Ground State

Ground State Definition Chemistry

So what is meant by the "ground state" of an atom?

The simplest definition of the ground state of an atom refers to:

To define this more broadly, we can say that the ground state is the state in which atoms are found if they are not charged or excited by external sources. These sources of excitation could be light (such as photons) or any other wavelength on the electromagnetic spectrum.

When discrete amounts of energy, such as quanta, excite the atom, it triggers certain subatomic rearrangements and a shift in the electronic configuration. But in this case, the ground state refers to the state where this process does not occur and concentrates on the atom in its usual "uncharged" state.

So what does the ground state mean in terms of the electrons within an atom? In fact, when speaking about the ground state of an atom, it is all about the electronic configuration and the energy states of the electrons present in the atom.

This means that in the ground state, the atom is not excited and subsequently none of the electrons are excited. The electrons are in their lowest possible energy state. What happens in the ground state is that all the electrons line up in such a way as to be at the lowest possible energy of their individual positioning within the atom, and also of the overall system.

There are a number of factors that determine the positioning of the electron within an atom, which we will cover in the next section. Yet it is crucial to remember that the electrons may occupy different states within an atom. The ground state will always refer to the state where the electrons are in their lowest possible energy configuration within the atom.

Ground State Electronic Configuration

So how can we visualize the ground state electronic configurations?

We can use electron configuration diagrams, such as the arrow and box diagrams. Here, we will explore what they are and how they can be used to depict atoms in the ground state. As the definition of the ground state of atoms refers to their electronic energy levels, depicting them will help us understand the inner workings of the atom.

Below, you will find a diagram of empty electron orbitals.

Fig. 1 - Empty electron orbitals

But how do electrons fill up these orbitals?

There are three sets of rules which you need to think about when considering such problems: the Aufbau principle, Pauli's exclusion principle, and Hund's rule. Here you will find summaries of what they mean.

So how does this relate to the notion of the ground state? You can take a look at how the electrons will preferentially line up in a ground state atom. Here, the way in which the atoms fill up naturally in an atom will be the ground state.

This can be useful for determining the ground state electronic configurations of any atom, since if you apply the above-mentioned three rules, you will determine the ground state of the particular element. This is due to the fact that when atoms are in an excited state (which we will soon cover), the electronic arrangement changes and deviates from the canonical rules of Aufbau, Pauli, and Hund. On the other hand, we can see how applying the rules will give us the ground state configurations of the electrons in a given atom, as that will be suggestive of the way the electrons would arrange themselves if there would be no external source of energy applied or any type of deviation possible. This would result in the configuration of the lowest possible energy levels, hence the ground state configuration.

Ground State of Atoms

You can apply the aforementioned definition of the ground state as well as the theories on the electronic configuration to atomic models now. As stated above, you can construct electronic diagrams to match the ground state. At the bottom of this article, you will find examples of the ground state.

Usually, the excited state of atoms is represented with an asterisk next to it. Below you will find an example:

A (ground state)

A* (excited state)

A + energy = A*

A* = A + energy

Thus, you can assume that molecules or atoms are only in their excited state if there is an asterisk next to them. This will help you to identify the ground states of atoms in equations.

Ground State vs Excited State Electron Configuration

Take a look at the two electronic configurations below. In this example, the model element is carbon.

Fig. 3 - Ground state and excited state electronic configuration diagram of carbon

Do you notice any differences between them? You can tell one of them clearly follows the three rules we set out earlier. As a reminder, these are the Aufbau principle, Pauli's exclusion principle, and Hund's rule.

The above diagram depicting the ground state depicts the electrons arranging themselves according to these three key principles. So how does it differ in the excited state? Particularly, you can see how an electron from the 2s orbital moves to the 2p orbital. As you can see, there is a 'hole' in the 2s orbital, which means that electrons do not occupy the lowest energy states. We would call this the excited state, as one of the electrons has enough energy to move up an energy level, in this case into the 2p orbital.

In the same way as it has gained energy to move up towards an excited state, the electron can reemit the energy and be lowered back down into the energy level it occupied before: the ground state.

Fig. 4 - Shift from an excited state to the ground state of an atom

Examples of Ground State

Here you will find a bunch of examples of the ground state electron configuration. Take a look at the figure below, which depicts the electronic configuration of the atoms from Boron to Oxygen.

Fig. 6 - Electronic configuration depicting ground state of the elements B, C, N, O

What can you observe in the diagram above? You can tell how the elements given in the example increase in atomic number by 1, hence their number of electrons will increase by 1.

Thinking about the gradual increase in electrons, take a look at what happens to the electronic configuration of the elements, and more importantly, how it changes from atom to atom. This way you will observe trends, and you'll see how Hund's rule plays a role in electronic configuration. All of this ultimately just shows the ground state of atoms being a process that is pattern-like and does not deviate from atom to atom. Using these examples, you can predict any electronic configuration of atoms in question, and determine whether they are in their ground state or excited state.