Electron Shells

atom.gif (1130 bytes) Electron Shells

Electrons are arranged around the Nucleus in SHELLS. For simplicity they can be thought of like mini-planets orbiting a central sun, but it is closer to the truth to think of them as "clouds" of electric charge around the Nucleus.

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The shells are numbered outward from the Nucleus. The maximum number of electrons found in each shell can be calculated by: where "n" is the number of the shell.

Shell Number	Maximum Number of Electrons in the Shell
1	2 x 1 = 2
2	2 x 4 = 8
3	2 x 9 = 18
4	2 x 16 = 32
5	2 x 25 = 50

The Octet Rule:
In general, atoms are most stable when they have 8 electrons in their outer-most shell. (Octet means 8.) The exception is the first shell which is most stable with TWO electrons. If you know the Atomic Number and Mass Number of an element and the maximum number of electrons in each electron shell you can draw a diagram of the element.

For example: Sodium has an Atomic Number of 11 and an Mass Number of 23 ie

This means an atom of Sodium has 11 Protons and therefore 11 electrons.

Since the number of Protons + Neutrons is 23 and there are 11 Protons there must be 12 Neutrons.

From the table above the electrons are arranged as: First Shell = 2, Second Shell = 8, Third Shell = 1 (Giving a total of 11.)

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Example 2: Chlorine has a Mass Number of 35.5 and an Atomic Number of 17.

This means an atom of Chlorine has 17 Protons and therefore 17 electrons.

Since the number of Protons + Neutrons is 35 and there are 17 Protons there must be 18 Neutrons.

From the table above the electrons are arranged as: First Shell = 2, Second Shell = 8, Third Shell = 7 (Giving a total of 17.)

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NOTE: if you are wondering where the ".5" from Chlorine's Mass Number went to check out: isotopes.htm

You should look up a Periodic Table of Elements containing information about the Mass Number and Atomic Number of a few elements and practice drawing the atoms. You will find links to a number of Periodic Tables on the Links page.

NOTE: Keep to the first 20, or so elements. Things get a bit more complex with the larger Atomic Numbers.

Electron Shells - QUIZ

Your Name and School -

Do you Certify the answers below to be all your own work? -

1. Apart from size, in what ways are electron shells different from the orbital path of Planets?. -

2.What is the theoretical maximum number of electrons in the sixth electron shell ?-

3. How many Electrons are there in the outer shell of an atom with an Atomic Number of 10? -

4. Describe the structure of an atom of Oxygen (Protons, Neutrons, electrons, shells, etc)? -

5. Which two elements do not obey the "octet" rule of stable electron structure? -

6. How many electron shells does the largest atom have? -

7. What is the URL of a Web page containing information about Electron Shells (apart from this one)?

Feedback, comments, questions -

(Make sure your NAME and SCHOOL are included.)

This form will be emailed to a 'marker'. You will receive feedback shortly. Satisfactory performance on this and other Quizzes is required for the granting of a "Completion Certificate" for this Course.