Valence Electron: Definition, Configuration & Example

Electrons are an essential part of an atom. Unlike protons and neutrons, valence electrons take part in the excitement of a chemical reaction. Learn how to find the number of valence electrons of any element in this lesson.

What Are Valence Electrons?

What makes cool chemical reactions work? Remember those fun experiments like making a volcano from baking soda and vinegar or a rocket from Mentos and soda? We wouldn’t have these reactions without valence electrons.

Valence electrons are the electrons located at the outermost shell of an atom. Why are these electrons special? Because when two atoms interact, the electrons in the outermost shells are the first ones to come into contact with each other and are the ones that determine how an atom will react in a chemical reaction.

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Let’s imagine a fast food, drive-through restaurant. We drive through the lane in our car, reach our hands out the window, and the employee reaches out and hands us the food. The whole interaction between the car and the restaurant rests just on the arms of the employee and driver at their windows. The arms of the driver and the arms of the employee are kind of like valence electrons.

Let’s look at some examples below to visualize valence electrons.

Diagrams of Oxygen and Neon Atoms

For the oxygen atom, you can see that the outermost shell has 6 electrons, so oxygen has 6 valence electrons. Neon’s outermost shell has 8 electrons. Neon therefore has 8 valence electrons.

The shells of an atom can only hold so many electrons. Each shell has a certain amount of subshells (s, p, d, etc) that have a certain amount of orbitals. Each orbital can hold 2 electrons. The first shell has one subshell, s, which has one orbital, so it can hold 2 electrons. The total number of electrons that each shell can hold is:

  • Shell 1 – has subshell s, which has one orbital. It can therefore hold 2 electrons.
  • Shell 2 – has subshells s and p. p has 3 orbitals, so can hold 6 electrons. Add the two that subshell s can hold, and we know that shell 2 can hold 8 total electrons
  • Shell 3 – has subshells s, p, and d. d has 5 orbitals, so can hold 10 electrons. Shell 3 can hold a total of 18 electrons.

Take magnesium, which has a total of 12 electrons. If we draw the electrons for magnesium, you’ll have 3 shells.

Diagram of Magnesium Atom

The first shell will take its maximum, 2, and so will the second shell, with 8. The remaining two electrons will occupy the third, outer, shell. Therefore, magnesium has 2 valence electrons.

Phosphorus, on the other hand, has 15 electrons.

Diagram of Phosphorus Atom

It will also have three shells, and the first and second shells are both fully occupied. The third shell will house the remaining 5 electrons, which means phosphorus has 5 valence electrons.

Electron Configuration

Before we dive into valence electron configuration, let us first review electron configuration. An electron configuration is the arrangement of electrons around the nucleus of an atom, just as we’ve been looking at so far. Each atom has its own position on the periodic table, and you can find the electron configuration by knowing where the atom is placed on the table.

The atomic number of an atom in the ground state is the same as the number of electrons. For example, sodium has an atomic number of 11 and magnesium has an atomic number of 12. So, sodium should contain 11 electrons in its electron configuration and magnesium should contain 12.

Each atom occupies an orbital in a certain order. We can continue drawing the electrons like we have been doing, but there is a shorter and easier way to do it. This is called the spdf notation. The periodic table is divided into s, p, d, and f blocks. We can use the picture to determine the electron configuration of an atom. It’s important to remember how many electrons occupy the subshells s, p, d, and f.

Let’s get the electron configuration for aluminum, which has an atomic number of 13. The two elements are indicated on the periodic table.

Location of Aluminum and Calcium in Blocks of Periodic Table

From their positions, we can determine the electron configuration. Aluminum is all the way in the third row, so its electrons occupy the first and second rows fully, and the third row partially. We count from left to right all the way to aluminum, writing each one down as we go. We can write its electron configuration as

1s^2 2s^2 2p^6 3s^2 3p^1

In ‘1s^2’ the ‘1s’ refers to the first shell’s subshell s, and the ‘2’ refers to the 2 electrons it will be holding. In ‘3p^1’, ‘3p’ refers to the third shell’s subshell p, and the ‘1’ means it’s only holding one electron. Though p subshells can hold a total of 6 electrons, aluminum only has 13 electrons, with the preceding subshells holding the rest of them.

The electron configuration of calcium.

Calcium has an atomic number of 20. It’s all the way on the fourth row. So we count the same way as aluminum, all the way until we reach calcium.

Another way to write the electron configuration is by using this pattern and remembering it.

Pattern of Electron Shells and Sub-shells

There is text written in red – this means that these subshells are hardly ever used. We use this pattern by filling the subshells with electrons, starting with 1s, then, going down to the next arrow, 2s, and then, 2p and so forth until we arrive to the final number of electrons.

Valence Electrons

Did you notice how writing electron configurations can be tedious? What if you have 100 electrons? That means you can be writing for some time. There is a shortcut to electron configuration, and we call this noble gas configuration. Using this shortcut, we can shorten the electron configuration.

How is this done? First, you locate your element that you are interested in. Next, you look at the last noble gas that comes before this element. The noble gases are located at the last column of the periodic table on the right most side. Then you put the noble gas in brackets, and then finish the standard electron configuration that come after. This method of writing electron configuration helps narrow down valence electrons.

Location of Chlorine in Periodic Table

Let’s take chlorine, which has 17 electrons. The noble gas just before chlorine is neon. Neon has 10 electrons so you can subtract 10 from the 17 electrons of chlorine. Write Ne in brackets, then use the old method. We have 7 electrons left, so count from Ne until all the electrons are taken care of. In this case, it will be neon 3s^2 3p^5. There are 7 valence electrons because the highest energy level, 3, has 7 total electrons (5 plus 2 is 7).

Location of Calcium in Periodic Table

For calcium, which has an atomic number of 20 and therefore 20 electrons, find calcium on the table. The gas just before it is argon, so subtract argon’s 18 electrons, and now you only have to fill 2 remaining electrons. Put argon in brackets, and follow it with 4s^2. There are 2 valence electrons for calcium, as it has 2 electrons in the highest energy level, 4.

Locations of Bromine and Argon in Periodic Table

You would do the same with bromine, as argon is also its immediately preceding noble gas. Bromine has 35 electrons. 35 minus 18 is 17, so you would write argon in brackets and follow it with 4s^2 3d^10 4p^5. The highest level is 4, and there are 7 electrons, so the number of valence electrons for bromine is 7.

If you do the electron configuration of all noble gases, you will see that except for helium, which has only 2, all noble gases have 8 valence electrons.

Lesson Summary

The electrons that occupy the outermost shell of an atom are called valence electrons. Valence electrons are important because they determine how an atom will react. By writing an electron configuration, you’ll be able to see how many electrons occupy the highest energy level. The electron configuration can be determined from where the atom is located in the periodic table and by using the spdf chart.

Luckily, there is a shorter way to write electron configurations called the noble gas configuration. Noble gases will always have 8 valence electrons, except for helium.

Key Terms

Periodic Table

Valence electrons – the electrons located at the outermost shell of an atom

Electron configuration – the arrangement of electrons around the nucleus of an atom

Spdf notation – a short, easy format for notating electron configuration using the periodic table and subshells s, p, d, and f.

Noble gas configuration – the electron configuration of noble gasses; can be used as a shortcut for figuring the electron configuration

Learning Outcomes

After this lesson, you should be able to write the electron configurations for a variety of atoms, applying spdf notation and noble gas configuration.


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