Structure of Atom

Hint: Number of orbitals in n^th shell= n²

Explanation: 

Number of orbitals in the n^th shell = n²

For n = 4

Number of orbitals = 16

If each orbital is taken fully, then it will have 1 electron with m_s value of .
The number of electrons with m_s value of $\left(–\frac{1}{2}\right)$ is 16.