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

$(i)$ Write the postulates of the Bohr model of an atom.
$(ii)$ Draw a sketch of the Bohr model of an atom with atomic number $15$.

Write five applications of isotopes in various fields.

Elements with valency $1$ are

Name any two noble gases.

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?