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 $u$)?

State three rules suggested by Bohr and Bury for the distribution of electrons into different orbits (shells) of an atom.

State three features of the nuclear model of an atom put forward by Rutherford.

Which of the following statements are true for an element?
$(i)$ Atomic number $=$ number of protons $+$ number of electrons
$(ii)$ Mass number $=$ number of protons $+$ number of neutrons
$(iii)$ Atomic mass $=$ number of protons $=$ number of neutrons
$(iv)$ Atomic number $=$ number of protons $=$ number of electrons

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?

Give one point of similarity and one point of difference between isotopes of ${ }_{6}^{14} C$ and ${ }_{6}^{12} C$.