Electron Configuration (AQA AS Chemistry): Revision Note

Deducing the Electron Configuration

• The Periodic Table is divided into four main blocks based on electron configuration

• Elements are classified as s-, p-, d-, or f-block elements according to the type of orbital that contains their outermost (valence) electron

  • s-block elements

    • These have their valence electron(s) in an s orbital

  • p-block elements

    • These have their valence electron(s) in a p orbital

  • d-block elements

    • These have their valence electron(s) in a d orbital

  • f-block elements

    • These have their valence electron(s) in an f orbital

• Principal quantum shells increase in energy as the principal quantum number increases.

  • For example, an electron in the n = 4 shell is higher in energy than one in the n = 2 shell

• Within a given shell, subshells increase in energy in the order: s < p < d < f

  • The main exception to this rule is that the 3d orbital is slightly higher in energy than the 4s orbital

  • As a result, the 4s orbital is filled before the 3d orbital

• All orbitals within the same subshell have the same energy and are described as degenerate.

  • For example, the three p-orbitals (sometimes called px, py, and pz orbitals) all have equal energy

• Electron configuration shows how electrons are arranged in the shells, subshells, and orbitals of an atom

• The subshells are filled in order of increasing energy

• Writing out the electron configuration shows how electrons in an atom or ion are arranged in their shells, subshells, and orbitals

• This can be written as either a full electron configuration or a shorthand electron configuration

  • The full electron configuration shows the arrangement of all electrons starting from the 1s subshell

  • The shorthand electron configuration uses the symbol of the nearest preceding noble gas to represent the electrons in filled inner shells

• Ions are formed when atoms lose or gain electrons

  • Negative ions are formed by adding electrons to the outer subshell, while positive ions are formed by removing electrons from the outer subshell

  • For transition metals, the 4s subshell fills before the 3d subshell

  • However, when ions are formed, electrons are lost from the 4s subshell first rather than the 3d subshell, because the 4s subshell is higher in energy once the 3d subshell contains electrons

Full Electron Configurations

• Hydrogen has one electron, which is found in the s orbital of the first shell

  • Its electron configuration is 1s¹

• Potassium has 19 electrons

  • The first two electrons fill the s orbital of the first shell, and electrons then fill subsequent orbitals and subshells in order of increasing energy

  • The 4s orbital is lower in energy than the 3d subshell, so it is filled first

  • The full electron configuration of potassium is: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹

Shorthand Electron Configurations

• Using potassium as an example, the nearest preceding noble gas is argon

  • This accounts for 18 of potassium’s 19 electrons.

  • The shorthand electron configuration of potassium is therefore:[Ar] 4s¹

Exceptions

• Chromium and copper have the following electron configurations, which are different from what you may expect:

  • Cr is [Ar] 3d⁵ 4s¹ not [Ar] 3d⁴ 4s²

  • Cu is [Ar] 3d¹⁰ 4s¹ not [Ar] 3d⁹ 4s²

• This is because the [Ar] 3d⁵ 4s¹ and [Ar] 3d¹⁰ 4s¹ configurations are energetically stable

Presenting the Electron Configuration

• Electrons can be described as having a property called spin, which can be represented as either “up” or “down”

• Electrons with the same spin repel each other

  • This is known as spin-pair repulsion

• To minimise this repulsion, electrons occupy separate orbitals within the same subshell before pairing up, and they do so with the same spin direction

  • For example, if there are three electrons in a p subshell, one electron will occupy each of the three p orbitals

• Electrons only pair up when no empty orbitals are remaining within a subshell

• When pairing occurs, the two electrons have opposite spins to minimise repulsion

  • For example, if there are four electrons in a p subshell, one p orbital contains two electrons with opposite spins, while the other two p orbitals each contain one electron

  • The first three electrons occupy the empty p orbitals one at a time, and the fourth electron pairs up in one of the p orbitals.

Box Notation

• Electron configuration can be represented using electron-in-boxes notation, also known as an orbital diagram

• Each box represents an atomic orbital, and the boxes are arranged in order of increasing energy from bottom to top

• Electrons are shown as arrows, with opposite directions representing opposite electron spins

  • For example, the box notation for titanium is shown below

  • Note that the 3d subshell in titanium is neither half-full nor full, so the second 4s electron is not promoted to the 3d subshell and remains in the 4s orbital