How are the elements arranged by atomic mass? - Answers

They are NOT normally arranged by Atomic Mass; instead they are arranged in the Periodic Table by their atomic number. The latter is the number of protons (or electrons) in a single atom of the element and it is a constant for the element. The atomic mass is an average of the isotopes of the element (in proportion to their abundance).While the atomic number is always an integer, increases in steps of 1 and elements that are a specific "distances" apart share similar chemical characteristic. On the other hand the atomic mass is rarely an integer, and occasionally it reverses the order laid out be the periodic table: for example, element 18, argon has an atomic mass of 39.948 which is greater than that of element 19, potassium with a mass of 39.098: this is because the predominant isotope of argon has 22 neutrons while that for potassium has 20 neutrons.