Lesson Video: Electronic Configurations Science

In this video, we will learn how to write electronic configurations for atoms or ions and identify an atom from its electronic configuration.

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Video Transcript

In this video, we will learn how to write electronic configurations for atoms or ions and identify an atom from its electronic configuration.

As we will learn how to write electronic configurations in this video, we should first define electronic configuration. The electronic configuration is the distribution of electrons in energy levels, where energy levels are defined as distances from the nucleus that describe the location of electrons. The four lowest energy levels of an atom are called K, L, M, and N. The electrons of an atom tend to occupy all possible states in the lowest energy levels before they start to fill higher levels. But each energy level holds a different number of electrons. The first level can hold a maximum of two electrons. The second can hold a maximum of eight electrons. The third can hold a maximum of 18 electrons. And the fourth can hold a maximum of 32 electrons.

Let’s take a look at the electronic configuration of nitrogen. The chemical notation for nitrogen tells us that nitrogen has an atomic number of seven. Thus, a neutral atom of nitrogen will have seven electrons. We can show which energy levels these electrons will fill using an energy level diagram. The electrons of an atom tend to occupy all possible states in the lowest energy levels before they start to fill higher levels. So the electrons will fill energy level K first. And as K holds a maximum of two electrons, nitrogen has two electrons in the energy level K. As seven minus two is five, there are five electrons left after filling energy level K. The remaining five electrons fill the second lowest energy level, L.

We have now drawn an energy level diagram. The diagram tells us that the electronic configuration of a neutral nitrogen atom is 2,5. Electronic configurations can also be written in a list form. So the electronic configuration for nitrogen can be written as two comma five. The occupied energy levels for an atom of nitrogen are K and L.

Let’s now look at the electronic configuration for sodium. The chemical notation for sodium tells us that it has an atomic number of 11. Thus, a neutral atom will have 11 electrons. The first two fill energy level K. As 11 minus two is nine, we are left with nine electrons. But energy level L can contain a maximum of eight electrons. So even with L full, there is one electron remaining. This remaining electron goes in energy level M. Therefore, sodium has an electronic configuration of 2,8,1.

Not only can we calculate the electronic configuration from knowing the identify of an atom, and its atomic number, but we can also do the reverse and identify an atom from its electronic configuration. Let’s look at an example where the electronic configuration of an atom is 2,8,3.

First of all, we need to add the numbers in the electronic configuration together. This gives a total of 13. Assuming the electronic configuration is for a neutral atom, as the number of electrons present is 13, the atom will have an atomic number of 13. The atom with atomic number 13 is aluminum, which has the chemical symbol Al. So the identity of the element with electronic configuration 2,8,3 is aluminum.

Now that we’ve looked at how to calculate the electronic configuration for atoms, let’s quickly look at how to calculate electronic configuration for ions.

An ion is a chemical species with an unequal number of protons and electrons. To help us further understand what an ion is, let’s look at an example of the electronic configuration of an atom and see how that atom can be converted to an ion. An atom of lithium has atomic number three. This means that a neutral atom of lithium has three protons and three electrons. Two of these electrons will fill energy level K. The remaining one electron goes in energy level L. So an atom of a lithium has the electronic configuration 2,1. Lithium tends to form a one plus ion, which means the ion forms after the atom loses one electron. So a lithium ion has three protons but two electrons. This matches the definition of an ion, which states that an ion has an unequal number of protons and electrons.

The electron will be lost from the outer energy level. As there is only one electron in the outer energy level of the atom, there will be no electrons in this energy level for the ion. Thus, the electronic configuration for the lithium ion is just two.

Now that we have looked at how to write electronic configurations for atoms and ions, let’s look at the impact that electronic configuration has on reactivity. Elements tend to be chemically active if they have fewer than eight electrons in their outer energy level. This is the case for oxygen and fluorine, for example. They have six and seven electrons in their outer energy level, respectively. Oxygen and fluorine react with many other substances because they do not have eight outer-level electrons. They are chemically active. There are also similarities between the way that fluorine reacts and the way that chlorine reacts.

The chemical notation for chlorine tells us that it has an atomic number of 17. So the first two electrons fill energy level K. As 17 minus two is 15, there are 15 electrons remaining. Eight of these 15 electrons fill energy level L, as energy level L holds a maximum of eight electrons. As 15 minus eight is seven, there are seven electrons remaining. These seven electrons fill energy level M. So although fluorine atoms contain nine electrons and chlorine atoms contain 17 electrons, both fluorine and chlorine have seven outer shell electrons. This is the key reason as to why fluorine and chlorine are quite reactive and react in similar ways.

