Electron Shell Diagrams & Modern Atomic Theory

First of all I want to point out that I have created a great set of notes by creating this blog. If I do find myself teaching Science 9 in the near future I will be much more prepared to tackle this topic.

Following up on the Bohr-Rutherford model I am going to show you how to draw a electron shell diagram. For the purpose of this blog I am going to pick one element to use as an example. However the same steps can be used for any of the elements in the Periodic Table of Elements. The element I will be using is  Chlorine (Cl).

The first thing you need to know when preparing to draw an electron shell diagram is how many protons/electrons the element you are dealing with has. The atomic number of an element tells you the number of electrons and protons found in each atom. When looking at the periodic table of elements box below the atomic number can be found above the Cl. You can use the atomic mass to determine the number of neutrons in an atom of the element as the atomic mass is the number of protons and number of neutrons. The atomic mass can be seen in the image below under the Cl.

To start an electron shell diagram you need to draw the nucleus and indicate inside the nucleus the number of protons and neutrons in the atom.

Cl nucleus: Protons = 17, Neutrons = 18

Then you can draw the first energy level. Based on the atomic number we know that Cl will have 17 electrons. According to Bohr's theory the first energy level can only hold a maximum of two electrons so you put two electrons on the first energy level.

First Energy Level

You have 15 more electrons to include in the diagram so you must draw a second energy level. The second energy level can hold a maximum of 8 electrons so you can fill this energy level. 

Second Energy Level

You still have 7 more electrons and they will go in the third energy level.

Third Energy Level

The key rules to remember when drawing electron shell diagrams is that the first energy level holds 2 electrons, the second and third energy levels hold a maximum of 8, and the fourth energy level can hold up to 18 electrons. Also due to the nature of electrons wanting to be in the lowest energy level possible it is important that in your diagram that the energy levels are full before drawing a new energy level. These diagrams show you the number of valence electrons an element has and helps you identify how the element bonds. Will the element want to receive an electron or give up one? A general rule is that an atom wants to be in a state where their energy levels are full. So in the case of a Cl atom you would expect it to accept an electron to make the third energy level full with 8 electrons. The opposite is true for an element like sodium that has only one valence electron in the third energy level. Na wants to give up its electron so that it can reach a state where its' first two energy levels are full. These diagrams help provide a visual element that can aid in someones understanding of the nature of particular elements to bond with other elements.

Here are some animations of the Bohr electron shell diagrams for the first 11 elements of the Periodic Table. They provide a great visual.

Atomic Structure Animations 

Modern Atomic Theory: Quantum Mechanical Model

LOUIS de BROGLIE: 1892 - 1987
EWIN SCHRODINGER: 1887 - 1961
WERNER HEISENBERG: 1901 - 1976

In 1924 Broglie purposed that all moving particles, including electrons, exhibit wave like behaviour. Schrodinger explored this idea and in 1926 described an electron as a wave function with the Schrodinger's equation. According to this theory it is impossible to know the exact position and momentum of an electron at the same time. This is known as the Uncertainty Principle and was theorized by Heisenberg. This model uses complex shapes of orbitals which there is likely to be an electron. This model helps explain the spectral phenomena that Bohr's model fails to explain.

Resources:

The Quantum model of the atom is extremely complex and is based on mathematics. I found that the description on the For Dummies web page was effective in explaining the model in a way that I could understand.

Atomic Structure: The Quantum Mechanical Model for Dummies