Chemistry Foundations

3.5.5 - How the Electronic Configurations of Atoms Relate to the Periodic Table

How the Electronic Configurations of Atoms Relate to the Periodic Table

It was mentioned on the previous page that for atoms of the first twenty elements, there is a faster way of working out the electronic configurations using the periodic table, which doesn't require us to start with the total number of electrons and then assign them to shells.

This faster method uses the fact that for the first twenty elements there is a very simple link between an element's position in the periodic table and the electronic configuration of its atoms.

Note that the information on this page applies only to atoms - not ions.

The periods represent the shells

If you count the number of elements in each of the first four periods, you will find that there are 2 elements in the first period, 8 elements in the second period, 8 elements in the third period and 18 elements in the fourth period.

These numbers - 2,8,8,18 - are the same as the capacities of the first four shells.

The first four periods of the periodic table. There are 2 elements in the first period, 8 in the second, 8 in the third, and 18 in the fourth. These numbers are the same as the capacities of the first four shells.

The reason for this is that the periods represent the shells.

We can understand how this works by examining the electronic configurations of atoms of the elements. The periodic table below shows the electronic configurations of atoms of the elements in the first three periods:

The electronic configurations of the atoms of the elements in the first three periods of the periodic table.

In the first period we have hydrogen (H) and helium (He). A hydrogen atom has one electron in the first shell. A helium atom has two electrons in the first shell.

A helium atom's first shell is full (since the first shell has a capacity of 2). Therefore when we go to an atom of the next element, lithium (Li), we have to put the new electron in the second shell. This gives a lithium atom an electronic configuration of 2,1.

Notice how we start a new period when we move from helium to lithium. This is because a new shell is added. In other words, every time we reach the capacity of the current shell and need to move to the next shell, we go on to a new period of the periodic table.

At the end of period 2 is neon (Ne). A neon atom has the electronic configuration 2,8. This means that the second shell is full. When we go on to the next element, sodium (Na), we have to add a new shell, which means that we start a new period in the periodic table.

This means that we can immediately tell how many shells an atom has just by looking at which period its element is in. For example, phosphorus (P) is in period 3, which tells us that a phosphorus atom has three shells.

At the moment, the periodic table goes up to period 7. This means that no elements have been discovered whose atoms have more than 7 shells.

The shells before the outer shell are always full

Because electrons always occupy the lowest available energy level, any shells before the outer shell will always be full.

For example, let's say we wanted to work out the electronic configuration of a sulphur atom. Sulphur is in period 3, which means a sulphur atom must have three shells.

Because the shells before the outer shell are always full, the first two numbers in a sulphur atom's electronic configuration must just be the capacities of the first two shells: 2 and 8.

Therefore, we know that a sulphur atom's electronic configuration must be 2,8,? where the question mark represents the number of electrons in the outer shell. Below, we will learn how to use group number to work out what this last number is.

Atoms of group 0 elements have full outer shells

All of the elements in group 0 (also known as group 8) have atoms which have full outer shells. We can use this fact to work out the electronic configuration of an atom of any element in group 0.

For example, let's say we want to work out the electronic configuration of an argon atom. Argon is in period 3, so there must be three shells, and all of the shells before the outer shell must be full. This tells us that the first shell contains 2 electrons and the second shell contains 8 electrons.

Since argon is in group 0, its outer shell (the third shell) must also be full. Therefore the third shell must contain 8 electrons.

This tells us that the electronic configuration of an argon atom is 2,8,8.

For atoms of group 1-7 elements, the group number equals the number of outer shell electrons

For atoms of group 1 elements, we are always starting a new shell. This means that they always have 1 electron in their outer shell.

As we move across the period, we add one electron each time until we reach the capacity of the shell, at which point we start a new period. This means that the number of electrons in the outer shell is always equal to the group number.

Using this information, we can work out the electronic configuration of an atom of any element in groups 1 to 7.

We worked out above that the electronic configuration of a sulphur atom is 2,8,? - where the question mark is the number of electrons in the third shell. To work out what that number is, all we have to do is look at sulphur's group number. Sulphur is in group 6, which tells us that a sulphur atom must have 6 electrons in its outer shell.

Therefore, a sulphur atom's electronic configuration is 2,8,6.

Note that this only works for atoms of the first twenty elements. After element twenty, we enter the d-block of the periodic table. Since the d-block isn't included in the numbering of the groups, the relationship between group number and number of outer shell electrons no longer exists. However, we can still use a similar approach to work out electronic configuration by looking at position in the periodic table.

Electronic configuration follows four rules (for atoms of elements 1-20)

To summarise, the following rules can be used to quickly work out the electronic configuration of an atom of any of the first 20 elements:

• The period number is equal to the number of shells.
• All shells before the outer shell are full.
• Elements in group 0 have a full outer shell.
• For groups 1-7, the number of electrons in the outer shell is equal to the group number.

Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.

1/3

How does an elementâ€™s period number relate to its electronic configuration?

2/3

How does an elementâ€™s group number relate to its electronic configuration?

3/3

How do you work out how many electrons are in the shells before the outer shell?