Periodic table

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The periodic table of the chemical elements is a tabular display of the chemical elements. It is perhaps the icon of Chemistry and expresses much about the physical and chemical properties of the known elements.

History

Main article: History of the periodic table

The original table was created without a knowledge of the inner structure of atoms, but rather by correlating physical and chemical properties of the elements with atomic mass. If the elements are ordered by atomic mass then a certain periodicity, or regular repetition, of physical and chemical properties can be observed. 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
Element Molar mass
(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 German Julius Lothar Meyer and the Russian chemistry professor Dmitri Ivanovich Mendeleev almost simultaneously 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 neighbours in the table. He also 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. The modern table is based on this understanding of the electronic structures.

In 1913, Henry Moseley rearranged the table according to atomic number to improve the observed periodicity in the chemical properties across the table. Todays table uses this ordering by atomic number (number of protons). Mendeleev's and Moseley's development of the periodic table was one of the greatest achievements in modern chemistry. Chemists were able to qualitatively explain the behavior of the elements, and to predict the existence of yet undiscovered ones.

In the 1940s Glenn T. Seaborg identified the transuranic lanthanides and the actinides, which may be placed within the table, or below (see the different possible arrangements below).

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
Chemical Series of the Periodic Table
Alkali metals Alkaline earth metals Lanthanides Actinides Transition metals
Poor metals Metalloids Nonmetals Halogens Noble gases


State at standard temperature and pressure

  • Elements numbered in red are gases.
  • Elements numbered in green are liquids.
  • Elements numbered in black are solids.

Natural occurrence

  • Elements without borders have not been discovered/synthesized yet.
  • Elements with dotted borders do not occur naturally (synthetic elements).
  • Elements with dashed borders naturally arise from decay of other chemical elements.
  • Elements with solid borders are older than the Earth (primordial elements).
    • Note: Although californium (Cf, 98) is not Earth-primordial, it (and its decay products) does occur naturally: its electromagnetic emissions are regularly observed in supernova spectra.

Other depictions

Structure of the Table

Each element appears in a box which contains the symbol of the element and its atomic number. Many tables also include the atomic mass, and some have additional information as well. The fundamental ordering of the elements is as a list according to their atomic number (number of protons). 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 laboratories. Following this basic order the elements are arranged in a table that contains specific columns and rows, knowns as groups and periods respectively.

Groups

The columns of the table are known as groups or families. All the elements in a group have similar properties. Placing elements in groups is one of the most important ways of classifying them. There is some variation in properties within a group, but the changes are relatively small and tend to vary continuously as you go down the group.

There are three ways of numbering the groups of the periodic table, one using Hindu-Arabic numerals and the other two using Roman numerals. The Roman numeral names are the original traditional names of the groups; the Arabic numeral names are those recommended by the International Union of Pure and Applied Chemistry (IUPAC) to replace the old names in an attempt to reduce the confusion generated by the two older, but mutually confusing, schemes.

There is considerable confusion surrounding the two old systems in use (old IUPAC and CAS) that combined the use of Roman numerals with letters. In the old IUPAC system the letters A and B were designated to the left (A) and right (B) part of the table, while in the CAS system the letters A and B were designated to main group elements (A) and transition elements (B). The former system was frequently used in Europe while the latter was most common in America. The new IUPAC scheme was developed to replace both systems as they confusingly used the same names to mean different things.


The periodic table groups are as follows (in the brackets are shown the old systems: European and American):


However, properties vary also with periods. This is because, with every new period, a new, full valence shell is added. This results in larger atoms, which can be polarised more easily and can disperse a ionic charge more efficiently. Inside a group, properties , without radical changes.

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 unreactive, monoatomic gases. 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, known as the halogens, elements are missing just one electron to fill their shell. Therefore, in chemical reactions they tend to acquire electrons (this 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 to disperse the charge.

Transition metals

In transition metals (groups from 3 to 12), things are not as clear-cut. The differences between groups are usually not very dramatic, and the reactions can be much more complicated. However, it is still possible to make useful predictions.

Lantanides and actinides

The chemical properties of these elements is even more similar 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.



The modern periodic table is arranged so that elements in each column of the table have the same number of valence electrons. Every time a valence shell is filled, a new row is started. Since the more external valence shells can accomodate many more electrons, the lower rows of the table are much wider.

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 position on the table. Properties, however, vary differently when moving vertically or horizontally.


Periodic table structure reflects electron configuration

The primary determinant of an element's chemical properties is its electron configuration, particularly the valence shell electrons. For instance, all atoms whose four valence electrons are found on the p shell will behave similarly, regardless of which energy level that last p shell is on. The shell in which the atom's outermost electrons reside determines the "block" to which it belongs. The number of valence shell electrons determines which family, or group, 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: 

1s 
2s              2p
3s              3p
4s          3d  4p
5s          4d  5p
6s      4f  5d  6p
7s      5f  6d  7p
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.

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.


Further resources

  • [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).

See also

  • 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

External links

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Periodic tables

Standard table | Vertical table | Table with names | Names and atomic masses (large) | Names and atomic masses (small) | Names and atomic masses (text only) | Inline F-block | Elements to 218 | Electron configurations | Metals and non metals | Table by blocks | List of elements by name
Groups:   1 -  2 -  3 -  4 -  5 -  6 -  7 -  8 -  9 - 10 - 11 - 12 - 13 - 14 - 15 - 16 - 17 - 18
Periods:  1  -  2  -  3  -  4  -  5  -  6  -  7  -  8
Series:   Alkalis  -  Alkaline earths  -  Lanthanides  -  Actinides  -  Transition metals  -  Poor metals  -  Metalloids  -  Nonmetals  -  Halogens  -  Noble gases
Blocks:  s-block  -  p-block  -  d-block  -  f-block  -  g-block
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List of biomolecules • List of inorganic compounds • List of organic compounds • Periodic table
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