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2014-11-23T09:05:48+08:00
They increase from left to right and top to bottom , they can be varied by isotopes,atomic mass, atomic groups .atomic uses
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2014-11-23T10:28:19+08:00

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Atomic masses in the periodic table are arranged from the left to right. The chemists calculate the mass of each element, and then they ranked the masses in order, simply atomic numbers. Atomic mass also used to arrange things in order. Also, atomic mass shows the  mass of each element.

Hope this helps =)
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