Draw a sketch of Bohr's model of an atom with three shells. 

Write the distribution of electrons in oxygen and sodium atoms. 

An atom has $2$ and $7$ electrons in the K and L shells respectively, then what would be the atomic number of the element? Name the element. 

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

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

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 $\mathrm{X}$.