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# Periodic table

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One can present data about the physical properties of the elements in various ways. The Periodic table of the chemical elements is a display of the known chemical elements, arranged by electron structure so that many chemical properties vary regularly across the table.

The original table was drawn up with no knowledge of the inner structure of atoms: if one orders the elements by atomic weight, and then plots certain other properties against atomic weight, one sees an undulation or periodicity to these properties as a function of atomic mass. These regularities had been recognised by others, but the russian chemist Dmitry Ivanovich Mendeleev developed the first periodic table, arranging the elements by mass (though he plotted a few elements out of strict mass sequence in to make a better match to the properties of their neighbours in the table - this was later vindicated by the discovery of the electronic structure of the elements in the late 19th and early 20th century. (see also atomic number)

Strictly speaking, we are referring here only to the chemical table as a "periodic table", but it is just one type of periodic table, more specifically, it is a periodic table of the elements . Omission of the important qualifier "of the elements" contributes perhaps to confusion over what is meant by "periodic table." As a counterexample, a chart of temperature or daylight hours versus the day of the year for someplace in the earth's temperate regions would also show periodicity, and hence could be termed a periodic table)

The following figure shows the currently known periodic table (rotated 90 degrees counter-clockwise from its usual representation, which makes it easier to render in text) Each element is listed by its chemical symbol, followed by its atomic number:

Key:

| Lanthanides?
| Actinides?
| [Transition metals]?

                   Period

1 | 2 | 3 | 4 | 5 | 6 | 7 | Group
H 1 | Li 3 | Na 11 | K 19 | Rb 37 | Cs 55 | Fr 87 | 1 (IA) | Alkali metals
| Be 4 | Mg 12 | Ca 20 | Sr 38 | Ba 56 | Ra 88 | 2 (IIA) | [Alkaline earths]?
|  |  |  |  | La 57 | Ac 89
|  |  |  |  | Ce 58 | Th 90
|  |  |  |  | Pr 59 | Pa 91
|  |  |  |  | Nd 60 | U 92
|  |  |  |  | Pm 61 | Np 93
|  |  |  |  | Sm 62 | Pu 94
|  |  |  |  | Eu 63 | Am 95
|  |  |  |  | Gd 64 | Cm 96
|  |  |  |  | Tb 65 | Bk 97
|  |  |  |  | Dy 66 | Cf 98
|  |  |  |  | Ho 67 | Es 99
|  |  |  |  | Er 68 | Fm 100 |
|  |  |  |  | Tm 69 | Md 101
|  |  |  |  | Yb 70 | No 102
|  |  | Sc 21 | Y 39? | Lu 71 | Lr 103 | 3 (IIIB)
|  |  | Ti 22 | Zr 40 | Hf 72 | Unq 104 | 4 (IVB)
|  |  | V 23 | Nb 41 | Ta 73 | Unp 105 | 5 (VB)
|  |  | Cr 24 | Mo 42 | W 74 | Unh 106 | 6 (VIB)
|  |  | Mn 25 | Tc 43 | Re 75 | Uns 107 | 7 (VIIB)
|  |  | Fe 26 | Ru 44 | Os 76 | Uno 108 | 8 (VIIIB)
|  |  | Co 27 | Rh 45 | Ir 77 | Une 109 | 9 (VIIIB)
|  |  | Ni 28 | Pd 46 | Pt 78 |  | 10 (VIIIB)
|  |  | Cu 29 | Ag 47 | Au 79 |  | 11 (IB)
|  |  | Zn 30 | Cd 48 | Hg 80 |  | 12 (IIB)
| B 5 | Al 13 | Ga 31 | In 49 | Tl 81 |  | 13 (IIIA)
| C 6 | Si 14 | Ge 32 | Sn 50 | Pb 82 |  | 14 (IVA) |
| N 7 | P 15 | As 33 | Sb 51 | Bi 83 |  | 15 (VA)
| O 8 | S 16 | Se 34 | Te 52 | Po 84 |  | 16 (VIA) | Chalcogens
| F 9 | Cl | Br 35 | I 53 | At 85 |  | 17 (VIIA) | Halogens
He 2 | Ne 10 | Ar 18 | Kr 36 | Xe 54 | Rn 86 |  | 18 (VIIIA) | Noble gases

(Here is the table in its usual form: /Alternate Table)

The number of electron shells an atom has determines what period it belongs to. 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 tend to be similar within groups.

See also http://www.webelements.com, which has more stuff...at the moment.

Could we establish a practice of listing, for each chemical element, the basic information from the periodic table (atom weight, symbol, ...), a pointer to the entry periodic table, some information about the element, and information about its commercial use (e.g. a pointer to an article on the uranium industry, if there is an industry for this element).

I think a standard form is good but is easily stifling if followed too carefully, depending on the element. For instance for hydrogen it makes more sense to discuss the pure form first, but for carbon second (because its structure makes more sense when compared to hydrocarbons). Some elements need some things that other should lack. So we could, but I think we should be careful.

Elements adjacent to one another within a group have similar physical properties, despite their significant differences in mass.

Elements adjacent to one another within a period have similar mass but different properties.

For example, very near to nitrogen in the second period of the chart are carbon and oxygen. Despite their similarities in mass (they differ by only a few atomic units), they have extremely difference properties, as can be seen by looking at their allotropes: diatomic oxygen is a gas that supports burning, diatomic nitrogen is a gas that does not support burning, and carbon is a solid which can be burnt (yes, diamonds can be burned!).

In contrast, very near to chlorine in the the next-to-last group in the chart are fluorine and bromine. Despite their dramatic differences in mass within the group, their allotropes have very similar properties: They are all highly corrosive (meaning they combine readily with metals to form MetalHalide? salts); chlorine and fluorine are gases, while bromine is a very low-boiling liquid; chlorine and bromine at least are highly colored (diatomic fluorine gas may be, too, though I'm not sure how many people have seen it--it's pretty nasty stuff I hear).

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Last edited August 8, 2001 2:51 am (diff)
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