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.
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The 1st shell can hold up to 2 electrons.
The 2nd shell can hold up to 8 electrons.
The 3rd shell can hold up to 18 electrons. However, for the first 20 elements, (from hydrogen to calcium), the 3rd can hold only 8 electrons.
Electronic configuration of the first 20 elements
The of hydrogen is 1. So, hydrogen is the 1st element in the Periodic Table.
The diagram shows the electronic configuration of a hydrogen atom. 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.Quick Quiz
The atomic number of aluminium is 13.
Which of the following correctly represents the electronic configuration of aluminium?
Patterns in the Periodic TableRecall
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.
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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 because they cannot gain or lose electrons very easily.Quick Quiz
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.