Periodic Table:

The periodic table of the chemical elements (also known as the periodic table or periodic table of the elements) is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus. While rectangular in general outline, gaps are included in the horizontal rows (known as periods) as needed to keep elements with similar properties together in vertical columns (known as groups), e.g. alkali metals, alkali earths, halogens, noble gases.

Although there were precursors, the current presentation's invention is generally credited to Russian chemist Dmitri Mendeleev’s, who developed a version of the now-familiar tabular presentation in 1869 to illustrate recurring ("periodic") trends in the properties of the then-known elements. The layout of the table has been refined and extended over time, as new elements have been discovered, and new theoretical models have been developed to explain chemical behavior.

Since the periodic table accurately predicts the abilities of various elements to combine into chemical compounds, use of the periodic table is now ubiquitous within the academic discipline of chemistry, providing a useful framework to classify, systematize, and compare many of the many different forms of chemical behavior. The table has found many applications not only in chemistry and physics, but also in such diverse fields as geology, biology, materials, engineering, agriculture, medicine, nutrition, environmental health, and astronomy. Its principles are especially important in chemical engineering.

One of the strengths of Mendeleev's presentation is that the original version accurately predicted some of the properties of then-undiscovered elements expected to fill gaps in his arrangement. For example: "eka-aluminium", expected to have properties intermediate between Aluminium and indium, was discovered with said properties in 1875 and named gallium. No gaps remain in the current 118-element periodic table; all elements from hydrogen to Plutonium except technetium, promethium and neptunium exist in the Earth in macroscopic or recurrently produced trace quantities. The three said exceptions do exist naturally, but only in trace amounts as the result of rare nuclear processes from decay of heavy elements. Every element through Copernicus, element 112, has been isolated, characterized, and named, and elements 113 through 118 have been synthesized in laboratories around the world.

While plutonium is now included among the 91 regularly occurring natural elements, and technetium, promethium, and neptunium also occur naturally in transient trace amounts, these four elements were first identified and characterized from technologically produced samples. Numerous synthetic radio nuclides of various naturally occurring elements have been produced as well.

Production of additional synthetic elements beyond atomic number 118 is being pursued; whether the next elements will neatly fill an eighth period or require modifications to the overall patterns of the present periodic table remains unknown.

Organizing principles

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. 111

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.

Elements, natural and synthetic.

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 un untrium, ununquadium, or un unpentium, 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.

Atomic number.

By definition, each chemical element has a unique atomic number, the number of protons in its nucleus. Different atoms of many elements have different numbers of neutrons, which differentiates between isotopes of an element. For example, all atoms of hydrogen have one proton, and no atoms of any other element have exactly one proton. On the other hand, a hydrogen atom can have one or two neutrons in its nucleus, or none at all, yet all of these cases are isotopes of hydrogen, not instances of some other element. (A hydrogen atom with no neutrons in addition to its sole proton is called protium, one with one neutron in addition to its proton is called deuterium, and one with two additional neutrons, tritium.

In the modern periodic table, the elements are placed progressively in each row (period) from left to right in the sequence of their atomic numbers, with each new row starting with the next atomic number following the last number in the previous row. No gaps or duplications exist. Since the elements can be uniquely sequenced by atomic number, conventionally from lowest to highest , sets of elements are sometimes specified by such notation as "through", "beyond", or "from ... through", as in "through iron", "beyond uranium", or "from lanthanum through lutetium". The terms "light" and "heavy" are sometimes also used informally to indicate relative atomic numbers (not densities!), as in "lighter than carbon" or "heavier than lead", although technically the weight or mass of atoms of an element (their atomic weights or atomic masses) do not always increase monotonically with their atomic numbers.

The significance of atomic numbers to the organization of the periodic table was not appreciated until the existence and properties of protons and neutrons became understood. Mendeleev's periodic tables instead used atomic weights, information determinable to fair precision in his time, which worked well enough in most cases to give a powerfully predictive presentation far better than any other comprehensive portrayal of the chemical elements' properties.