The ion of an element has $3$ positive charges. The mass number of the atom is $27$ and the number of neutrons is $14$. What is the number of electrons in the ion?

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 $50^o$.

Sulphur dioxide $(SO_{2})$ is a colourless,pungent-smelling gas and is a major air pollutant.
$(a)$ Write the electronic configuration of its constituent elements,sulphur and oxygen.
(Given: $_{16}^{32}S, _{8}^{16}O)$
$(b)$ Write the valency of sulphur and oxygen.
$(c)$ Are sulphur and oxygen isotopes of the same element? Explain your answer.

$(a)$ An ion $M^{2+}$ contains $10$ electrons and $12$ neutrons. What is the atomic number and mass number of the element $M$?
$(b)$ Is it possible in an atom to have $12$ protons and $13$ electrons? Explain.
$(c)$ Why is helium gas inert?

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.

Why is the atomic mass of chlorine taken as $35.5\, u$ and not a whole number like $35\, u$ or $36\, u$? Explain.