May 14, 2013
The Modern Periodic Table
The modern Periodic Table is arranged in order of atomic (proton) numbers. This was first done when protons, neutrons and electrons were first discovered early in the 20th century. This order solved the problems of elements being placed in the wrong places due to the order of atomic weights.
The modern Periodic Table arranges elements in terms of their electronic structures (which affect their properties).
Elements in the same group have the same number of electrons in their outer shell (highest energy level).
Elements in the same period have the same number of shells (energy levels).