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?

$(a)$ Both helium and beryllium have two electrons in their valence shells. Helium is a noble gas whereas beryllium is a metal. Justify.
$(b)$ Hydrogen exists in three isotopic forms. Why are the isotopes of hydrogen chemically alike?

Identify the most stable atom from the following. Also give a reason for your answer.
$Na_{11}^{23}, Cl_{17}^{35}, Al_{13}^{27}, Ar_{18}^{40}$

If an ion $M^{3+}$ contains $10$ electrons and $14$ neutrons, what are the atomic number and mass number of the element $M$?

What information do you get from the figure about the atomic number,mass number,and valency of atoms $X$,$Y$,and $Z$? Give your answer in a tabular form.

Rutherford designed an experiment to determine the structure of an atom. Answer the following in relation to this experiment:
$(a)$ Name the element he selected for this purpose.
$(b)$ Which subatomic particle was used by him?
$(c)$ What was the drawback of Rutherford's model of the atom?