Assertion (A): Lithium, sodium and potassium form a Dobereiner's triad.

Reason (R): The atomic mass of the middle element is the average of the masses of the other two elements.

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): Chlorine with atomic number 17 is placed in group 17 and period 3.

Reason (R): The number of shells signifies a period, while the number of valence electrons indicates the group.

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): Hydrogen can be placed in two groups i.e., with Alkali metals and with halogens.

Reason (R): Hydrogen has two electrons in its valence shell.

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): Cobalt (atomic mass 58.94) is placed before Nickel (atomic mass 58.68) in Mendeleev's periodic table.

Reason (R): Physical and chemical properties of the elements are a periodic function of their atomic masses.

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.