Periodic table
See the standard periodic table below.The
periodic table of the chemical elements is a
tabular method of displaying the
chemical elements, first devised in
1869 by the
Russian
chemist Dmitri Mendeleev. Mendeleev intended the table to illustrate recurring ("periodic") trends in the properties of the elements. The layout of the table has been refined and extended over time, as many new elements have been discovered since Mendeleev's time, and new theoretical models have been developed to explain chemical behavior. Various different layouts are possible to emphasize different aspects of behavior; the most common forms, however, are still quite similar to Mendeleev's original design.
The periodic table is now ubiquitous within the academic discipline of
chemistry, providing an extremely useful framework to classify, systematize and compare all the many different forms of
chemical behavior. The table has also found wide application in
physics,
biology,
engineering, and
industry. The current standard table contains 116 confirmed elements, and two further speculated elements.
Earlier attempts to list the elements to show the relationships between them had usually involved putting them in order of
atomic mass. Mendeleev's key insight in devising the periodic table was to lay out the elements to illustrate recurring ("periodic") chemical properties (even if this meant some of them were not in mass order), and to leave gaps for "missing" elements. Mendeleev used his table to predict the properties of these "missing elements", and many of them were indeed discovered and fitted the predictions well.
With the development of theories of
atomic structure (for instance by
Henry Moseley) it became apparent that Mendeleev had listed the elements
in order of increasing atomic number (i.e. the number of
protons in the
atomic nucleus). This sequence is nearly identical to that resulting from ascending atomic mass.
In order to illustrate recurring properties, Mendeleev began new rows in his table so that elements with similar properties fell into the same vertical columns (
"groups").
With the development of modern
quantum mechanical theories of
electron configuration within atoms, it became apparent that each horizontal row (
"period") in the table corresponded to the filling of a quantum shell of electrons.In Mendeleev's original table, each period was the same length. Modern tables have progressively longer periods further down the table, and group the elements into
s-,
p-,
d- and
f-blocks to reflect our understanding of their electron configuration.
In printed tables, each element is usually listed with its
element symbol and
atomic number; many versions of the table also list the element's
atomic mass and other information, such as its abbreviated
electron configuration,
electronegativity and most common
valence numbers.
As of 2005, the table contains 116 chemical elements whose discoveries have been confirmed. 94 are found naturally on Earth, and the rest are
synthetic elements that have been produced artificially in
particle accelerators.
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.
Groups and Periods
Groups and Periods are the two main ways to view the
Periodic Table of the Elements. Take
Group 2 for example, all of the elements in that group have similar characteristics as the others.
Groups
*A
group, also known as a
family, is a vertical column in the periodic table of the elements.
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 â€" these groups tend to be given trivial (non-scientific) names, e.g. the
alkali metals,
alkaline earth metals,
transition metals,
halogens and
noble gases. Some other groups in the periodic table display fewer similarities and/or vertical trends (for example Groups 14 and 15). Modern
quantum mechanical theories of atomic structure explain that elements within the same group have the same electron configurations in their
valence shell, which is the largest factor in accounting for their similar chemical properties.
Periods
*A
period is a horizontal row in the periodic table of the elements.
Although groups are the most common way of classifying elements, there are some regions of the periodic table where the horizontal trends and similarities in properties are more significant than vertical group trends. This can be true in the
d-block (or "
transition metals"), and especially for the
f-block, where the
lanthanoids and
actinoids form two substantial horizontal series of elements. The period number also shows how many electron shells there are in an element.
Examples
Noble gases
All the elements of group 18 (8 or 0 if discounting transition elements), the
noble gases, have full valence shells. This means they do not need to react with other elements to attain a full shell, and are therefore unreactive.
Helium is the most inert element among noble gases, since reactivity, in this group, increases with the periods: it is possible to make heavy noble gases react since they have much larger electronic shells. However, their reactivity remains low in absolute terms.
Halogens
In group 17, (7 if discounting transition metals) known as the
halogens, elements are missing just one electron to fill their shell. Therefore, in chemical reactions they tend to acquire electrons (the tendency to acquire electrons is called
electronegativity). This property is most evident for
fluorine (the most electronegative element of the whole table), and it diminishes with increasing period.
As a result, all halogens form acids with hydrogen, such as
hydrofluoric acid,
hydrochloric acid,
hydrobromic acid and
hydroiodic acid, all in the form
HX. Their
acidity increases with higher period, since a large I
- ion is more stable in solution than a small F
-, that has less volume in which to disperse the charge.
Transition metals
In
transition metals (groups 3 to 12, see
transition metal), the differences between groups are usually not dramatic, and the reactions involve coordinated species. However, it is still possible to make useful predictions.
Lanthanides and actinides
The chemical properties of the
lanthanides (elements 57-71) and the
actinides (elements 89-103) are even more similar to each other than in
transition metals, and separating a mixture of these can be very difficult. This is important in the
chemical purification of
uranium, important for
nuclear power.
Standard periodic table
Other depictions
* The
standard table (same as above) provides the basics.
* A
vertical table for improved readability in web browsers.
* The
big table provides the basics and full element names.
* The
huge table provides the basics plus full element names and
atomic masses.
* A table with an
inline F-block inserts the
lanthanides and
actinides into their correct place in the table.
*
Electron configurations*
Metals and non-metals*
Periodic table filled by blocks*
Table in Chinese*
List of elements by name*
List of elements by symbol*
List of elements by atomic number*
List of elements by boiling point*
List of elements by melting point*
List of elements by density*
List of elements by atomic mass*
List of elements by electronegativityOther
alternative periodic tables exist as well.
The primary determinant of an element's chemical properties is its
electron configuration, particularly the
valence shell electrons. For instance, any atoms with four valence electrons occupying p orbitals will exhibit with some similarity. The type of orbital in which the atom's outermost electrons reside determines the "block" to which it belongs. The number of
valence shell electrons determines the family, or group, to which the element belongs.
