Which property of an element can be used for its identification?

An atom with atomic number $9$ and mass number $19$ will have the following constituents:

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?

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?

The atomic numbers of calcium and argon are $20$ and $18$ respectively,but the mass number of both these elements is $40$. What is the name given to such a pair of elements?

$(a)$ 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.