The electronic structure of an atom describes the arrangement of electrons in its energy levels, which orbit the nucleus. In a neutral atom, the number of electrons is equal to the number of protons. So, the atomic number also tells us the number of electrons in an atom.
Electrons occupy specific energy levels, also known as electron shells. Each energy level has a maximum number of electrons it can hold.
Electrons occupy energy levels in a specific order. They occupy the lowest available energy level first, which is the level closest to the nucleus. When the lowest energy level is full, the electrons will then occupy the next energy level, and so on.
For an atom to be considered stable, it must have a full outer energy level. If the outer energy level is not full, then the atom will be unstable and will likely react with another atom to gain or lose electrons until it becomes stable. For example:
Many single atoms, like sodium, are unstable and tend to react to form molecules or compounds. However, there are exceptions, such as the group 0 elements, which have a full outer shell. The group 0 elements can be found in the far right column of the periodic table. They are stable, so they don’t react with anything.
You could be asked to work out the electronic structure of the first 20 elements. Let’s work out the electronic structure of chlorine.
1. First, find the element on the periodic table.
2. Determine the atomic number of the element, which tells you how many electrons the element has.
3. Fill up the electron shells with electrons.
To fill up the electron shells, you can start with the first energy level and move to the next one once it is full. Here’s the process for filling the electron shells of chlorine:
With this process, you can predict the electronic structure of any element. Chlorine has 17 electrons arranged in the following manner: 2 electrons in the first shell 8 electrons in the second shell 7 electrons in the third shell
So, we get:
We can also write the electronic structure using numbers to represent the number of electrons in each energy level. For example, the electronic structure of chlorine can be written as 2,8,7. Each comma separates one energy level from the next.
The electronic structure of an atom can also tell us its position in the periodic table. The number of electrons in the outer energy level of an atom tells us the group number, while the number of energy levels tells us the period number. For example: