# Lesson Video: Electrons and Energy Levels Science

In this video, we will learn how to describe and identify energy levels in atoms and determine the number of electrons each energy level can contain.

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### Video Transcript

In this video, we will learn how to describe and identify energy levels in atoms and determine the number of electrons each energy level can contain.

Atoms are the smallest unit of ordinary matter that forms a chemical element. They are composed of subatomic particles. There are three types of subatomic particles: positively charged protons, neutral neutrons, and negatively charged electrons. Despite containing both positively and negatively charged particles, atoms are neutral overall. This is because an atom contains the same number of positively charged protons and negatively charged electrons. The positively charged protons and neutral neutrons are found in the nucleus in the center of the atom. The negatively charged electrons stay in a different part of the atom called energy levels. But what exactly are energy levels?

Energy levels, sometimes called electron shells, are regions around the nucleus in which electrons move according to their energies. The seven lowest energy levels in an atom are represented by the blue arcs in this diagram. Each has a different energy. The energy level closest to the nucleus has the lowest energy. The energy increases the farther away the energy level is from the nucleus.

Each energy level can be identified by a number or a single capital letter. The energy level closest to the nucleus is K, followed by L, M, N, and so on. Not only do the energy levels have a different energy, but each also has a different maximum number of electrons that it can contain. The K energy level can only contain a maximum of two electrons. But the N energy level can contain a maximum of 32 electrons.

We can determine how many electrons an energy level can hold by using the equation two 𝑛 squared. In this equation, 𝑛 indicates the energy level, with K being energy level one, L being two, and so on. Using this equation, we can determine the maximum number of electrons that the L energy level can contain. First, we substitute two for 𝑛. Two squared is four, and two times four is eight. Therefore, the L energy level can hold a maximum of eight electrons. What about the M energy level? This is energy level three. Three squared is nine, and two times nine is 18. So the M energy level can hold a maximum of 18 electrons.

We can see that the maximum number of electrons that an energy level can hold dramatically changes from one energy level to the next. We might say that the number of electrons an energy level can hold increases exponentially. The equation two 𝑛 squared can be used to determine the maximum number of electrons in the K through N energy levels. But we don’t typically apply this equation to energy levels O and beyond. This is because atoms tend to split into two when an energy level contains more than 32 electrons. We now know what energy levels are and how many electrons each can hold. But which energy levels do electrons in an atom occupy?

It turns out that electrons in an atom tend to fill the lowest possible energy levels before filling higher ones. This means that all of the electrons in an atom will ordinarily have the lowest possible energy state. To see this in practice, let’s take a look at an atom of helium, lithium, and magnesium.

An atom of helium has two protons. Therefore, it must also have two electrons. The electrons will fill or occupy the lowest possible energy level. The lowest energy level is the K energy level, which can hold two electrons. So both of the helium electrons, each represented by a blue dot, will be found in the K energy level.

An atom of lithium has three protons and therefore three electrons, one more electron than an atom of helium. We know that electrons will occupy the lowest possible energy level first. But the K energy level can only hold two electrons, and a lithium atom has three. So the first two electrons will fill the K energy level, and the third electron will occupy the next highest energy level, which is the L energy level.

An atom of magnesium has 12 protons and 12 electrons. The first two electrons fill the K energy level. The L energy level can only hold eight electrons. So the next eight electrons will fill the L energy level. So far, the diagram only shows 10 electrons. The remaining two electrons must be in the next highest energy level, which is the M energy level.

All three of these atoms are in what we call the ground state. The ground state is the lowest possible energy state of an atom. It is achieved when all of the electrons are in the lowest possible energy levels. Electrons tend to stay in the lowest possible energy levels. So atoms are typically in the ground state. However, it is possible for an electron to move to a higher energy state.

Let’s consider a hydrogen atom that has one proton and one electron. In the ground state, its single electron will be found in the lowest energy level, K. Electrons like this one can move to a higher energy level if they absorb a quantum with the right energy. A quantum is a unit or packet of energy. The amount of energy that an electron needs in order to move depends on the energy of the energy level it starts at and the energy of the energy level it moves to. The amount of energy an electron needs in order to move between two energy levels can be calculated by subtracting the energy of the level that the electron currently occupies from the energy of the level that the electron will move to.

Let’s determine how much energy this electron will need in order to move to the M energy level. The energy of the M energy level is negative 1.5 energy units. The energy of the K energy level that the electron is currently in is negative 13.6 energy units. By subtracting negative 13.6 energy units from negative 1.5 energy units, we can determine how much energy the electron needs in order to move from the K energy level to the M energy level. This gives us a value of 12.1 energy units. So, if an electron in the K energy level absorbs a quantum with 12.1 energy units, it can move to the M energy level.

Because the electron has moved to a higher energy level, this atom is no longer in the ground state but is in what we call an excited state. The electrons in an excited-state atom are not in the lowest possible energy states. Excited-state atoms tend to be unstable and do not stay excited for very long. The excited electron will quickly move back to the ground state.

In order for the electron to move back to the ground state, energy must be emitted or released. The amount of energy released is the same as the amount of energy the electron absorbed. In this example, the electron absorbed a quantum with 12.1 energy units and then transitioned from the K energy level to the M energy level. So, in order for this electron to move from the M energy level back to the K energy level, it must emit 12.1 energy units.

We’ve learned a lot about electrons and energy levels in this video. But before we summarize what we’ve learned, let’s take a look at a few questions.

What is the maximum number of electrons that can occupy energy level X shown in the diagram?

Energy levels are regions around the nucleus in which electrons move according to their energies. The energy levels each have a different energy. The energy level closest to the nucleus has the lowest energy. And the energy increases as the energy levels get farther from the nucleus.

In addition to having a different energy, each energy level can hold a different maximum number of electrons. For energy levels one through four, we can calculate the maximum number of electrons each can contain using the equation two 𝑛 squared. 𝑛 in this equation represents the energy level.

To answer the question, we need to determine the maximum number of electrons that the energy level marked X can contain. The X energy level is energy level number three. So we can substitute three for 𝑛 in the equation. Three squared is nine, and two times nine is 18. So the maximum number of electrons that can occupy energy level X in the diagram is 18 electrons.

Which of the diagrams shows the ground state for an atom that contains two electrons?

Electrons in an atom occupy energy levels. Energy levels are regions around the nucleus in which electrons move according to their energies. The energy level closest to the nucleus in an atom is the K energy level. The second energy level is the L energy level. The third energy level is the M energy level. And the fourth is the N energy level. With this information, we can eliminate answer choices (D) and (E) as the energy levels are not labeled correctly. Let’s remove these answer choices from the screen to finish discussing the question.

We are told that the atom in the question is in the ground state. The ground state is the lowest possible energy state of an atom. The ground state is achieved when all of the electrons in an atom are in the lowest possible energy levels. Of the four energy levels shown, the energy level with the lowest energy is energy level K. The energy levels increase in energy the farther they are from the nucleus. So we should expect that for an atom to be in the ground state, the electrons should fill the K energy level first, followed by L then M then N.

We now know the order in which the electrons will fill the energy levels. But we need to know how many electrons each energy level can hold. The K energy level can contain a maximum of two electrons. The L energy level can hold up to eight, M up to 18, and N up to 32. The atom in the question, which is in the ground state, contains two electrons. The first electron will occupy the lowest energy level, level K. The second electron will also be found in the K energy level, as this energy level can hold up to two electrons. So an atom that contains two electrons will be in the ground state when both of those electrons occupy the K energy level. This is represented by the diagram shown in answer choice (C).

Now let’s summarize what we’ve learned with the key points. Atoms are electrically neutral because they contain the same number of positively charged protons and negatively charged electrons. Electrons occupy and move in regions outside of the nucleus, called energy levels, that are labeled K through Q. The energy of the energy levels increases the farther the energy level is from the nucleus. The maximum number of electrons an energy level can hold can be calculated using the equation two 𝑛 squared.

Electrons in an atom fill the lowest possible energy levels first before filling higher ones. The ground state is the lowest possible energy state of an atom, where all of the electrons are in the lowest possible energy levels. Electrons can move to a higher energy level if they absorb a quantum with the right amount of energy. Atoms are in the excited state when electrons are not in the lowest possible energy states.