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AP Chem

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Unit 1

1.5 Atomic Structure and Electron Configuration

7 min read•december 27, 2022

Jeremy Kiggundu

Dalia Savy

Jeremy Kiggundu

Dalia Savy

AP Chemistry 🧪

269 resources

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We're back to atoms! Remember how small they are? ⚛️

Recall that an atom is made up of three subatomic particles:

Subatomic Particle	Location	Mass (amu)	Charge	Extra Information
Protons	Nucleus	~1	+1	Represented by the atomic number of an element and makes up part of the mass number.
Neutrons	Nucleus	~1	0	Makes up part of the mass number of an element.
Electrons	Orbitals	~0	-1	Represented by the atomic number of an element of zero charge.

John Dalton and the Atomic Theory

One of the principles that chemists use to understand atoms is Dalton’s Atomic Theory, which has four parts.

Each element is made up of indivisible and indestructible atoms.
All atoms of a given element carry the same properties.
Atoms combine in whole-number ratios to form compounds.
A chemical reaction changes the way atoms are bound together. In other words, a chemical reaction is simply a rearrangement of atoms.

The structure of an atom! We can see the protons and neutrons in the nucleus, with electrons orbiting it.

Coulomb's Law

Now that we know the structure of an atom, we’ll need to be able to calculate the force, or attraction, between two atoms. This is where Coulomb's Law comes in:

Image Courtesy of APlusPhysics

Breaking down Coulomb's Law

The formula above is made up of the following variables:

Fe represents the calculated electric force between the two particles.
k represents Coulomb's constant.
q1 and q2 represent the charges of the two particles.
r represents the distance between the nuclei of the two particles.

Understanding Coulomb's Law

You don't have to memorize this formula, but you should understand that the strength of the forces depends on two factors:

Magnitude of charge - The greater the charge, the stronger the attraction.
Distance between the nuclei of the particles - The closer the two particles, the stronger the attraction.

The smaller the distance and the higher the charge, the stronger the attraction but don't worry about this yet! We'll come back to Coulomb's Law in a future unit.

The Atom: Electrons

We're back to electrons! We know that each element has a certain number of electrons, but how do we represent them? In this section, we also learn about how to properly write out the electron configuration of an element.

The Bohr Model

Let's begin with the basic Bohr Model. Neils Bohr predicted that electrons orbit the nucleus in a circular orbit just like how the planets in our solar system orbit the Sun. ☀️🪐

However, unlike the planets in our solar system, Bohr's orbits exist only at specific, fixed distances from the nucleus. This causes the energy of each orbit to be fixed, quantized, or stationary.

The Bohr Model of Sodium

Let's look at the Bohr model of sodium, which has 11 electrons.

Image Courtesy of Wikimedia

The atomic number of sodium is 11, which indicates that there are both 11 protons and electrons. This is why there are 11 electrons represented in the above diagram.

Bohr understood that electrons in an atom are arranged in a set of electron shells, or energy levels, around the nucleus. Each energy level corresponds to a specific energy state of the electron, which is again, fixed.

He also made the connection that the closer an electron is to the nucleus, the less energy the electron has. Therefore, the valence electrons, or the outermost electrons, have the most energy. Valence electrons are found on the valence shell of an atom, or the outermost energy level.

Taking a look at the above diagram, you can see that there is only one valence electron in the valence shell.

The Electron Configuration of an Atom

Electron configuration refers to the arrangement of electrons in an atom or molecule. The idea behind electron configuration is quite similar to drawing out the shells in the Bohr model, in that each shell only holds a certain number of electrons.

How is the electron configuration derived?

Not only are the electrons in different energy levels, or shells, but they are also located in different subshells. The four different subshells are s, p, d, and f. The maximum number of electrons in each subshell, respectively, are 2, 6, 10, and 14.

Core and Valence Electrons

Outer electrons are called valence electrons, while inner electrons are called core electrons.

Valence electrons occupy the outer s and p orbitals.
Core electrons occupy the inner d and f orbitals.

