Identify the four quantum numbers of 19^th electron of an element having atomic number 24. [2]

The element with atomic number 24 is chromium (Cr). Its electronic configuration is written as: $$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{1}3d^5$$

The 19th electron enters the 3d subshell (since the 4s subshell fills after 3p). The four quantum numbers for this electron are:

• Principal quantum number (n): 3 (since it is in the 3rd shell)
• Azimuthal quantum number (l): 2 (for the d subshell)
• Magnetic quantum number (m_l): $-2,-1,0,+1,+2$ (any one of these five values is acceptable; here we choose m_l = 0 for simplicity)
• Spin quantum number (m_s): $+1/2$ or $-1/2$ (we choose +1/2)

Thus, the four quantum numbers for the 19th electron are: $n=3,l=2,m_l=0,m_s=+\frac{1}{2}$.