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⚛️Unit 1

•  ⏱️5 min read

1.5 Atomic Structure and Electron Configurations

jeremy kiggundu

dalia savy

dylan black

(editor)

⏱️ September 18, 2020

📅

Dalton's Theory

One of the principles that Chemists use to understand atoms is Dalton’s Atomic Theory⚛️. Dalton concluded that atoms are indivisible and indestructible. Dalton also found that all of the atoms of a single element carry the same properties.

We also learn in this section that an atom contains negatively charged electrons and a nucleus which holds positively charged protons and neutrons.

Coulomb's Law

Now that we know the structure of an atom, we’ll need to be able to calculate the force between particles, or the intermolecular forces. There is where Coulomb's Law comes in. It states that force is equal to the product of the charges, divided by the distance between the particles, multiplied by the Coulomb constant.

The Coulomb constant is equal to 8.9875517923×10⁹ kg⋅m³⋅s⁻⁴⋅A⁻².

You don't have to memorize this formula, but you should understand that the strength of the forces depends on the charge of the atoms and the distance between the nuclei of the atoms. 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.

Electrons

Well, 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 electronic configuration of an element.

The Bohr Model

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

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

Image Courtesy of Wikimedia

The number of electrons used in the diagram is derived from the atomic number, or # of protons/electrons. That is why there are 11 electrons.

Bohr understood that the electrons orbited in different energy levels and 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.

Electron Configuration

The idea behind electron configuration is quite similar to drawing out the shells in valence electrons, in that each shell only holds a certain number of electrons. 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, respectively, are 2, 6, 10, and 14. Here is a breakdown of 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.

Configuration Rules

The Aufbau Principle states that you must fill electrons in order of increasing sublevel energies (1s-2s-2p-3s-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.

Hund's Rule says that unpaired electrons must fill an unoccupied orbital before pairing up with a single electron in a previous orbital.

How do I do this?

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.

To start, you should put your finger at 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 suborbitals 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 is 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.

Noble Gas Shortcut

The noble gas shortcut becomes especially helpful if you are asked to write the configuration of element 86. 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:

Image Courtesy of Chegg

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

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

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

Another Example - Element 26 (Fe)

Here is Fe on the periodic table:

Fe actually includes the d block in its electron configuration. Here it is:

Just make sure to include the d block! You got this, just keep practicing.

Valence/Core Electrons

Just a quick note:

Valence Electrons = outermost electrons = s+p orbitals

Core Electrons = innermost electrons = d+f orbitals

Given the following information, how many valence electrons does 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! Then you just add up the electrons in the 4s orbital and the 4p orbital, and you get a total of 5 valence electrons.

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