Name the particles which determine the mass of an atom.

The average atomic mass of a sample of an element $y$ is $35.5 \,u$. What are the percentages of isotopes $_{17}^{37}y$ and $_{17}^{35}y$ in the sample?

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

$(a)$ Name the three subatomic particles of an atom.
$(b)$ An atom of an element has $5$ electrons in the $L$ shell.
$(i)$ What is the atomic number of the element?
$(ii)$ State its valency.
$(iii)$ Identify the element and write its name.

Which of the $Na^+$ and $He$ has completely filled $K$ and $L$ shells? Give a reason to support your answer.

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.