An element $X$ has a mass number $4$ and atomic number $2$. What is the valency of this element?

What is the valency of $(a)$ aluminium in aluminium sulphate,and $(b)$ sodium in sodium sulphate?

Atomic models have been improved over the years. Arrange the following atomic models in their chronological order:
$(i)$ Rutherford's atomic model
$(ii)$ Thomson's atomic model
$(iii)$ Bohr's atomic model

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 the atom is empty
Which of the above statements are correct?

Which of the following in the figure do not represent Bohr's model of an atom correctly?

Given that a natural sample of iron $(Fe)$ has isotopes $_{26}^{54}Fe$,$_{26}^{56}Fe$,and $_{26}^{57}Fe$ in the ratios of $5\%$,$90\%$,and $5\%$ respectively. What will be the average atomic mass of iron $(Fe)$ (in $u$)?