Essay on the Periodic Table

The main value of the periodic table is the ability to predict the chemical properties of an element based on its location on the table. It should be noted that the properties vary differently when moving vertically along the columns of the table than when moving horizontally along the rows.

The layout of the periodic table demonstrates recurring ("periodic") chemical properties. Elements are listed in order of increasing atomic number (i.e., the number of protons in the atomic nucleus). Rows are arranged so that elements with similar properties fall into the same columns (groups or families). According to quantum mechanical theories of electron configuration within atoms, each row (period) in the table corresponded to the filling of a quantum shell of electrons. There are progressively longer periods further down the table, grouping the elements into s-, p-, d- and f-blocks to reflect their electron configuration.

In printed or other formally presented periodic tables, each element is provided a formatted cell that provides selected information, usually including its element symbol and atomic number, and often each element's name, atomic mass ("atomic weight"), and selected other information, such as its density, melting and boiling points, abbreviated electron configuration, electronegativity, and most common valence numbers.Only chemical elements, not mixtures, compounds, or subatomic particles, are included in the periodic table. Each element has a single entry, even if it has multiple isotopes.

As of June 2011, the periodic table includes 118 chemical elements whose discoveries have been confirmed. Of these, 91 are regularly occurring primordial or recurrently produced elements found naturally on the Earth, at least in transient trace amounts, and three others occur naturally, but only incidentally. The 24 other known elements (those from americium through ununoctium) are synthetic, produced by human technology but not regularly or incidentally occurring naturally. Various synthetic elements, as well as synthetic isotopes of naturally occurring elements, are now also present in the environment from such sources as nuclear weapons explosions, nuclear waste processing, and disposal of materials including industrial and medical nucleotides. For example, americium and its decay product neptunium are incidentally present in household and commercial waste from disposal of unwanted americium-containing smoke detectors.

Formal naming of the chemical elements is overseen by the International Union of Pure and Applied Chemistry (IUPAC). Provisional names, such as ununtrium, ununquadium, or ununpentium, are provided for elements that have been discovered but not yet been formally named; these names are based on the three digits of their atomic numbers.