However, most noble gas atoms are essentially inert because they have eight electrons in their outer energy level. An example of this is neon. Neon is a noble gas with eight electrons in its outer energy level. Neon gas is chemically stable and cannot react with other substances unless the reaction conditions are incredibly unusual. The case is similar for argon, another noble gas. Each neutral atom has 18 electrons. 10 of them fill the lowest energy levels K and L. The remaining eight electrons are found in energy level M. Argon atoms are chemically stable and essentially inert. They don’t tend to react with any other substances because they have eight outer shell electrons.

Helium, however, is a noble gas but only has two outer shell electrons. Despite this, helium atoms are chemically inert. Although helium does not have eight electrons in its outer energy level, it does have a full outer energy level. Helium atoms are light, small, and incredibly stable. They do not usually react with any other substances.

Now that we have learnt how to write electronic configurations for atoms and learnt how this affects their reactivity, let’s look at some example questions.

In total, how many electrons are needed for both the K and L energy levels to be full?

Energy levels are defined as distances from the nucleus that describe the location of electrons. The first four energy levels are called K, L, M, and N. Each energy level holds a different maximum number of electrons. The maximum number of electrons for energy level K is two, L is eight, M is 18, and N is 32.

The question asks how many electrons are needed for both the K and L energy levels to be full. As two electrons are needed for K to be full and eight electrons are needed for L to be full, then two plus eight electrons are needed for K and L to be full. Two electrons plus eight electrons equals 10 electrons. Thus, the answer to the question “how many electrons are needed for both the K and L energy levels to be full?” is 10 electrons.

An atom contains five electrons. How many of these electrons occupy the L energy level?

Energy levels are distances from the nucleus that describe the location of electrons. The first energy level is called K, the second energy level is L, and the third and fourth are M and N, respectively. Energy level K can hold a maximum of two electrons, L holds a maximum of eight, and M and N hold a maximum of 18 and 32, respectively. Electrons fill from the lowest energy level first, which is K.

The question tells us that the atom contains five electrons. Two of these electrons fill energy level K. The remaining three electrons fill the second lowest energy level L. Thus, the answer to the question “How many of these electrons occupy the L energy level?” is three electrons.

An atom of oxygen has eight protons in its nucleus. Which energy level diagram shows the correct electronic configuration for oxygen?

We’ve been given five energy level diagrams labeled from (A) to (E). We need to identify which diagram shows the correct electronic configuration for oxygen. And we are told in the question that oxygen has eight protons. So a neutral atom of oxygen will also have eight electrons. We can immediately eliminate answer choice (E) as the diagram shows that oxygen has 10 electrons, rather than eight.

The other answer choices have the correct number of electrons, but the electrons aren’t necessarily in the correct energy levels. Each energy level has a different maximum number of electrons. K is two, L is eight, M is 18, and N is 32. The maximum number of electrons found in energy level K is two. Thus, answer choices (C) and (D) must be incorrect. Additionally, electrons fill the lowest energy level first, which is K. So two of oxygen’s electrons will be found in energy level K. And the remaining six will be found in the second lowest energy level L. As answer choice (A) has an electronic configuration of 2,6, it is the correct answer. The energy level diagram which shows the correct electronic configuration for oxygen is answer choice (A).

Which of the following energy level diagrams corresponds to the least reactive atom?

We’ve been given five answer choices labeled from (A) to (E). Energy levels are distances from the nucleus that describe the location of electrons. The first four energy levels are called K, L, M, and N, where K is the closest energy level to the nucleus. Electrons fill this energy level first. They then fill energy level L.

All of the answer choices have two electrons in K and eight in L. The difference lies in the number of electrons in energy level M, their outer energy level. Elements tend to be chemically active if they have fewer than eight electrons in their outer energy level. So the atoms in (A), (B), (D), and (E) are likely to be chemically active. However, elements tend to be chemically stable and inert if they have eight electrons in their outer energy level. Thus, it’s likely that atom (C) is inert. Therefore, the energy level diagram which corresponds to the least reactive atom is answer choice (C).

Let’s summarize the key points of this video. Electrons tend to fill energy levels in order of increasing energy state so would fill energy level K first, then L, then M, and so on. The electronic configuration is the distribution of electrons in energy levels. Atoms are ordinarily active if they do not have eight outer-level electrons. Most noble gas atoms are inert because they have eight outer-level electrons.

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