Explain with examples: Atomic number.

Explain with examples: Isotopes. Give any two uses of isotopes. 

Explain with examples: Isobars.

The mass number of an element is $40$. It contains $19$ electrons. What is the number of protons and neutrons in it? What is the atomic number of the element? 

If both $\mathrm{K}$ and $\mathrm{L}$ shells of an atom are full, what is the total number of electrons in the atom? 

If bromine atom is available in the form of two isotopes  ${}_{35}{}^{79}\mathrm{Br}\left(49\%\right) \mathrm{and} {}_{35}{}^{81}\mathrm{Br}\left(51\%\right)$. Calculate the average atomic mass of the bromine atom.