Unit 10: Electron Configurations

Unit Overview:

In the last unit, you continued your examination of the development of the model of the atom, focusing on deepening your understanding of the structural components of the atom. You clarified the nuclear model by considering the planetary model.

The Nuclear Model:	What it’s based on: ● Rutherford’s Gold Foil Experiment ● The discovery of the nucleus	Unanswered Questions of this model: 1. How do we describe the nucleus of the atom? 2. Why do the electrons stay outside of the nucleus of an atom?
The Planetary Model:	What it’s based on: ● An extension of Rutherford’s Gold Foil Experiment → protons. ● Chadwick’s Beryllium Experiment → neutrons. ● Bohr’s Hydrogen Model → energy levels.	How it answers the Questions of the Nuclear Model: 1. It explains that the nucleus is made up of protons and neutrons. 2. It explains that electrons are found within energy levels outside of the nucleus.

In this unit, you will further deepen your understanding of the electrons within the energy levels of an atom by considering the development of the quantum model of the atom.

What is the flaw of the Planetary Model of the Atom?

Although the planetary model more accurately represents the nucleus of the atom by indicating that the nucleus is comprised of both protons and neutrons, its representation of the electrons is overly-simplistic. Recall that Niels Bohr examined the Hydrogen atom, creating a mathematical model to explain its emission spectrum. This model accurately describes the movement of hydrogen’s single electron between specific energy levels in the atom.

However, as scientists attempted to extend this mathematical model to larger atoms, they noticed that it did not perfectly predict the emission spectra of larger atoms. Although it generally predicted its spectral lines, it did not accurately predict the wavelength of each line. This inability to perfectly predict the exact spectra motivated scientists implied that the planetary model’s explanation that the electron travels in a fixed path around the nucleus of the atom. This inaccuracy motivated scientists to continue to explore the location of the electron within the electron cloud, once again changing the model of the atom over time.

Three important contributions helped us to understand the flaws of the planetary model:

1. Dual Wave-Particle Nature of the Electron

In 1924, Louis de Broglie hypothesized that because of the extremely small size of the

Electron it is not sufficient to conceive of them simply as particles. Instead, he

suggested that electrons also have wave properties associated with them. This

hypothesis helps to explain the inaccuracy of the planetary model by offering a reason for it - the electron is not simply a particle.

In order to hear a verbal description about this dual wave-particle nature, watch the following video: Video

2. Wave Equation

In 1926, Erwin Schrodinger used mathematical equations to describe the probability of finding an electron in a certain position outside of the nucleus. These equations provide a three-dimensional description of the electron cloud. This explanation applied the wave nature of the electron to the atomic model to understand areas of probability within energy levels where it is likely to find an electron. Video

3. Uncertainty Principle

In 1927, Werner Heisenberg explained that it was impossible to know the exact position and momentum of the electron. In other words, because of the dual wave-particle nature of the electron, it is impossible to know its exact location within the atom at a given moment. This explanation helps to understand that the electron is not on a fixed path like the planetary model indicates.

In order to hear a verbal description and watch a laser experiment about Heisenberg’s Uncertainty Principle, watch the following video: Video

What is the Quantum Mechanical Model of the Atom?

Based on Schrodinger’s wave equation, scientists changed the model of the atom - removing the electron from the fixed orbit of the planetary model, and instead describing the electron within the most probable areas of the electron cloud where it can be found. This most current model of the atom is referred to as the quantum mechanical model of the atom. This model is described by quantum numbers.

The quantum numbers are a set of 4 numbers that describe the electron and its location in the electron cloud of the atom. In order to understand these numbers, we are going to define them and use an analogy to understand them. In this analogy, we will compare the quantum numbers to an apartment building.

The electron cloud is the general region outside of the nucleus in which all of the electrons of an atom are found. In our analogy, an apartment building is similar to the electron cloud. The four quantum numbers further divide the electron cloud to better describe the location of the electron.

