Help Window: The Periodic Table of the Elements

Topics Covered in This Section

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What Is the Periodic Table?

The Periodic Table is a listing of the elements according to increasing atomic number and similarities in chemical behavior. It was developed in the 19th century by the Russian chemist Mendeleev, before knowledge of modern atomic theory.

The Table is a chart having rows and columns.


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The Table Itself!



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Reading Electron Configurations from the Periodic Table

It is not necessary to memorize the filling-order of electrons in atoms since these can be read from the Periodic Table. In fact, the reason that elements in the same Group have similar chemical behavior is because their electronic configuration is similar -- only the valence level electrons are at different energy levels.

To use the Table this way it is necessary to remember:

Some Examples

Li has atomic number 3. The n = 1 shell is filled with electrons in the same arrangement as He, so these can be represented as [He]. Since Li is in Group 1, period 2, it has its one valence electron at the n = 2 quantum level in an s orbital. We write [He] 2s1.

Al has atomic number 13. Its first 10 electrons are arranged like Ne. There are thus 3 valence electrons in Al at the n = 3 energy level. Al is in the third column, so the first two electrons are in an s orbital, and the last in a p orbital. We write: [Ne] 3s2, 3p1.

Br has atomic number 35. The first 18 electrons are arranged like Ar. The remaining 17 start filling orbitals at the energy level 4s (because Br is in period 4). We write [Ar] 4s2, 3d10, 4p5. (Remember that there are three p-type orbitals, each of which may describe only 2 electrons at most. The designation 4p5 means 4px2, 4py2, 4pz1.)


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Copyright 1997, Larry McGahey, The College of St Scholastica, Duluth, Minnesota.
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