2.2.3 The Electronic Configuration (Demo)

The Electron Shell and Electronic Configuration

Electrons form layers around the nucleus known as electron shells. The innermost shell, nearest to the nucleus, represents the lowest energy level. As the distance from the nucleus increases, the energy levels of the shells increase.

Click the tabs to learn more…

Electronic configuration of the first 20 elements

The atomic numbernumber of protons in an atom of hydrogen is 1. So, hydrogen is the 1st element in the Periodic Table.

The diagram shows the electronic configuration of a hydrogen atom. A hydrogen atom has 1 electronevery atom has an equal number of protons and electrons so, the electron occupies the 1st shell.

Click the button on the element to learn more about the electronic configuration of the first 20 elements.

The atomic number of aluminium is 13.

Which of the following correctly represents the electronic configuration of aluminium?

Patterns in the Periodic Table

Elements in the Periodic table are arranged into groups and periods. You can use the electronic configuration to identify the group and period of an element.

Click the tabs to learn more.

The period number

The number of outer (last) shell electrons tells you about the group the element belongs to.

Look at the electronic configuration of sodium.

Sodium atom has 3 occupied shells, so sodium belongs to period 3 in the periodic table.

Quick Quiz

Select all the elements that belong to period 3 in the Periodic Table.

The group number

The number of outer (last) shell electrons tells you about the group the element belongs to.

As you can see from the diagram, there is 1 electron in the outer shell of the sodium atom. Therefore, sodium belongs to group I in the Periodic Table.

Quick Quiz

The outer shell electrons

Atoms of elements in the same group have the same number of electrons in their outer shell. Outer shell electrons are also called valence electrons.

Elements with the same number of valence electrons have similar chemical propertiesreact in a similar way

The diagram shows that lithium, sodium, and potassium belong to group I in the Periodic Table. They have the same number of valence electrons and therefore, react in a similar way.

Quick Quiz

Group VIII, the noble gases

The Group VIII elements have a unique electronic configuration. They have a complete set of electrons in their outermost shell.

The diagram below compares the outer shells of fluorine, neon and sodium atoms.

• Neon is unreactive because its atom is stable with a full outer shell of 8 electrons.
• Sodium and fluorine are reactive because the outer shell of their atoms are not full.

Noble gases (helium, neon, and argon) are unreactivedo not react because their atoms have a full outer shell because they cannot gain or lose electrons very easily.

Elements after Calcium 

After the element calcium, which is the 20th element in the Periodic Table, the arrangement of electron shells will be more complex. 

Let’s look at strontium (Sr). Strontium is the 38th element in the Periodic Table, so it has 38 protons and 38 electrons.