1. Principal Quantum Number - describes the energy level

● Symbol: n

● Possible values: n = 1, 2, 3, 4, 5, 6, 7 {positive integer values}

● Special notes:

○ The higher the value of n, the higher the energy level.

○ The higher the energy level, the further away from the nucleus.

● Analogy: The principal quantum number divides the electron cloud into energy levels, just like an apartment building is divided into floors.

2. Angular Momentum [Azimuthal] Quantum Number - describes the sublevel within the energy level

● Symbol: l

● Possible values: l = 0 - (n-1) {a range of values}

● Special notes: also describes the general shape and number of the orbitals within that sublevel

○ When l = 0, there is 1 orbital in the sublevel, it is referred to as an s-orbital, and has a spherical shape

○ When l = 1, there are 3 orbitals in the sublevel, they referred to as a p-orbital, and have a 2-lobe shape

○ When l = 2, there are 5 orbitals in the sublevel, they referred to as a d-orbital, and have a 4-lobe shape

○ When l = 3, there are 7 orbitals in the sublevel, they referred to as a f-orbital, and have a 8-lobe shape

● Analogy: The angular momentum quantum number divides the energy levels into sublevels, just like the floors of an apartment building are divided into apartments.

3. Magnetic Quantum Number - describes the orbital within the sublevel

● Symbol: m_l =

● Possible values: m_l = -l - 0 - +l {a range of values}

● Special notes:

○ The orbital is the specific area within the electron cloud in which it is most likely to find the electron.

○ Orbitals within a sub-level have equal energy.

● Analogy: The magnetic quantum number divides the sublevel into orbitals, just like an apartment is divided into separate rooms.

4. Spin Quantum Number - describes the spin of the electron within the orbital

● Symbol: m_s

● Possible values: m_s = +½ or -½

● Special notes:

○ There are only 2 possible values of the spin quantum number

■ There are only 2 different directions in which the electron can spin.

■ These 2 directions are represented as an up and a down arrow.

○ There is a maximum of 2 electrons in any one orbital.

○ If 2 electrons occupy the same orbital, they must have opposite spins.

● Analogy: The spin quantum number describes the electron(s) within the orbital, just like people can be in the rooms of an apartment; the maximum occupancy of any room is 2.

Visualizing our Analogy:

Electron-Land Apartments:

l = 0 n= 4 ⇅ ml = 0

l = 1 ⇅ ml = -1

⇅ ml = 0

⇅ ml = +1

l = 2 ⇅ ml = -2

⇅ ml = -1

⇅ ml = 0

⇅ ml = +1

⇅ ml = +2

l=3 ⇅ ml = -3

⇅ ml = -2

⇅ ml = -1

⇅ ml = 0

⇅ ml = +1

⇅ ml = +2

⇅ ml = +3

l = 0 n = 3⇅ ml = 0

l = 1 ⇅ ml = -1

⇅ ml = 0

⇅ ml = +1

l = 2 ⇅ ml = -2

⇅ ml = -1

⇅ ml = 0

⇅ ml = +1

⇅ ml = +2

l = 0 n = 2 ⇅ ml = 0

l = 1 ⇅ ml = -1

⇅ ml = 0

⇅ ml = +1

l = 0 n = 1 ⇅ ml = 0

In order to hear a verbal description and see more visual aids to help you to understand the quantum mechanical model of the atom, watch the following video: Video

Because of the abstract nature of this model of the atom, you may find it helpful to watch a second video that represents the orbitals within an atom: Video

Practice: Complete this online quiz about the quantum mechanical model of the atom.

What are electron configurations?

Although the planetary model is not accurate because we know that the electrons are not traveling on fixed paths, it is a simpler representation of the atom to draw - a drawing of the quantum mechanical model is too complex to draw because it is difficult to draw the electrons in the numerous orbitals inside that exist in the concentric energy levels. Therefore, the quantum model is more often represented as electron configurations.

