## Aufbau’s principle: Electronic conﬁguration

Electronic conﬁguration is the arrangement of electrons in an atom, molecule, or other physical structure.

Electrons are arranged in energy levels around the nucleus of an atom. Electrons that are in the energy level that is closest to the nucleus, will have the lowest energy and those further away will have a higher energy.

Each energy level can only hold a certain number of electrons, and an electron will only be found in the second energy level once the ﬁrst energy level is full. The same rule applies for the higher energy levels.

In this tutorial, we are going to look at how to write electronic configuration of electron using Aufbau’s principle but first, you will need to learn the following rules:

• The 1st energy level can hold a maximum of 2 electrons
• The 2nd energy level can hold a maximum of 8 electrons
• The 3rd energy level can hold a maximum of 8 electrons
• If the number of electrons in the atom is greater than 18, they will need to move to the 4th energy level.

In the following examples, the energy levels are shown as concentric circles around the central nucleus.

Lithium Lithium (Li) has an atomic number of 3, meaning that in a neutral atom, the number of electrons will also be 3. The ﬁrst two electrons are found in the ﬁrst energy level, while the third electron is found in the second energy level

Fluorine Fluorine (F) has an atomic number of 9, meaning that a neutral atom also has 9 electrons. The ﬁrst 2 electrons are found in the ﬁrst energy level, while the other 7 are found in the second energy level

There are diﬀerent orbital shapes, and each of these orbital shape have maximum number of electron that they can contain but in this tutorial we will be discussing on two only ( S and P)

s-orbital = 2 electrons

p- orbital = 6 elecrons

d- orbital = 10 electrons

f – orbital = 14 electrons

The ﬁrst energy level contains only one ’s’ orbital, the second energy level contains one ’s’ orbital and three ’p’ orbitals and the third energy level also contains one ’s’ orbital and three ’p’ orbitals.

Within each energy level, the ’s’ orbital is at a lower energy than the ’p’ orbitals. This arrangement is shown in the example figure below.

Aufbau’s Principle

Aufbau comes from the German word “Aufbauen” which means “to build”. In essence when writing electron configurations we are building up electron orbitals as we proceed from atom to atom.

As we write the electron configuration for an atom, we will fill the orbitals in order of increasing atomic number

• Each orbital can only hold two electrons. Electrons that occur together in an orbital are called an electron pair. These electrons spin in opposite directions around their own axes.
• An electron will always try to enter an orbital with the lowest possible energy.
• An electron will occupy an orbital on its own, rather than share an orbital with another electron. An electron would also rather occupy a lower energy orbital with another electron, before occupying a higher energy orbital. In other words, within one energy level, electrons will ﬁll an ’s’ orbital before starting to ﬁll ’p’ orbitals.

An element’s electron conﬁguration can be represented using Aufbau diagrams or energy level diagrams. An Aufbau diagram uses arrows to represent electrons. You can use the following steps to help you to draw an Aufbau diagram

Aufbau’s diagram for fluorine

STEPS

• Determine the number of electrons that the atom has.
• Fill the ’s’ orbital in the ﬁrst energy level (the 1s orbital) with the ﬁrst two electrons.
• Fill the ’s’ orbital in the second energy level (the 2s orbital) with the second two electrons.
• Put one electron in each of the three ’p’ orbitals in the second energy level (the 2p orbitals), and then if there are still electrons remaining, go back and place a second electron in each of the 2p orbitals to complete the electron pairs.
• Carry on in this way through each of the successive energy levels until all the electrons have been drawn.

NOTE : When there are two electrons in an orbital, the electrons are called an electron pair. If the orbital only has one electron, this electron is said to be an unpaired electron. Electron pairs are shown with arrows in opposite directions. This is because when two electrons occupy the same orbital, they spin in opposite directions on their axis.