Electron Arrangements: Study with Video Lessons, Practice Problems & Examples

The electron arrangement in an atom determines the number of electrons in each energy level, with higher energy levels (n) accommodating more electrons. As n increases, both the size and energy of atomic orbitals rise. The energy levels correspond to the periods in the periodic table, illustrating the relationship between electron configuration and elemental properties. Understanding these concepts is crucial for grasping atomic structure and behavior in chemical reactions.

Electron Arrangements Concept 1

The electron arrangement of an atom gives the number of electrons in each energy level. Now recall, as the value of n increases, then both the size and energy level of an atomic orbital will also increase. And we're going to say as we increase the energy levels, the number of electrons within a given orbital will also increase. So for example, if we have electrons in shells 2₅, 5 is a higher energy level so we'd expect it to have more electrons than an energy level of 2. Now the energy level, shell numbers, of an atom can be tied to the period or rows of the periodic table. So these are things that we've examined before when it comes to the atom itself, but now we're going to apply them to electron arrangements. So now click on the next video and let's take a look at an example question.

Electron Arrangements Example 1

So here we have to complete the electron arrangements for the following elements of the periodic table. Alright. So we're going to start out with hydrogen, which has an atomic number of 1, which means it has only 1 electron. So its electron arrangement is simply just 1. Helium is 2 because it has 2 electrons because its atomic number is 2. Alright. So now what's going to start happening is we're gonna start adding more and more electrons. Remember, in the first shell, we can hold a maximum of 2 electrons, and that's because of the formula 2n2. In the second shell, we can theoretically hold up to 8 electrons. So now we're at lithium. Lithium has an atomic number of 3. The first two electrons are in the first shell. So now we're talking about electrons in the second shell, so how many is that? 1. And it's 1 because again its total atomic number is 3, which means it has in total 3 electrons. We've accounted for the first 2 in the first shell, and then this third one is in the second shell. Then we move over, we go from lithium, then beryllium, beryllium will be 2-2. Now let's go to boron. Boron here would be 2-3, right? Because its atomic number is 5, so it can have 5 total electrons. 2 are in the first shell because the first shell can only hold a maximum of 2. The remaining 3 that we need are in the second shell. Then we have carbon, nitrogen, oxygen. Let's look at fluorine here. Fluorine's atomic number is 9. That means it has 9 total electrons. 2 of them are in the first shell and then the other 7 are in the second shell.

Let's keep going. Alright. So for sodium, sodium has an atomic number of 11 on the periodic table. The first 2 are in the first shell, the next 8 are in the second shell. We need one more electron, and it will be here in the 3rd shell. Let's keep going. Skip over to aluminum. Aluminum has an atomic number of 13. So we have 2 electrons in the first, 8 electrons in the second shell, and 3 in the 3rd shell. Alright. When it comes to depicting the number of electrons in each of these shells, we're going to say that the second shell can have up to 8. And when we're doing electron arrangements, we're going to say, yes, theoretically, the 3rd shell can hold up to 18 electrons, but here, for the purposes of an electron arrangement, we're going to say we go up to 8. So potassium has an atomic number of 19. So it has 2 electrons in the first shell, 8 in the second, 8 in the third, and we need one more to get to 19, so it has one in the 4th shell. So here we'd have our electron arrangements of different elements on the periodic table. And what's important to know is that electron arrangements are simple as long as we keep them that way. We're going to say here that elements beyond an atomic number of 20 can have partially filled orbitals and are beyond the scope of this course. So really only need to know up to calcium. All the remaining elements, you don't need to worry about. Right? So keep this in mind. When it comes to electron arrangements, the maximum we can hold in the first shell is 2, in the second shell 8, in the 3rd shell 8, and in the 4th shell 2. Those are the maximum numbers of electrons that we can have in each one of those 4 shells.

Write the electron arrangement for the following element:Calcium (Z = 20)

• A. 2-2-6-2-6-2
• B. 2-8-8-2
• C. 2-16-2
• D. 2-8-8-1
• E. 1-2-2-8-2

Here’s what students ask on this topic:

What is the electron configuration of an atom and how is it determined?

The electron configuration of an atom describes the distribution of electrons in the atomic orbitals. It is determined by the principles of quantum mechanics, specifically the Aufbau principle, Pauli exclusion principle, and Hund's rule. The Aufbau principle states that electrons fill orbitals starting from the lowest energy level to the highest. The Pauli exclusion principle asserts that no two electrons can have the same set of quantum numbers, meaning each orbital can hold a maximum of two electrons with opposite spins. Hund's rule states that electrons will fill degenerate orbitals (orbitals with the same energy) singly before pairing up. For example, the electron configuration of oxygen (atomic number 8) is 1s² 2s² 2p⁴.

How do energy levels and sublevels relate to the periodic table?

Energy levels and sublevels are directly related to the periodic table. The principal energy levels (n) correspond to the periods (rows) of the periodic table. Each energy level contains sublevels (s, p, d, f) that are filled with electrons in a specific order. For instance, the first energy level (n=1) has only an s sublevel, while the second energy level (n=2) has s and p sublevels. As you move across a period, electrons fill these sublevels according to the Aufbau principle. This arrangement explains the periodicity of elemental properties, as elements in the same group (column) have similar valence electron configurations, leading to similar chemical behaviors.

What is the significance of the Pauli exclusion principle in electron arrangements?

The Pauli exclusion principle is crucial in determining electron arrangements because it states that no two electrons in an atom can have the same set of four quantum numbers. This principle ensures that each electron in an atom occupies a unique position in terms of its energy and spatial distribution. As a result, each atomic orbital can hold a maximum of two electrons with opposite spins. This principle helps explain the structure of the periodic table and the chemical properties of elements, as it dictates how electrons fill orbitals and how atoms bond with each other.

How does Hund's rule affect the electron configuration of atoms?

Hund's rule affects the electron configuration of atoms by stating that electrons will fill degenerate orbitals (orbitals with the same energy) singly before pairing up. This minimizes electron-electron repulsion within an atom, leading to a more stable configuration. For example, in the case of carbon (atomic number 6), the 2p sublevel has three degenerate orbitals. According to Hund's rule, the electron configuration is 1s² 2s² 2p², with the two 2p electrons occupying separate orbitals (2p_x and 2p_y) rather than pairing up in one orbital. This rule is essential for understanding the magnetic properties and reactivity of elements.

Why do higher energy levels accommodate more electrons?

Higher energy levels accommodate more electrons because they have more sublevels and orbitals available for electron occupancy. As the principal quantum number (n) increases, the number of sublevels within that energy level also increases. For example, the first energy level (n=1) has only one sublevel (1s), while the second energy level (n=2) has two sublevels (2s and 2p). The third energy level (n=3) has three sublevels (3s, 3p, and 3d), and so on. Each sublevel contains a specific number of orbitals, and each orbital can hold two electrons. Therefore, higher energy levels can accommodate more electrons due to the increased number of available orbitals.