Chemistry Foundations

3.5.4 - Electronic Configuration

# Electronic Configuration

Electronic configuration (also known as electronic structure) is the arrangement of electrons within an atom or ion.

It is usually expressed as a list of numbers separated by commas. The first number is the number of electrons in the first shell, the second number is the number of electrons in the second shell, and so on.

For example, a chlorine atom has 2 electrons in the first shell, 8 electrons in the second shell and 7 electrons in the third shell. Therefore, a chlorine atomâ€™s electronic configuration is 2,8,7.

A chlorine atom has the electronic configuration 2,8,7. This means that there are 2 electrons in the first shell, 8 in the second, and 7 in the third.

## To work out an atom's electronic configuration, we need to know its atomic number

The work out the electronic configuration of an atom, we need to know how many electrons it has. The number of electrons in an atom is always equal to the number of protons, which is called the atomic number. Therefore, to work out an atom's electronic configuration, we need to start by finding out its atomic number.

For example, if we wanted to work out the electronic configuration of a silicon atom, we would start by looking up silicon's atomic number in the periodic table (shown below).

The periodic table.

Silcon's atomic number is 14, meaning that a silicon atom has 14 protons and therefore 14 electrons. By using the capacities of the shells, we can work out that the first shell will contain 2 electrons, the second shell will contain 8 electrons, and the third shell will contain the final 4 electrons.

Therefore, the electronic configuration of a silicon atom is 2,8,4.

(Note: for atoms of the first twenty elements, there is actually a faster way to work out the electronic configuration using the periodic table. It will be explained on the next page).

## To work out an ion's electronic configuration, we need to know its atomic number and relative charge

In an ion, the number of electrons is not equal to the number of protons. Therefore, in order to work out the electronic configuration of an ion, we need to know not just its atomic number, but also its relative charge.

For example, let's say we want to work out the electronic configuration of an Na+ ion (a sodium ion with a relative charge of +1).

Sodium's atomic number is 11, meaning that the ion has 11 protons.

Since the ion's relative charge is +1, the number of protons must be one more than the number of electrons. Therefore, we can deduce that an Na+ ion has 10 electrons.

Using the capacities of the shells, we can work out that there must be 2 electrons in the first shell and 8 in the second shell. Therefore, an Na+ ion has an electronic configuration of 2,8.

A sodium ion with a relative charge of +1 has an electronic configuration of 2,8.

As another example, let's work out the electronic configuration of an S2- ion (a sulphide ion with a relative charge of -2).

Sulphur's atomic number is 16, meaning that the sulphide ion has 16 protons.

Since the ion's relative charge is -2, the number of electrons must be 2 more than the number of protons. Therefore, there must be 18 electrons.

Using the capacities of the shells, we can work out that there must be 2 electrons in the first shell, 8 in the second shell and 8 in the third shell.

This tell us that the electronic configuration of an S2- ion is 2,8,8.

A sulphide ion with a relative charge of -2 has an electronic configuration of 2,8,8.

## Elements do not have electronic configurations

When we talk about electronic configuration, we need to make sure we are specific about which particle (atom or ion) we are talking about.

Sometimes you may hear people say things like, "Copper's electronic configuration is...". The problem with this is that copper is an element, and there are atoms of copper (Cu) as well as ions of copper (such as Cu+ and Cu2+).

A copper atom has the electronic configuration 2,8,8,11.

A Cu+ ion has the electronic configuration 2,8,8,10.

A Cu2+ ion has the electronic configuration 2,8,8,9.

Therefore, the element copper does not have an electronic configuration - it depends on what kind of copper particle we are talking about (Cu, Cu+, or Cu2+).

Usually, when people say, "Copper's electronic configuration", what they actually mean is, "A copper atom's electronic configuration". It is common for people to just say the name of the element when they are actually referring to an atom of that element. While people may know what you mean if you do this, it is better to avoid any potential misunderstanding by always stating which specific particle of that element you are talking about.

## 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.

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What is electronic configuration? What is it also known as?

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How do you work out the electronic configuration of an atom?

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How do you work out the electronic configuration of an ion?