Calculate the number of neutrons present in the nucleus of an element $X$ which is represented as $^{31}_{15}X$.

An atom with $3$ protons and $4$ neutrons will have a valency of

In response to a question,a student stated that in an atom,the number of protons is greater than the number of neutrons,which in turn is greater than the number of electrons. Do you agree with the statement? Justify your answer.

Two elements are denoted as follows:
$_{20}^{40}A$ and $_{18}^{40}B$
$(a)$ Find the number of electrons present in $A$ and $B$.
$(b)$ Find the number of nucleons present in $A$ and $B$.
$(c)$ Explain the special 'term' used to represent $A$ and $B$ and define the same.

Dalton's atomic theory successfully explained which of the following laws?
$(i)$ Law of conservation of mass
$(ii)$ Law of constant composition
$(iii)$ Law of radioactivity
$(iv)$ Law of multiple proportion

Why did Rutherford select a gold foil in his $\alpha$-ray scattering experiment?