A period (row) indicates the number of energy levels.
So period 1 which contains Hydrogen and Helium both have 1 energy level/ outermost shell
Period 2 which contains 8 elements (Lithium, Beryllium, Boron, Carbon, Nitrogen, Oxygen, Fluorine, Neon) all have 2 energy levels.
So you have plenty of examples. Just any three elements in the same row and they will all have a number of energy levels equal to the row they are in. For example, Sodium(Na), Aluminium (Al) and Chlorine (Cl) in period 3 all have a number of energy levels equal to 3
Nov 13th, 2014
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