Periodic table

The periodic table of the chemical elements, also called the Mendeleev periodic table, is a tabular display of chemical elements, first created in 1869 by Russian chemist Dmitri Mendeleev. The table has been extended as new elements were discovered, and is able to include any possible element.

Arrangement

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.

Each element is listed with at least the number of protons in its nucleus (the atomic number) and its element symbol; many versions of the table also include 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 laboratories.

The elements were originally arranged by the number of nucleons (approximately atomic mass). Mendeleev shuffled cards with the elements' names and properties until he found a pattern. In 1913, Henry Moseley rearranged the table according to atomic number so that many chemical properties followed a regular pattern across the table. 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.

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.

Groups and periods

A periodic table group, also known as a family, is a vertical column in the periodic table of the elements. A row is instead known as a period. Elements in a group have the same external electronic configurations in their valence shell, which gives them similar properties.

Adding or subtracting an electron from the shell has dramatic consequences for the chemical properties, that can change radically from one element to the next. Therefore, groups are considered the most important way of classifying the elements.

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 tend to vary continuously with the periods, 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.

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

• 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 back into the table.
• Electron configurations
• Metals and non-metals
• Periodic table filled by blocks
• List of elements by name with atomic number and atomic mass
• List of elements by electronegativity

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.

History

Main article: History of the periodic table

The original table was created without a knowledge of the inner structure of atoms: 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

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, 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).

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

Wikimedia Commons has media related to:

Periodic table

• "Periodic table (professional edition)". WebElements.
• The IUPAC periodic table
• "Visual Periodic Table". ChemSoc.org.
• Counterman, Craig, "Periodic Table of the Elements : For each of many properties a separate periodic table and a graph showing the relation with the atomic number". MIT Course 3.091.
• Heilman, Chris, "The Pictorial Periodic Table". (Includes alternate styles: Stowe, Benfey, Zmaczynski, Giguere, Tarantola, Filling, Mendeleev)
• "Periodic table". Los Alamos National Laboratory's Chemistry Division.
• Dayah, Michael, "Periodic Table". Large full-color scalable text-only rendering.
• The periodic table for magnetic resonance.
• The Periodic Table of Comic Books - clicking on elements brings up comic book panels dealing with that element.
• The Periodic Table of Haiku - clicking on an element brings up a haiku of its properties.

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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