Electron Configurations in Atomic Energy Levels

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  • 0:07 Electrons
  • 1:20 Quantities of Electrons
  • 2:47 Electron Patterns
  • 6:36 Electron Configurations
  • 9:20 Making Predictions
  • 9:55 Lesson Summary
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Lesson Transcript
Kristin Born

Kristin has an M.S. in Chemistry and has taught many at many levels, including introductory and AP Chemistry.

Expert Contributor
Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

This lesson will explain what the electrons are doing inside the atom. Tune in to find out how we specify where they are located and how this location description will help us predict an element's properties.


The diagram shows multiple electron patterns
Periodic Table Electron Patterns

Of the three subatomic particles (protons, neutrons, and electrons), which do you think is the most important? You may think that the protons are the most important because they are the ones that determine what type of element an atom is. Or you may think that the neutrons are important because they're the ones that can make atoms of the same element heavier or lighter by being found in larger or smaller quantities in the nucleus. But when it really comes down to chemistry and chemical reactions, the subatomic particle that is the most important is the electron. Electrons are the stars of the chemistry show. That is why we are going to dedicate several lessons to this negatively charged, super speedy, very tiny, elusive particle.

You will find out in a later lesson why I call the electron elusive. But why is it the star? An atom's chemical properties rely heavily on the arrangement of its electrons. So before we can discuss just how its chemical properties can be predicted, we first need to learn how the electrons in an atom are arranged.

Quantities of Electrons

Remember that electrons are negatively charged, have almost no mass, and are located in the electron cloud, meaning that they are found in the extremely large location outside of the nucleus. They are the ones taking up all the space in an atom, and the amount of space they take up will depend on how much energy they have (more on that later).

First, it is important to know how many electrons each atom has. We are only going to be discussing neutral atoms in this lesson, so each atom will always have the exact same number of electrons as it has protons.

By now, you should be a whiz at determining the number of protons an atom has. If not, let's do a quick review. Find tin on the periodic table. It is on the middle right side and has the symbol Sn. How many protons does an atom of tin have? The answer is 50, which also means that a neutral atom of tin will have 50 electrons. So as the elements increase in atomic number (the number of protons), they are also going to increase in the number of electrons. An atom of hydrogen will have one electron, an atom of helium will have two electrons, an atom of lithium will have three electrons, and so on.

Using the diagram shows that a krypton atom has 36 electrons
Krypton 36 Electrons

Electron Patterns

Now, the thing that makes an atom behave a certain way is not how many electrons it has, but the way they are arranged. Luckily, the arrangement of the electrons in the electron cloud is quite predictable. Let's take a look at this diagram.

It may look just like a bunch of numbers and letters in blocks arranged somewhat like the periodic table, but look closer. Do you see any patterns? First, you may notice that the large numbers increase as you move down. This number indicates the energy level of the electron. The higher the number, the more energy an electron will have! You may also notice that in the center of the table, in that sunken-in region, the numbers follow the same pattern but are one less than the numbers on the 'towers' on either side.

Let's see if we can see a pattern with the letters. The letters seem to have a more organized pattern. With the exception of only a few, all the ss are blocked together on the left 'tower' we will call the s-block, all of the ds are in the sunken-in center part we will call the d-block, the ps are on the right 'tower' we will call the p-block, and the fs are all in the lower 'island,' which you may have guessed is called the f-block.

The final pattern you may notice is with the tiny superscript numbers. You should see that in the s-block they increase from 1 to 2, in the d-block they increase from 1 to 10, in the p-block they increase from 1 to 6, and in the f-block they increase from 1 to 14. The little superscripts are just counting numbers; they represent the quantity of electrons in each number/letter combination.

So what does this all mean? Each of these little clusters of numbers and letters represents information about the location of an electron. We go into more detail on what kinds of information in a later lesson. For now, just focus on the numbers and letters. For example, hydrogen (in the very top left corner box) has one electron. That electron is called a 1s electron. Helium (in the very top right corner box), labeled 1s2, has two electrons. Both of them are 1s electrons. Lithium is in the box that is labeled 2s1. An atom of lithium has three electrons: two 1s electrons and a 2s electron. Are you starting to see the pattern?

The electron configuration for an atom of krypton
Krypton Electron Configuration

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Additional Activities

Electron Configuration

Each atom has electrons in its electron cloud. The number of electrons in an atom's electron cloud is that element's atomic number. These electrons are arranged in specific energy levels surrounding the nucleus. In this activity, we are going to use the periodic table to determine the electron configuration of a few elements. You will need to have a periodic table for this activity.

Labeling the Periodic Table

The first two columns on the left side of the periodic table are the s-block elements. The middle section up to group 12 is the d-block. The remaining groups comprise the p-block elements except for helium. It is in the s-block. We won't work with the f-block, which are the bottom two rows that are separated from the main two groups.

The rows tell us what energy level the electron is in. The group number within the block tells us how many electrons are in that subshell. We'll start at the element we are working with and work backward.

For example, lithium's electron configuration is 1s2 2s1. We start at lithium on the periodic table, and we see it is in the second row and the first column of the s-block, so it's electron configuration ends in 2s1. All inner orbitals are filled, which means 1s2% is complete. This gives us 2s1 1s2. Reversing this order gives us the correct electron configuration of 1s2 2s1.

Electron Configuration Practice

  1. Write the electron configuration for beryllium.
  2. Write the electron configuration for sulfur.
  3. Write the electron configuration for calcium.


  1. 1s2 2s2
  2. 1s2 2s2 2p6 3s2 3p4
  3. 1s2 2s2 2p6 3s2 3p6 4s2

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