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Ionization Energy

Posted by Penny Marsh
Grade level: undergrad
School: Ealing Tertiary College
City: London State/Province: Ealing
Country: England
Area of science: Chemistry


Why does ionization energy increase across any period in the periodic table?

Electrons are arranged in orbitals around the atomic nucleus. To answer your question, you can think of an orbital as a shell with slots in it, one slot for each electron. Any atom has an infinite number of orbitals around it, but only a finite number of electrons to fill them. The first orbital has only two slots in it. Neutral hydrogen has one electron, so one slot in this first orbital is filled, and one remains empty. Helium has two electrons, so both slots are filled. Lithium has three, so the first orbital is filled, and the second has a single electron in it, and so on, through the periodic table.

So the rows of the table of elements represent orbital filling. The elements on the left have one electron to put into the outermost orbital, and the ones all the way on the right have filled orbitals. But, as you go across a row, the number of protons in the nucleus increases. The positive charge on the protons is what binds the negatively charged electrons to the atom. The more protons you have, the stronger the pull on the electrons. In other words, it takes more energy to pull an electron off the atom (the 'ionization energy') if there are more protons in the nucleus.

Please note that this is a highly simplified answer to a fairly complex situation. You might want to look around the web for more information. A good place to start is the Online Periodic Table of Elements. Another good place is a list of links to online periodic tables.

©2008 Phil Plait. All Rights Reserved.

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