When elements are listed in order by the masses of their atoms, the
same sequence of properties appears over and over again in the list.
| NSES Content Standard B
Physical Science: Structure and properties of matter Grades 9-12, page 178 An element is composed of a single type of atom. When elements are listed in order according to the number of protons (called the atomic number), repeating patterns of physical and chemical properties identify families of elements with similar properties. This "Periodic Table" is a consequence of the repeating pattern of outermost electrons and their permitted energies. |