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Periodic table

2007 Schools Wikipedia Selection. Related subjects: General Chemistry

   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 layouts are possible to emphasize different aspects of
   behaviour; 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
   behaviour. The table has also found wide application in physics,
   biology, engineering, and industry. The current standard table contains
   117 confirmed elements as of October 16, 2006 (while element 118 has
   been synthesized, element 117 has not).

Methods for displaying the periodic table

Standard periodic table

    Group →  1  2  3   4  5  6  7  8  9 10 11 12  13  14  15  16  17  18
   ↓  Period
       1     1
             H
                                                                      2
                                                                      He
       2     3
             Li 4
                Be
                                                  5
                                                  B   6
                                                      C   7
                                                          N   8
                                                              O   9
                                                                  F   10
                                                                      Ne
       3     11
             Na 12
                Mg
                                                  13
                                                  Al  14
                                                      Si  15
                                                          P   16
                                                              S   17
                                                                  Cl  18
                                                                      Ar
       4     19
             K  20
                Ca 21
                   Sc 22
                      Ti 23
                         V  24
                            Cr 25
                               Mn 26
                                  Fe 27
                                     Co 28
                                        Ni 29
                                           Cu 30
                                              Zn  31
                                                  Ga  32
                                                      Ge  33
                                                          As  34
                                                              Se  35
                                                                  Br  36
                                                                      Kr
       5     37
             Rb 38
                Sr 39
                   Y  40
                      Zr 41
                         Nb 42
                            Mo 43
                               Tc 44
                                  Ru 45
                                     Rh 46
                                        Pd 47
                                           Ag 48
                                              Cd  49
                                                  In  50
                                                      Sn  51
                                                          Sb  52
                                                              Te  53
                                                                  I   54
                                                                      Xe
       6     55
             Cs 56
                Ba *
                      72
                      Hf 73
                         Ta 74
                            W  75
                               Re 76
                                  Os 77
                                     Ir 78
                                        Pt 79
                                           Au 80
                                              Hg  81
                                                  Tl  82
                                                      Pb  83
                                                          Bi  84
                                                              Po  85
                                                                  At  86
                                                                      Rn
       7     87
             Fr 88
                Ra **
                      104
                      Rf 105
                         Db 106
                            Sg 107
                               Bh 108
                                  Hs 109
                                     Mt 110
                                        Ds 111
                                           Rg 112
                                              Uub 113
                                                  Uut 114
                                                      Uuq 115
                                                          Uup 116
                                                              Uuh 117
                                                                  Uus 118
                                                                      Uuo
   * Lanthanides      57
                      La 58
                         Ce 59
                            Pr 60
                               Nd 61
                                  Pm 62
                                     Sm 63
                                        Eu 64
                                           Gd 65
                                              Tb  66
                                                  Dy  67
                                                      Ho  68
                                                          Er  69
                                                              Tm  70
                                                                  Yb  71
                                                                      Lu
   ** Actinides       89
                      Ac 90
                         Th 91
                            Pa 92
                               U  93
                                  Np 94
                                     Pu 95
                                        Am 96
                                           Cm 97
                                              Bk  98
                                                  Cf  99
                                                      Es  100
                                                          Fm  101
                                                              Md  102
                                                                  No  103
                                                                      Lr

   Notes
     * Lanthanides are also known as " rare earth elements", a deprecated
       term. Regarding group membership of these elements, see here.
     * Alkali metals, alkaline earth metals, transition metals, actinides,
       lanthanides, and poor metals are all collectively known as
       "metals".
     * Halogens and noble gases are also non-metals.


   CAPTION: Chemical series of the periodic table

   Alkali metals Alkaline earth metals Lanthanides Actinides Transition
                                                             metals
   Poor metals   Metalloids            Nonmetals   Halogens  Noble gases

   CAPTION: State at standard temperature and pressure (0 °C and 1 atm)

   Gases Liquids Solids

   CAPTION: Natural occurrence

   Undiscovered Synthetic From decay Primordial

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 electronegativity

   Other alternative periodic tables exist as well.

Arrangement

   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 fit 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. Ninety are found naturally on Earth,
   and the rest are synthetic elements that have been produced
   artificially in particle accelerators. Elements 43 (technetium) and 61
   (promethium), although of lower atomic number than the naturally
   occurring element 92, uranium, are synthetic; elements 93 (neptunium)
   and 94 (plutonium) are listed with the synthetic elements, but have
   been found in trace amounts on earth.

Periodicity of chemical properties

   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

     * 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 — these groups tend
   to be given trivial (non-scientific) names, e.g. the alkali metals,
   alkaline earth 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), and these have no trivial names and are
   referred to simply by their group numbers. Modern quantum mechanical
   theories of atomic structure explain group trends by proposing that
   elements within the same group have the same electron configurations in
   their valence shell, which is the most important factor in accounting
   for their similar properties.
     * 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 lanthanides and actinides form
   two substantial horizontal series of elements.

Examples

Noble gases

   All the elements of Group 18, 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 much less reactive than other
   groups. 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 electron
   shells. However, their reactivity remains low in absolute terms.

Halogens

   In Group 17, known as the halogens, elements are missing just one
   electron each to fill their shells. 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,
   for example, with regard to iodine and fluorine, since a large I^- ion
   is more stable in solution than a small F^-, there is less volume in
   which to disperse the charge.

Transition metals

   In transition metals (Groups 3 to 12), 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
   concerning nuclear power.

Nonmetals

   A nonmetal is distinguished from a metal or metalloid on the basis of
   ionization and chemical bonding properties. Nonmetals are
   electronegative, meaning that they accept valence electrons from other
   atoms more readily than they give them up. There are 18 naturally
   occurring nonmetals including the halogens and noble gases. In general,
   nonmetals are not able to conduct electricity or heat very well (
   Graphite, an allotrope of carbon, is an exception). Unlike metals, the
   solid non-metallic elements are very brittle and cannot be rolled into
   wires or pounded into sheets. Many nonmetals are gases at room
   temperature and atmospheric pressure (for example oxygen) while bromine
   is one of only two elements in the periodic table that is liquid under
   such conditions (the other being the metal, mercury). The nonmetals
   have no metallic luster, and do not reflect light. They have oxidation
   numbers of ±4, -3, and -2.

Structure of the periodic table

   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 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

   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.

History

   In Ancient Greece, it was believed that there were four elements: air,
   fire, earth and water. All of these "elements" could be reacted to
   create another one; e.g., earth and fire combined to form lava.
   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:

   CAPTION: Some triads

    Element  Molar mass
             (g/mol)   Density
                       (g/cm³)  Quotient
                                (cm³/mol)
   chlorine  35.453    0.003214 11030
   bromine   79.904    3.122    25.6
   iodine    126.90447 4.93     25.7

   calcium   40.078    1.55     26.0
   strontium 87.62     2.54     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). Element 106, seaborgium, is the only element that was
   named after a living person. (Seaborg has since died.)

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