State and explain the Aufbau principle. Write electronic configuration of $\mathrm{C},\mathrm{Cr},\mathrm{Fe},\mathrm{Cu}$.

Aufbau principle:

• Aufbau Principle states that in the ground state of an atom, the electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels i.e. orbitals are filled in the increasing order of their energies.
• Or, the Aufbau principle states that the accessible atomic orbitals with the lowest energy levels are occupied first, followed by those with higher energy levels.
• The graphic below depicts the sequence in which atomic orbitals are filled.
• The primary quantum number is denoted by '$\mathrm{n}$' while the azimuthal quantum number is denoted bxy '$\mathrm{l}$'

• The $(\mathrm{n}+\mathrm{l})$ rule may be used to identify the sequence in which the energy of orbitals grows, where the sum of the main and azimuthal quantum numbers defines the energy level of the orbital.
• Lower $(\mathrm{n}+\mathrm{l})$ values indicate lower orbital energy. If two orbitals have equivalent $(\mathrm{n}+\mathrm{l})$ values, the orbital with the lower $\mathrm{n}$ value is said to have lower energy.
• The orbitals are filled with electrons in the following order: $1\mathrm{s},2\mathrm{s},2\mathrm{p},3\mathrm{s},3\mathrm{p},4\mathrm{s},3\mathrm{d},4\mathrm{p},5\mathrm{s},4\mathrm{d},5\mathrm{p},6\mathrm{s},4\mathrm{f},5\mathrm{d},6\mathrm{p},7\mathrm{s},5\mathrm{f},6\mathrm{d},7\mathrm{p}..$ and so on.

Electronic Configurations according to Aufbau Principle:

• Electronic Configuration of Carbon ($\mathrm{C}$) according to Aufbau rule:

$\mathrm{C}(\mathrm{Z}=6)=1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^2$

• Electronic Configuration of Chromium ($\mathrm{Cr}$) according to Aufbau rule:

$\mathrm{Cr}(\mathrm{Z}=24)=\left[\mathrm{Ar}\right]3{\mathrm{d}}^{5}4{\mathrm{s}}^1$

• Electronic Configuration of Zinc ($\mathrm{Zn}$) according to Aufbau rule:

$\mathrm{Fe}(\mathrm{Z}=26)=\left[\mathrm{Ar}\right]3{\mathrm{d}}^{6}4{\mathrm{s}}^2$

• Electronic Configuration of Copper ($\mathrm{Cu}$) according to Aufbau rule:

$\mathrm{Cu}(\mathrm{Z}=29)=\left[\mathrm{Ar}\right]3{\mathrm{d}}^{10}4{\mathrm{s}}^1$