Electron Configuration


Recall: In atoms, we learnt that electrons can be considered as “orbiting” the nucleus in various energy levels or shells. We will now take a look at how the electrons are distributed among the various energy levels or shells. (i.e. electron configuration)

There are some properties of the energy levels:

  • As the energy level increases, the energy possessed by an electron in said energy level increases. (I.e. An electron in third energy level has more energy than an electron in first energy level)
  • Each energy level can hold a maximum number of electrons
  • First energy level can hold a maximum of 2 electrons
  • Second energy level can hold a maximum of 8 electrons
  • Third energy level can hold a maximum of 8 electrons
  • Each energy level must be filled in order of increasing energy. (I.e. The first energy level is filled first before going to subsequent higher levels.)
  • The outermost shell is the highest energy level that is occupied with electrons
  • Electrons in outermost shell are called valence electrons

 

Example to familiarise you with the electron configuration

Helium has a proton number of 2. This means that:

  • Number of electrons: 2
  • 2 electrons in first shell
  • Electron configuration for helium is 2
  • Valence electrons: 2

Carbon has a proton number of 6. This means that:

  • Number of electrons: 6
  • 2 electrons in first shell
  • 4 electrons in second shell
  • Electron configuration for carbon is 2.4
  • Valence electrons: 4

Note: The fullstop is not a decimal point. It is just a way to separate the numbers.

Aluminium has a proton number of 13. This means that:

  • Number of electrons: 13
  • 2 electrons in first shell
  • 8 electrons in second shell
  • 3 electrons in third shell
  • Electron configuration for aluminium is 2.8.3
  • Valence electrons: 3

 

Electron configuration table for first twenty elements

You can practice writing out the electron configurations for the first twenty elements and check them with this table

ElementSymbolProton No.Electron Config.
HydrogenH11
HeliumHe22
LithiumLi32.1
BerylliumBe42.2
BoronB52.3
CarbonC62.4
NitrogenN72.5
OxygenO82.6
FluorineF92.7
NeonNe102.8
SodiumNa112.8.1
MagnesiumMg122.8.2
AluminiumAl132.8.3
SiliconSi142.8.4
PhosphorusP152.8.5
SulphurS162.8.6
ChlorineCl172.8.7
ArgonAr182.8.8
PotassiumK192.8.8.1
CalciumCa202.8.8.2

 

When the outermost electron shell is completely filled (i.e. 2 for first energy level, 8 for second energy level and so on), the atom would have a stable noble gas configuration or a stable octet structure.

  • Elements with stable noble gas configuration are referred to as inert. They have very low chemical reactivities.
  • Atoms of other elements strive to achieve the noble gas configuration by undergoing chemical reactions to form chemical bonds with other atoms/elements.

These chemical bonds that we are interested in:

  • Ionic Bonds
  • Covalent Bonds
  • Metallic Bonds

 


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