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

Give reasons for the following:
$(a)$ Isotopes of an element are chemically similar.
$(b)$ An atom is electrically neutral.
$(c)$ Noble gases show least reactivity.
$(d)$ Nucleus of an atom is heavy and positively charged.
$(e)$ Ions are more stable than atoms.

An atom of an element has $2$ electrons in the $M$ shell, i.e., the third shell. What will be the atomic number of this element? Name this element. Find the valency of this element. Also, find the number of neutrons in the atom of this element (assume mass number is $24$).

Rutherford's $\alpha-$ particle scattering experiment showed that
$(i)$ Electrons have negative charge
$(ii)$ The mass and positive charge of the atom is concentrated in the nucleus
$(iii)$ Neutron exists in the nucleus
$(iv)$ Most of the space in atom is empty
Which of the above statements are correct?

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?

$(a)$ Explain why did Rutherford select a gold foil in his alpha scattering experiments?
$(b)$ What observations in $\alpha$-scattering experiment led Rutherford to make the following conclusions?
$(i)$ Most of the space in an atom is empty.
$(ii)$ Nucleus is positively charged.
$(c)$ Mention any two drawbacks of Rutherford's model.