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

The electron arrangement in an atom is defined by the number of electrons in each energy level, with higher principal quantum numbers (n) indicating larger size and energy. As energy levels increase, the capacity for electrons also rises, linking to the periodic table's rows. For instance, energy level 5 can accommodate more electrons than level 2. Understanding these arrangements 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 n25, n5 is a higher energy level so we'd expect it to have more electrons than an energy level of n2. 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 have 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 going to start adding more and more electrons. Remember, in the first shell, we can hold a maximum of 2 electrons, and that's because 2 times n squared. 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, this space here. So now we're talking about electrons in the second shell, so dash, 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 2₁2₂. Now let's go to, boron here. So boron here would be 2₁3₂, 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. Ford'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.

All Right. 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, that the 3rd shell can hold up to 18 electrons, but here, in 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 that we can have electrons 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?

The electron configuration of an atom describes the distribution of electrons in the atomic orbitals. It follows the Aufbau principle, which states that electrons fill orbitals starting from the lowest energy level to the highest. For example, the electron configuration of carbon (atomic number 6) is 1s² 2s² 2p². This means that carbon has two electrons in the 1s orbital, two in the 2s orbital, and two in the 2p orbital. Understanding electron configurations helps predict an element's chemical properties and its behavior in reactions.

How do you determine the number of valence electrons in an atom?

To determine the number of valence electrons in an atom, you need to look at its electron configuration and identify the electrons in the outermost energy level (shell). For example, the electron configuration of oxygen (atomic number 8) is 1s² 2s² 2p⁴. The outermost shell is the second energy level (2s and 2p), which contains 6 electrons. Therefore, oxygen has 6 valence electrons. Valence electrons are crucial because they determine an element's chemical reactivity and bonding behavior.

What is the significance of the principal quantum number (n) in electron arrangements?

The principal quantum number (n) indicates the main energy level or shell of an electron in an atom. As n increases, the energy and size of the orbital also increase. For example, n=1 represents the first energy level, n=2 the second, and so on. Higher energy levels can accommodate more electrons: the first level can hold 2 electrons, the second can hold 8, and the third can hold 18. The principal quantum number helps in understanding the electron distribution and the atom's overall structure.

How are electron arrangements related to the periodic table?

Electron arrangements are closely related to the periodic table. The rows (periods) of the periodic table correspond to the principal quantum numbers (n) of the electron shells. For instance, elements in the second period have electrons filling up to the second energy level (n=2). The number of valence electrons, which determine an element's chemical properties, can be inferred from the group (column) in which the element is located. This relationship helps predict and explain the chemical behavior of elements.

Why do higher energy levels accommodate more electrons?

Higher energy levels accommodate more electrons because they have more sublevels and orbitals. Each energy level (n) contains n² orbitals, and each orbital can hold 2 electrons. For example, the first energy level (n=1) has 1 orbital (1s), holding 2 electrons. The second energy level (n=2) has 4 orbitals (2s and 2p), holding 8 electrons. As n increases, the number of orbitals and thus the capacity for electrons increases, allowing higher energy levels to accommodate more electrons.