An atom with atomic number $9$ and mass number $19$ will have the following constituents:

Which will be chemically more reactive: element $A$ with atomic number $17$ or element $B$ with atomic number $16$? Give a reason as well.

State whether the following statement is true or false:
Each isotope of an element is an impure substance.

In a sample of ethyl ethanoate $(CH_3COOC_2H_5)$,the two oxygen atoms have the same number of electrons but different number of neutrons. Which of the following is the correct reason for it?

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 $u$)?

The nucleus of a hydrogen atom is known as: