Which of the following sets of elements show variable valencies?

1. Iron and calcium
2. Copper and sodium
3. Lead and mercury
4. Silver and potassium

Which of the following has zero valency?

1. Hydrogen
2. Chlorine
3. Carbon
4. Neon

Which of the following represents the correct symbol of potassium ion ?

1. K
2. K⁺
3. K²⁺
4. K³⁺

Assertion (A): When an atom gains one or more electrons, it becomes negatively charged.

Reason (R): The number of electrons in an atom exceeds the number of protons upon gaining electrons.

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.