The nucleus of an atom has $3$ protons and $4$ neutrons. Calculate the number of valence electrons per atom of this element.

The nucleus of an atom has $3$ protons and $4$ neutrons. Calculate valency.

What are valence electrons? Where are valence electrons situated in an atom?

The atomic number of an element is $16$. Find the number of valence electrons in its atom. Predict the valency of this element.

Naturally occurring boron consists of two isotopes whose atomic masses are $10$ and $11$. The mass of natural boron is $10.80$. Calculate the percentage abundance of each isotope in natural boron.

Copper consists of two isotopes, ${}^{63}\mathrm{Cu}$ (having relative abundance $69\%$) and ${}^{65}\mathrm{Cu}$ (having relative abundance $31\%$). What is the relative atomic mass of copper?

The atomic species $\mathrm{A}$ and $\mathrm{B}$ have different number of protons but the same number of nucleons. On the other hand, the atomic species $\mathrm{X}$ and $\mathrm{Y}$ have the same number of protons but a different number of nucleons. Which pair is an example of isobars? Why?