The nucleus of an atom has $7$ protons and $8$ neutrons. What would be the number of electrons?

The nucleus of an atom has $7$ protons and $8$ neutrons. What would be the number of valence electrons? 

What is the general name of the elements having $8$ electrons in the outermost shells of their atoms? 

The atomic number of an element $\mathrm{X}$ is $15$. Write down the electronic configuration of $\mathrm{X}$. 

The atomic number of an element $\mathrm{X}$ is $15$. What will be the valency of $\mathrm{X}$? 

Name the three subatomic particles present in an atom and compare their properties.

What atom does not contain neutron?