Aufbau Principle

State Aufbau Principle

The Aufbau principle states that in an atom’s or ion’s ground state, electrons start filling the atomic orbitals from the lowest free energy levels before they occupy higher energy levels. Below we have discussed a few examples which include the hydrogen and helium atom.

Rules of Aufbau Principle

Some rules outlined by the Aufbau principle are used in determining the organization of electrons into various subshells and shells surrounding the atomic nucleus.

  • The electrons occupy the subshell which has lowest possible energy.
  • An orbital holds a maximum of 2 electrons in accordance with the Pauli Exclusion Principle.
  • Electrons should obey Hund’s rule.

Aufbau Principle Expectations

Completely filled and Half-filled f and d subshells add up stability to atoms therefore, the f and d block elements do not follow the principle always. For instance, the anticipated Aufbau configuration for Chromium is 4s23d4, but the perceived configuration is 4s13d5. This results in the reduction of electron-electron repulsion inside the atom because every electron in its atom occupies own seat in the subshell.

Aufbau and the Elements of Period 1:

Hydrogen – The simplest element with only one electron.

As per the Aufbau principle, the hydrogen atom’s electron is placed in the lowest free energy level, which is 1s. It is the ground state of hydrogen and is written as 1s1.

It is one of the unique elements as its atom has only one electron. Therefore, its energy levels are also unique. There is no repulsion between electrons hence the energy is equal in its sublevels. For instance,

  • The energy in 2s and 2p is equal
  • The energy in 3s, 3p, and 3d is equal.

Helium- The two electrons of this element fill the 1s orbital.

Spin pairing takes place in helium. In spin pairing, one electron is spin-up and the other spin-down in obedience to the Pauli Exclusion Principle.

The quantum numbers of helium atom’s electrons are given below:

n = 1, l = 0, ml = 0, ms = +½

The other electron’s quantum numbers:

n = 1, l = 0, ml = 0, ms = −½

There are some limitations to the Aufbau principle. One among them is that this principle is not useful in predicting the electron configuration when atoms are ionized.

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