Electron Subshells on the Periodic Table

Here is a breakdown of the different subshells on the periodic table:

This will be super helpful when we begin writing the electron configurations from scratch, but first, there are some rules to cover for writing them.

Electron Configuration Rules

The Aufbau Principle states that you must fill electrons in order of increasing sublevel energies. Remember that Bohr discovered that the outermost electrons in the valence shell have the greatest amount of energy, so the order of increasing sublevel energies is as follows: 1s-2s-2p-3s-3p-etc.

The Pauli Exclusion Principle states that no two electrons in the same suborbital can have the same spin. One must spin clockwise and the other must spin counterclockwise. We'll see what that looks like in the orbital diagrams below.

Hund's Rule says that unpaired electrons must fill an unoccupied orbital before pairing up with a single electron in a previous orbital. This rule is a result of electrons trying to fill the lowest energy orbitals first.

How do we write the electron configuration of an atom?

Let's begin with an easy example: boron (element 5).

If you compare boron's spot on the periodic table to the labeled one above, you would see that Boron is in the "2p" spot. You must memorize the labeled periodic table in order to write out the electron configuration of atoms.

To start, you should put your finger on the element you are trying to find (boron). Then, start at Hydrogen (1s) and read the periodic table as if you are reading a book. Therefore, you would go to helium, and then down to lithium all the way to boron.

To know the electron configuration, note all of the subshells that you passed on your way to boron, which in this case, would be 1s, 2s, and 2p.

Now, how many elements did you pass in each block?

1s: H, He = 2

2s: Li, Be = 2

2p: B = 1

These numbers represent electrons and are noted as superscripts in the electron configuration. Putting it all together, Boron's electron configuration is:

To understand this conceptually, the superscripts are an electron. Boron's atomic number of 5 indicates that it has 5 electrons, and all the superscripts added up are equal to 5. The electron configuration is telling us that 2 electrons occupy the 1s orbital, 2 electrons occupy the 2s orbital, and one electron occupies the 2p orbital.

The Noble Gas Shortcut

The noble gas shortcut becomes especially helpful if you are asked to write the configuration of an element really far into the periodic table, such as element 86. Let's start practicing using the noble gas shortcut with boron.

To do this, you would go to the noble gas before Boron and then start reading the periodic table from there instead of from Hydrogen. Since Helium is the noble gas before Boron, the electron configuration would read:

You could use either method to write electron configurations, just make sure you put brackets around the noble gas if you choose the shortcut.

Where do the rules apply?

You may also see electron configurations represented like this, in orbital diagrams:

Image Courtesy of Chegg

Each arrow here represents a singular electron. The Aufbau Principle is easily seen here since the electrons are filling up orbitals in the order of increasing energies (1s ➡️ 2s ➡️ 2p).

Pauli's Exclusion Principle is represented here as well by the arrows facing opposite directions. No two electrons can face the same way, or in reality, spin the same way in a single subshell.

Hund's Rule isn't actually represented here since there is only one electron in the 2p orbital, but here is a good visual:

Image Courtesy of Chemistry 301

The left is correct since the electrons are filling unoccupied orbitals before pairing up with one another. Remember that this occurs so that the electron fills the lowest energy orbital first! Everything in chemistry strives for the lowest energy possible.

Writing the Electron Configuration of Element 26 (Fe)

Here is Fe on the periodic table:

Fe actually includes the d block in its electron configuration, and it is listed after the 4s orbital. Here it is:

Just make sure to include the d block! You got this, just keep practicing. It is very unlikely that you will be asked to write the electron configuration of an atom in the f block.

If you wanted to use the Nobel gas shortcut for iron, you would have to use argon in brackets!

Understanding Core and Valence Electrons

Given the following electron configuration of As, how many valence electrons does one atom of As have?

First, you always want to look at the outermost shell, which in this case, is n=4. Remember, only the electrons in the s and p orbital are valence electrons! Therefore, you just add up the electrons in the 4s orbital and the 4p orbital, giving you a total of 5 valence electrons.

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