Dmitri Mendeleev was the first scientist to create a periodic table of the elements similar to the one we use today. You can see Mendeleev's original table (1869). This table showed that when the elements were ordered by increasing atomic weight, a pattern appeared where properties of the elements repeated periodically. This periodic table is a chart that groups the elements according to their similar properties. The current standard table contains 118 elements to date. The periodic table is now ubiquitous within the academic discipline of chemistry, providing a useful framework to classify, systematize, and compare all of the many different forms of chemical behavior. 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.

As of 2010, the table contains 118 chemical elements whose discoveries have been confirmed. Ninety-four are found naturally on Earth, and the rest are synthetic elements that have been produced artificially in particle accelerators. Elements 43 (technetium), 61 (promethium) and all elements greater than 83 (bismuth), beginning with 84 (polonium) have no stable isotopes. The atomic mass of each of these element's isotope having the longest half-life is typically reported on periodic tables with parentheses. Isotopes of elements 43, 61, 93 (neptunium) and 94 (plutonium), first discovered synthetically, have since been discovered in trace amounts on Earth as products of natural radioactive decay processes.

The primary determinant of an element's chemical properties is its electron configuration, particularly the valence shell electrons. For example, the noble gases are found in group 18, on the far right of each period. The reluctance of the noble gases to undergo chemical reactions indicates that the atoms of these gases strongly prefer their own electron configurations - featuring a full outer shell of electrons - to any other. In contrast to the noble gases, the elements with the highest reactivity are those with the greatest need to gain or lose electrons in order to achieve a full outer shell of electrons. Elements sitting in the same group (e.g. the alkali metals in Group 1) all have the same number of outer electrons, leading to similar chemical properties. Likewise the halogens in Group 17 also have similar properties to one another. When halogens react, they gain an electron to form negatively charged ions. Each ion has the same electron configuration as the noble gas in the same period. The ions are therefore more chemically stable than the elements from which they formed. There is a progression from metals to non-metals across each period. The block of elements in groups 3 - 12 contains the transition metals. These are similar to one another in many ways; they produce colored compounds, have variable valency and are often used as catalysts. The rare earth elements can be divided into lanthanides (elements 58 - 71) and actinides (elements 90 - 103). The naturally occurring rare earths are found on earth in only very small amounts. The actinides include most of the well-known elements that take part in or are produced by nuclear reactions. No element with atomic number higher than 92 occurs naturally in large quantities. Tiny amounts of plutonium and neptunium exist in nature as decay products of uranium. These elements, and higher elements, are also produced artificially in nuclear reactors or particle accelerators. Moreover, the elements ununtrium, ununquadium, ununpentium, etc. are elements that have been discovered, but so far have not received a trivial name yet. There is a system for naming them temporarily.

A group or family is a vertical column in the periodic table. Groups are considered the most important method of classifying the elements. In some groups, the elements have very similar properties and exhibit a clear trend in properties down the group. These groups tend to be given trivial (unsystematic) names, e.g., the alkali metals, alkaline earth metals, halogens, pnictogens, chalcogens, and noble gases. Some other groups in the periodic table display fewer similarities and/or vertical trends (for example Group 14), and these have no trivial names and are referred to simply by their group numbers.

As you are probably well aware, in the periodic table, elements are arranged in order of increasing atomic number. The 18 vertical columns of the table are called groups or families, while the seven horizontal rows are called periods and correspond to the seven principal quantum energy levels, n = 1 through n = 7.

On the right side of the periodic table is a dividing line resembling a staircase. To the left of the staircase lie the metals, and to the right of the staircase lie the nonmetals. Many of the elements that touch the staircase are called metalloids, and these exhibit both metallic and nonmetallic properties.

Metals are malleable, ductile, and have luster; most of the elements on the periodic table are metals. They oxidize (rust and tarnish) readily and form positive ions (cations). They are excellent conductors of both heat and electricity. The metals can be broken down into several groups.

Transition metals (also called the transition elements) are known for their ability to refract light as a result of their unpaired electrons. They also have several possible oxidation states. Ionic solutions of these metals are usually colored, so these metals are often used in pigments. The actinides and lanthanides are collectively called the rare earth elements and are filling the f orbitals. They are rarely found in nature. Uranium is the last naturally occurring element; the rest are man-made.

Nonmetals lie to the right of the staircase and do not conduct electricity well because they do not have free electrons. All the elemental gases are included in the nonmetals. Notice that hydrogen is placed with the metals because it has only one valence electron, but it is a nonmetal.

Here are some specific families you should know about, within the three main groups (metals, nonmetals, and metalloids): Alkali metals (1A)—The most reactive metal family, these must be stored under oil because they react violently with water! They dissolve and create an alkaline, or basic, solution, hence their name. Alkaline earth metals (2A)—These also are reactive metals, but they don’t explode in water; pastes of these are used in batteries. Halogens (7A)—Known as the “salt formers,” they are used in modern lighting and always exist as diatomic molecules in their elemental form. Noble gases (8A)—Known for their extremely slow reactivity, these were once thought to never react; neon, one of the noble gases, is used to make bright signs.