The electron configuration is a simplified representation of the filling of the electrons in the electron cloud of an atom.

The writing of electron configurations is guided by several principals:

1. The Aufbau Principle states that electrons fill sublevels in order of increasing energy. It is often referred to as the “building up principle” because it emphasizes that electrons fill lower energy levels before they begin filling higher energy levels. So, in general, the 1st energy level fills before the second, which fills before the third, etc. And, within energy levels, the s sublevel is filled before the p sublevel, which fills before the d sublevel, which fills before the f sublevel. However, there are some exceptions to his principle when filling the d and f sublevels because of their complexity. Fortunately, the periodic table reminds us when these exceptions happen.

2. Hund’s Rule states that equal-energy orbitals fill one at a time. In other words, each orbital will hold 1 electron before any orbitals hold a second electron.

3. Pauli’s Exclusion Principle states that no two electrons within any one atom can have the exact same set of 4 quantum numbers. Therefore, when a second electron goes into an orbital, it must have the opposite spin.

Consider the element oxygen in the following examples. Oxygen has 8 electrons in its electron cloud. There are two primary ways in which an electron configuration can be written:

1. Orbital Notation - this notation represents the electrons as arrows in simplified diagrams of the orbitals

Example:

● Description: Each box represents an orbital. Each arrow represents an electron. Notice:

○ There is a maximum of 2 electrons in any 1 orbital, 2 arrows in any one box.

○ If there are 2 electrons, they have opposite spins, an up and a down arrow in any one box.

○ Equal energy orbitals are represented as boxes that are touching.

2. Electron Configuration Notation - this notation represents the number of electrons in each sublevel

● Example: 1s²2s²2p⁴

● Notes: The superscript represents the total number of electrons that are in the designated sublevel.

In order to hear verbal descriptions and see more visual aids to help you to understand electron configurations of the atom, watch the following video:

Practice: Complete this online practice about electron configurations, also referring to any of the related pool of videos to help you better understand this complex concept.

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ChemLab: Electron Configuration

Overview:

The quantum mechanical model of the atom provides detailed descriptions of the electron cloud. In this lab, you will create the electron configuration of elements by filling electron orbitals. You will notice patterns in how the electrons fill the electron cloud.

Directions:

1. Download the Student Exploration and Vocabulary sheets for Electron Configuration.

2. Familiarize yourself with the words on the vocabulary sheet.

3. Log-in to your Explore Learning account.

4. Click on “Electron Configuration” and launch the gizmo.

5. Answer the Prior Knowledge Question.

6. Practice using the Gizmo, using the Gizmo warm-up instructions.

7. After you are comfortable using the Gizmo, begin the activity. Use the lab sheet as a guide to complete the 2 activities:

a. Activity A: Smaller Atoms

b. Activity B: Larger Atoms

BrainPOP Activity

Overview:

This unit has been very abstract - trying to visualize and represent the model of the atom at such a small scale - a scale so small that we cannot actually see these orbitals that we are describing. These representations are also very complex, making it difficult to even draw it. So, it is important to “pull ourselves back out” and remember why it is important to understand atoms at this scale. In this activity, you will explore some applications of our atomic understanding by considering nanotechnology.

Directions:

1. Go to BrainPOP and watch the video on Nanotechnology. If a login is required, please enter the following:

a. username: jcesc

b. password: qfaf9361

2. Choose between the Graphic Organizer and the Worksheet. Complete at least one of them. Be sure to save your document, so that you will be able to upload it to your log.

3. Choose between the Make-a-Map and Related Reading. Complete at least one of them. Be sure to save your document, so that you will be able to upload it to your log.

a. If you choose Make-a-Map, use the words and/or images to relate the ideas of this unit in a way that makes sense to you.

b. If you choose Related Reading, choose at least 1 of the readings to write a 3-sentence summary of.

4. To check your understanding, complete the quiz.

5. IMPORTANT: This is considered an off-line activity, be sure to keep track of the time that you spend on BrainPOP.