The correct electronic configuration of sodium is:

1. 2, 8, 1
2. 1, 2, 8
3. 8, 2, 1
4. 8, 1, 2

Assertion (A): Matter consists of very small particles called atoms.

Reason (R): Atoms are indivisible.

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 nucleus of an atom is a centrally located mass containing protons only.

Reason (R): The nucleus of an atom is positively charged.

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 sum of the number of protons and neutrons present in the nucleus of an atom of an element is called its mass number.

Reason (R): The number of protons in the nucleus of an atom of an element is called its atomic number.

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.