State whether the following statement is true or false:
One atomic mass unit is a mass unit equal to exactly one-twelfth $(1/12)$ the mass of one atom of carbon-$12$.

Elements with valency $1$ are

State whether the following statement is true or false:
In water,the ratio of the mass of hydrogen to the mass of oxygen is always $1:8$,regardless of the source of water.

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

Give a brief account of the development of the presently accepted model of the atom with the contributions made by different scientists. Write two features of this atomic model.

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 $u$)?