Assertion (A): Magnesium burns in oxygen to produce a compound magnesium oxide with the molecular formula MgO.

Reason (R): Magnesium oxide is a white amorphous solid.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Assertion (A): The valency of inert gases, helium, neon and argon is zero.

Reason (R): The outermost shell of inert gases is complete.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Fill in the blanks:

(a) A symbol represents …………… of an element.

(b) An atom is neither created nor destroyed during a …………… .

(c) …………… are positively charged ions.

(d) Total number of atoms of all the elements present in Al₂O₃ is …………… .

(e) A …………… is the symbolic representation of a chemical reaction using symbols and formulae of the substances taking part in the reaction.

Write 'true' or 'false' for the following and correct the false statements.

(a) When an atom gains one or more electrons, it becomes positively charged.

(b) The valency of nitrogen in ammonia gas is 3.

(c) The sulphide ion is a polyatomic ion.

(d) The nitrate ion is represented as NO₂^-.

(e) The element cobalt is represented as CO.