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

State whether the following statement is true or false:
In water,the ratio of the mass of hydrogen to the mass of oxygen is always $1:8$,regardless of the source of water.

Write two differences between isobars and isotopes.

$(a)$ Define the following terms with one example each:
$(i)$ Isotope
$(ii)$ Isobar
$(b)$ Name the elements whose isotopes are used in:
$(i)$ Nuclear Reactor
$(ii)$ Treatment of cancer

Which of the following statements about Rutherford's model of the atom are correct?
$(i)$ Considered the nucleus as positively charged.
$(ii)$ Established that the $\alpha$-particles are four times as heavy as a hydrogen atom.
$(iii)$ Can be compared to the solar system.
$(iv)$ Was in agreement with Thomson's model.

In the Gold foil experiment of Geiger and Marsden,which paved the way for Rutherford's model of an atom,$\sim 1.00\%$ of the $\alpha$-particles were found to deflect at angles $> 50^o$. If one mole of $\alpha$-particles were bombarded on the gold foil,compute the number of $\alpha$-particles that would deflect at angles less than $50^o$.