Electron Configuration (Oxford AQA International A Level Chemistry)

Electron Configurations of Atoms & Ions

Energy Levels

• The arrangement of electrons in an atom is called the electronic configuration

• Electrons are arranged around the nucleus in energy levels (shells)

• There are four main energy levels: 1,2, 3 and 4

• Principal quantum numbers (n) are used to number these energy levels

  • The lower the principal quantum number, the closer the shell is to the nucleus

    • E.g. The first shell which is the closest to the nucleus is n = 1

  • The higher the principal quantum number, the greater the energy of the shell and the further away from the nucleus

• Each shell has a fixed number of electrons it can hold:

  • n = 1 : up to 2 electrons

  • n = 2 : up to 8 electrons

  • n = 3 : up to 18 electrons

  • n = 4 : up to 32 electrons

• The energy levels get closer together as you move further away from the nucleus with an overlap between the third and fourth energy levels

Atomic Orbitals

• The main energy levels (or shells) are split into sub-shells

• Sub-shells consist of one or more atomic orbitals

  • These are called s, p, and d

    • Elements with more than 57 electrons also have an f shell

    • The energy of the electrons in the sub-shells increases in the order s < p < d < f

• These orbitals represent a volume of space in which there is a high probability of finding an electron

• Each shell contains a different numbers of orbitals:

  • 1st energy level

    • Has 1 s- orbital

  • 2nd energy level

    • Has 1 s-orbital

    • Has 3 p- orbitals

  • 3rd energy level

    • Has 1 s-orbital

    • Has 3 p- orbitals

    • Has 5 d-orbitals

• Each atomic orbital can be occupied by a maximum of two electrons

• The orbitals have specific 3-D shapes

Summary table of electrons in shells and sub-shells

s orbital shape

• The s orbitals are spherical

• The size of the s orbitals increases with increasing shell number

  • E.g. the s orbital of the third shell (n = 3) is bigger than the s orbital of the first shell (n = 1)

p orbital shape

• The p orbitals have a dumbbell shape

• Every shell has three p orbitals except for the first one (n = 1)

• The p orbitals occupy the x, y and z axes and point at right angles to each other, so are oriented perpendicular to one another

• The lobes of the p orbitals become larger and longer with increasing shell number

An overview of the shells, subshells and orbitals in an atom

Filling electron orbitals

• There are some rules to remember when assigning electrons to atomic orbitals:

  • Electrons fill atomic orbitals of lower energy first

  • Each orbital can hold a maximum of 2 electrons

  • Atomic orbitals with the same energy, such as the 2p orbital, fill singly before occupying shells in pairs

• Remember, there is an overlap between the third and fourth energy levels

  • The 4s orbital is of lower energy than the 3d so it will be filled first

• We can show the atomic orbitals being filled and deduce the electronic configuration for an element using the box notation

  • Each box represents an atomic orbital

  • The boxes are arranged in order of increasing energy from bottom to top

  • The electrons are represented by opposite arrows to show the spin of the electrons

  • Some examples of box notations are below:

Nitrogen

Sodium

Chlorine

Titanium

Writing electronic configurations

• We can write electronic configurations using the notation below:

• This tells us how the electrons in an atom or ion are arranged in their shells, sub-shells and orbitals

Hydrogen

• Hydrogen has 1 single electron

  • The electron is in the s orbital of the first shell

  • Its electron configuration is 1s¹

Potassium

• Potassium has 19 electrons

  • The first 2 electrons fill the s orbital of the first shell, 1s²

  • They then continue to fill subsequent orbitals and sub-shells in order of increasing energy

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

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

  • To simplify this electronic structure, we can use the shorthand electronic configuration

    • The nearest preceding noble gas to potassium is argon

    • This accounts for 18 electrons of the 19 electrons that potassium has

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

• Elements can be classified as an s-block element, p-block element and so on, based on the position of the outermost electron:

  • s block elements

    • Have their outer electron(s) in an s orbital

  • p block elements

    • Have their outer electron(s) in a p orbital

  • d block elements

    • Have their outer electron(s) in a d orbital

  • f block elements

    • Have their outer electron(s) in an f orbital