Periodic Table

The Periodic Table is a chart which organises chemical elements into groups based on their various properties - ascending by atomic number and grouped by things such as reactivity and valency. While it was first organised by these macroscopic properties, it actually forms a map of the atomic orbital structure indicating how quantum mechanical concepts relate to chemical properties. The Table was invented by Russian chemist Dmitri Mendeleyev in the second half of the 19th century, though others were part of a push towards order in the chemical world.

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For more information, see: List of elements in the periodic table

Origins

Chemical behaviour is generally very messy, complicated and irregular.[1] Chemists, in contrast, enjoy making neat and tidy lists of things. By the 17th century, chemists had begun to use the concept of an "element" in its modern sense (i.e. a substance which cannot be broken down into a simpler substance using chemical means) - which was nice and tidy - but, distressingly, they had also begun to discover dozens of new elements besides those which had been known for centuries - this was distressing untidy. A glimmer of hope was at hand, however: it was noticed that many of these newly discovered elements appeared to exhibit similar properties, seeming almost to belong to 'families'. Moreover, these similar properties often seemed to exhibit trends when the elements were placed in order of atomic weight. Examples included "triads" such as chlorine, bromine and iodine, and copper, silver and gold.

The race was on amongst chemists to draw up some kind of nice tidy list which would arrange the ever-increasing numbers of these pesky new elements in some kind of elegant way which brought order to the increasing chemical mess. The breakthrough was finally made by Mendeleev. His Table managed to set out the elements in rows and columns, in order (more or less) of atomic weight, so that those elements with similar properties fell into nice tidy vertical groups. Mendeleev made two particular innovations over previous pedants:

• in a couple of cases where listing the elements in order of atomic weight would have messed up his groups, he swapped elements round so that they fitted the pattern
• in further cases where listing the elements in any order wouldn't fit the pattern at all, he declared there must be a "missing element" and left a gap for it in the table.

However, Mendeleev couldn't, at the time, provide any physical explanation of why there should be missing elements, or why atomic weights occasionally needed swapping around. The trends were evident, but the knowledge of atomic structure and the identity of the missing elements was still far off, meaning that there was no verifiable proof that the periodic table was as Mendeleev hypothesised. Still, he stuck to his hypothesis, and the testable predictions of what properties these missing elements ought to have if and when they were discovered. Luckily for him, his predictions were correct!

Later, the discovery of atomic structure (protons, neutrons and electrons) provided a sound rationalising for Mendeleev's quirky ad hoc tweaks, and vindicated his periodic table. It was discovered that the shared properties of similar elements was due to these their having the same number of electrons in their outer shells. Thus, silicon has similar reactivities to carbon, although their difference in physical size also affects these properties. These observed facts are combined in their arrangement in the modern Periodic Table of the Elements.

Elements in the table are sorted by the increasing number of protons in their nuclei, from 1 to somewhere around 120. The structure of the table is based on the electronic structure of the atom: in the "p" block for example, there are elements for which the outermost electron is in a "p" orbital.

And here it is!

Group #	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
Period
1	1 H Hydrogen																	2 He Helium
2	3 Li Lithium	4 Be Beryllium											5 B Boron	6 C Carbon	7 N Nitrogen	8 O Oxygen	9 F Fluorine	10 Ne Neon
3	11 Na Sodium	12 Mg Magnesium											13 Al Aluminium	14 Si Silicon	15 P Phosphorus	16 S Sulfur	17 Cl Chlorine	18 Ar Argon
4	19 K Potassium	20 Ca Calcium	21 Sc Scandium	22 Ti Titanium	23 V Vanadium	24 Cr Chromium	25 Mn Manganese	26 Fe Iron	27 Co Cobalt	28 Ni Nickel	29 Cu Copper	30 Zn Zinc	31 Ga Gallium	32 Ge Germanium	33 As Arsenic	34 Se Selenium	35 Br Bromine	36 Kr Krypton
5	37 Rb Rubidium	38 Sr Strontium	39 Y Yttrium	40 Zr Zirconium	41 Nb Niobium	42 Mo Molybdenum	43 Tc Technetium	44 Ru Ruthenium	45 Rh Rhodium	46 Pd Palladium	47 Ag Silver	48 Cd Cadmium	49 In Indium	50 Sn Tin	51 Sb Antimony	52 Te Tellurium	53 I Iodine	54 Xe Xenon
6	55 Cs Caesium	56 Ba Barium	*	72 Hf Hafnium	73 Ta Tantalum	74 W Tungsten	75 Re Rhenium	76 Os Osmium	77 Ir Iridium	78 Pt Platinum	79 Au Gold	80 Hg Mercury	81 Tl Thallium	82 Pb Lead	83 Bi Bismuth	84 Po Polonium	85 At Astatine	86 Rn Radon
7	87 Fr Francium	88 Ra Radium	**	104 Rf Rutherfordium	105 Db Dubnium	106 Sg Seaborgium	107 Bh Bohrium	108 Hs Hassium	109 Mt Meitnerium	110 Ds Darmstadtium	111 Rg Roentgenium	112 Cn Copernicium	113 Nh Nihonium	114 Fl Flerovium	115 Mc Moscovium	116 Lv Livermorium	117 Tn Tennessine	118 Og Oganesson
*			57 La Lanthanum	58 Ce Cerium	59 Pr Praseodymium	60 Nd Neodymium	61 Pm Promethium	62 Sm Samarium	63 Eu Europium	64 Gd Gadolinium	65 Tb Terbium	66 Dy Dysprosium	67 Ho Holmium	68 Er Erbium	69 Tm Thulium	70 Yb Ytterbium	71 Lu Lutetium
**			89 Ac Actinium	90 Th Thorium	91 Pa Protactinium	92 U Uranium	93 Np Neptunium	94 Pu Plutonium	95 Am Americium	96 Cm Curium	97 Bk Berkelium	98 Cf Californium	99 Es Einsteinium	100 Fm Fermium	101 Md Mendelevium	102 No Nobelium	103 Lr Lawrencium

Metals						Metalloids	Nonmetals			Unknown chemical properties
Alkali metals	Alkaline earth metals	Inner transition metals		Transition metals	Post-transition metals		Other nonmetals	Halogens	Noble gases
		Lanthanides	Actinides

External links

• Interactive version
• The world's most awesome Periodic Table display.
• The Periodic Table Song
• It should really look like this.

References

1. More specifically:
   • Inorganic Chemist: "Inorganic chemistry is messy, complicated and irregular and therefore interesting. Organic chemistry is boring and predictable."
   • Organic Chemist: "Organic chemistry is messy, complicated and irregular and therefore interesting. Inorganic chemistry is boring and predictable."