (like H₂ or O₂) that is made up of atoms of the same kind.

(iii) FALSE. Correct statement: One molecule of nitrogen gas (N₂) contains two nitrogen atoms, not three.

(iv) TRUE. Water (H₂O) is indeed made of two hydrogen atoms covalently bonded with one oxygen atom.

Question 2.

Write the chemical formulae for the following compounds.

(i) Aluminium nitrate

(ii) Calcium oxide

(iii) Ferric oxide

Answer:

(i) Aluminium nitrate: Al³⁺ (valency 3) and NO₃^- (valency 1). Formula: Al(NO₃)₃

(ii) Calcium oxide: Ca²⁺ (valency 2) and O^2- (valency 2). Since valencies are the same, simplifies to: Formula: CaO

(iii) Ferric oxide: Fe³⁺ (ferric, valency 3) and O^2- (valency 2). Formula: Fe₂O₃