Chemistry / Unit 2 Chem 0273 short lesson, about 20 seconds
The Periodic Law
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The idea
Under the periodic law, elemental properties repeat at regular intervals when the elements are ordered by atomic number rather than atomic mass.
In practice
Arranging elements by increasing atomic number places lithium, sodium, and potassium in one column, each showing similar reactive chemical behavior.
Where people slip
Mendeleev's table used atomic mass; the later shift to atomic number resolved ordering problems the mass-based table could not fix.
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