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Organization
In 1913, the modern periodic table of the elements was created when Henry Moseley arranged the elements according to the number of protons in the nucleus. A table organized by increasing atomic number offered more precise and accurate ordering. The elements are charted in columns called groups or families. One or two letters are used to designate each element. Each element in the family shares chemical and physical properties with the other members.
Groups
Each column in the periodic table is classified as a group of similar elements with many common characteristics. Group 1 contains soft, silvery, alkali metals such as lithium, sodium and potassium. In rare instances, such as in Jupiter's core, hydrogen may become an alkali metal. Group 2 comprises the alkaline earth metals, including magnesium and radium. Other members of the main groups are: boron, carbon, nitrogen, chalcogen and halogen, as well as the noble gases. The other groups are called transition metals.
Patterns
The periodic table is further organized into three blocks of elements with similar valence configurations, or electrical positive or negative charges of the electrons of the atom's outer shell of orbits. The charges determine reactivity with other elements to form compounds. The horizontal rows of elements are called periods. Each element across the row contains one more electron and proton. Moving across the row, the extra electron affects the element's reactivity. Elements periodically morph from metallic to non-metallic properties until a stable, unreactive electron configuration is reached. The addition of another electron makes the element reactive and metallic again.
Periodicity
The arrangement of the elements in the periodic table illustrates the periodicity of elemental properties. The ability of elements to form co-valent, ionic bonds with other elements are based on periodic repetitions of valence electron configurations. Atoms form stable compounds by giving up, sharing or accepting a valence electron until both atoms form a complete, full outer shell. An element's electronegativity and reactivity decreases as you move vertically down a column. Highly electronegative atoms readily accept bonding electrons but don't readily share them for reactive covalent bonds. Physical properties such as melting and boiling points rise as you go down a group column.