$A, B$ and $C$ are three metals with atomic numbers $4, 11$ and $13$ respectively. Arrange the metals in the increasing order of their valency.

The isotope of cobalt used in the treatment of cancer and iodine used in the treatment of goitre respectively are

State whether the following statement is true or false:
Each isotope of an element is an impure substance.

Give reasons for the following:
$(a)$ Isotopes of an element are chemically similar.
$(b)$ An atom is electrically neutral.
$(c)$ Noble gases show least reactivity.
$(d)$ Nucleus of an atom is heavy and positively charged.
$(e)$ Ions are more stable than atoms.

$(a)$ Write the names of three isotopes of hydrogen.
$(b)$ State one use of an isotope of uranium.
$(c)$ Isotopes of an element have similar chemical properties. Why?

Which of the following statements is always correct?