In 1789, Antoine Lavoisier published a list of 33 chemical elements. Although Lavoisier grouped the elements into gases, metals, non-metals, and earths, chemists spent the following century searching for a more precise classification scheme. The development of the periodic table begins with German chemist Johann Dobereiner (1780-1849) who grouped elements based on similarities. Calcium (atomic weight 40), strontium (atomic weight 88), and barium (atomic weight 137) possess similar chemical prepares. Dobereiner noticed the atomic weight of strontium fell midway between the weights of calcium and barium:
Ca Sr Ba (40 + 137) ÷ 2 = 88
40 88 137
Was this merely a coincidence or did some pattern to the arrangement of the elements exist? Dobereiner noticed the same pattern for the alkali metal triad (Li/Na/K) and the halogen triad (Cl/Br/I).
Li Na K Cl Br I
7 23 39 35 80 127
In 1829 Dobereiner proposed the Law of Triads: Middle element in the triad had atomic weight that was the average of the other two members. Soon other scientists found chemical relationships extended beyond triads. Fluorine was added to Cl/Br/I group; sulfur, oxygen, selenium and tellurium were grouped into a family; nitrogen, phosphorus, arsenic, antimony, and bismuth were classified as another group.

German chemist August Kekulé had observed in 1858 that carbon has a tendency to bond with other elements in a ratio of one to four. Methane, for example, has one carbon atom and four hydrogen atoms. This concept eventually became known as valency. In 1864, fellow German chemist Julius Lothar Meyer published a table of the 49 known elements arranged by valency. The table revealed that elements with similar properties often shared the same valency.

English chemist John Newlands (1837-1898), having arranged the 62 known elements in order of increasing atomic weights, noted that after interval of eight elements similar physical/chemical properties reappeared. Newlands was the first to formulate the concept of periodicity in the properties of the chemical elements. In 1863 he wrote a paper proposing the Law of Octaves: Elements exhibit similar behavior to the eighth element following it in the table.

Then in 1869, Russian chemist Dimitri Mendeleev (1834-1907) proposed arranging elements by atomic weights and properties (Lothar Meyer independently reached similar conclusion but published results after Mendeleev). Mendeleev's periodic table of 1869 contained 17 columns with two partial periods of seven elements each (Li-F & Na-Cl) followed by two nearly complete periods (K-Br & Rb-I). In 1871 Mendeleev revised the 17-group table with eight columns (the eighth group consisted of transition elements). This table exhibited similarities not only in small units such as the triads, but showed similarities in an entire network of vertical, horizontal, and diagonal relationships. The table contained gaps but Mendeleev predicted the discovery of new elements. In 1906, Mendeleev came within one vote of receiving the Nobel Prize in chemistry.

Lord Rayleigh (1842-1919) and William Ramsey (1852-1916) greatly enhanced the periodic table by discovering the "inert gases." In 1895 Rayleigh reported the discovery of a new gaseous element named argon. This element was chemically inert and did not fit any of the known periodic groups. Ramsey followed by discovering the remainder of the inert gases and positioning them in the periodic table. So by 1900, the periodic table was taking shape with elements were arranged by atomic weight. For example, 16g oxygen reacts with 40g calcium, 88g strontium, or 137g barium. If oxygen used as the reference, then Ca/Sr/Ba assigned atomic weights of 40, 88, and 137 respectively. Rayleigh (physics) and Ramsey (chemistry) were awarded Nobel prizes in 1904. The first inert gas compound was made in 1962 (xenon tetrafluoride) and numerous compounds have followed (see xenon compounds)--today the group is more appropriately called the noble gases.

Soon after Rutherford's landmark experiment of discovering the proton in 1911, Henry Moseley (1887-1915) subjected known elements to x-rays. He was able to derive the relationship between x-ray frequency and number of protons. When Moseley arranged the elements according to increasing atomic numbers and not atomic masses, some of the inconsistencies associated with Mendeleev's table were eliminated. The modern periodic table is based on Moseley's Periodic Law (atomic numbers). At age 28, Moseley was killed in action during World War I and as a direct result Britain adopted the policy of exempting scientists from fighting in wars.

The last major change to the periodic table resulted from Glenn Seaborg's work in the middle of the 20th century. Starting with plutonium in 1940, Seaborg discovered transuranium elements 94 to 102 and reconfigured the periodic table by placing the lanthanide/actinide series at the bottom of the table. In 1951 Seaborg was awarded the Nobel Prize in chemistry and element 106 was later named seaborgium (Sg) in his honor.

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. And the table has found many applications in chemistry, physics, biology, and engineering, especially chemical engineering.