The total number of
electron shells an atom has determines the period to which it belongs. Each shell is divided into different subshells, which as atomic number increases are filled in roughly this order (the
Aufbau principle):
| Subshell: | S | G | F | D | P |
| Period | | | | |
| 1 | 1s | | | |
| 2 | 2s | | | | 2p |
| 3 | 3s | | | | 3p |
| 4 | 4s | | | 3d | 4p |
| 5 | 5s | | | 4d | 5p |
| 6 | 6s | | 4f | 5d | 6p |
| 7 | 7s | | 5f | 6d | 7p |
| 8 | 8s | 5g | 6f | 7d | 8p |
| 9 | 9s | 6g | 7f | 8d | 9p |
Hence the structure of the table. Since the outermost electrons determine chemical properties, those with the same number of valence electrons are grouped together.
Progressing through a group from lightest element to heaviest element, the outer-shell electrons (those most readily accessible for participation in chemical reactions) are all in the same type of orbital, with a similar shape, but with increasingly higher energy and average distance from the nucleus. For instance, the outer-shell (or "valence") electrons of the first group, headed by
hydrogen all have one electron in an s orbital. In hydrogen, that s orbital is in the lowest possible energy state of any atom, the first-shell orbital (and represented by hydrogen's position in the first period of the table). In
francium, the heaviest element of the group, the outer-shell electron is in the seventh-shell orbital, significantly further out on average from the nucleus than those electrons filling all the shells below it in energy. As another example, both carbon and lead have four electrons in their outer shell orbitals.
Note that as
atomic number (i.e. charge on the
atomic nucleus) increases, this leads to greater
spin-orbit coupling between the nucleus and the electrons, reducing the validity of the quantum mechanical
orbital approximation model, which considers each atomic orbital as a separate entity.
Because of the importance of the outermost shell, the different regions of the periodic table are sometimes referred to as
periodic table blocks, named according to the sub-shell in which the "last" electron resides, e.g. the
s-block, the
p-block, the
d-block, etc.
In Ancient Greece, it was believed that there were four elements. These comprised Air, Fire, Earth and Water. All of these elements could be reacted to create another one...eg. earth and fire made air. However, this theory was dismissed when the real chemical elements started being discovered. Scientists needed an easily accessible, well organized database through which the elements could be recorded and accessed. This was to be known as the periodic table.
The original table was created before the discovery of
subatomic particles or the formulation of current
quantum mechanical theories of
atomic structure.If one orders the elements by
atomic mass, and then plots certain other properties against atomic mass, one sees an undulation or
periodicity to these properties as a function of atomic mass.The first to recognize these regularities was the German chemist
Johann Wolfgang Döbereiner who, in 1829, noticed a number of
triads of similar elements:
Some triads| (g/mol)> | Density
(g/cm³) | Quotient
(cm³/mol) |
|---|
| chlorine | 35.4527 | 0.003214 | 11030 |
| bromine | 79.904 | 3.122 | 25.6 |
| iodine | 126.90447 | 4.93 | 25.7 |
| |
| calcium | 40.078 | 1.54 | 26.0 |
| strontium | 87.62 | 2.64 | 33.2 |
| barium | 137.327 | 3.594 | 38.2 |
This was followed by the English chemist
John Newlands, who noticed in 1865 that the elements of similar type recurred at intervals of eight, which he likened to the
octaves of music, though his
law of octaves was ridiculed by his contemporaries. Finally, in 1869 the Russian chemistry professor
Dmitri Ivanovich Mendeleev and four months later the German
Julius Lothar Meyer independently developed the first periodic table, arranging the elements by mass. However, Mendeleev plotted a few elements out of strict mass sequence in order to make a better match to the properties of their neighbors in the table, corrected mistakes in the values of several atomic masses, and predicted the existence and properties of a few new elements in the empty cells of his table. Mendeleev was later vindicated by the discovery of the electronic structure of the elements in the late
19th and early 20th century.
In the 1940s
Glenn T. Seaborg identified the
transuranic lanthanides and the actinides, which may be placed within the table, or below (as shown above).
* [
1] Scerri, E.R., references to several scholarly articles by this author.
* Mazurs, E.G.,
"Graphical Representations of the Periodic System During One Hundred Years". University of Alabama Press, Alabama. 1974.
* Bouma, J., An Application-Oriented Periodic Table of the Elements,
J. Chem. Ed.,
66, 741 (1989).
*
Atomic electron configuration table*
Isotope table (complete)*
Isotope table (divided)*
Discoveries of the chemical elements*
Abundance of the chemical elements*
Tom Lehrer's song The Elements*
IUPAC's
systematic element names
*
Cosmochemical Periodic Table of the Elements in the Solar System*
Table of chemical elements*
Extended periodic table*
"WebElements" periodic table
*
The IUPAC periodic table*
"Visual Elements". ChemSoc.org.
*
Periodic Table in Java - Java Applet of the Periodic Table of Elements.
*
Stephen Hawking's Universe - 03 - PBS documentary on the history of the periodic table and the cosmic evolution of the elements.
* Heilman, Chris,
"Alternate Layouts".
*
"Periodic table". Los Alamos National Laboratory's Chemistry Division.
* Dayah, Michael,
"Periodic Table". Large full-color scalable text-only rendering.
*
Colorful table with pictures of usages - Printable PDF with complete data and understandable pictures of usages
*
List of periodic tables.
*
The Wooden Periodic Table Table An actual table containing samples of each naturally occurring element.
*
Interactive periodic table with photos.
*
Detailed Periodic Table in Flashzh-yue:å…ƒç´ é€±æœŸè¡¨
