Modern periodic law and the long form

In 1913 Moseley (English physicist) found that √ν of an element's characteristic X-rays gives a straight line against atomic number Z, not against atomic mass. Z is therefore the more fundamental property.

Modern Periodic Law: the physical and chemical properties of elements are periodic functions of their atomic numbers.

Z equals the number of protons, which equals the number of electrons in a neutral atom. Periodicity is really the repeating pattern of electronic configurations, which fix the properties of elements and their compounds.

Nature supplies 94 elements. Pitchblende, a uranium ore, contains traces of neptunium and plutonium as well as actinium and protactinium.

The long form is the most widely used table. Horizontal rows are periods; vertical columns, holding elements with similar outer configurations, are groups or families. IUPAC numbers the groups 1 to 18, replacing IA…VIIA, VIII, IB…VIIB and 0.

There are seven periods, and the period number is the highest principal quantum number n present. Period lengths are 2, 8, 8, 18, 18 and 32; the seventh, like the sixth, can hold at most 32.

The 14 lanthanoids (period 6) and 14 actinoids (period 7) sit in two separate rows at the bottom. Seaborg's work on plutonium (1940) and the transuranium elements 94-102 led to this layout; he received the 1951 Nobel Prize and element 106 is named seaborgium (Sg).