ELEMENTS, CHEMICAL PreviousNext


ELEMENTS, CHEMICAL, substances that cannot be decomposed, or broken into more elementary substances, by ordinary chemical means. Elements were at one time believed to be the fundamental substances but are now known to consist of a number of different Elementary Particles (see also CHEMISTRY; ELECTRON; NEUTRON; PROTON).

The total number of known chemical elements is 115. The naturally occurring elements are those with atomic numbers from 1 (hydrogen) to 92 (uranium). Elements with atomic numbers higher than 92 are called Transuranium Elements. Except for traces of neptunium (at.no. 93) and plutonium (at.no. 94), the rest of all the known transuranium elements (23 as of 1999) have been produced artificially by bombarding the atomic nuclei of other elements with charged nuclei or nuclear particles. Such bombardment can take place in an accelerator such as the cyclotron, in a nuclear reactor, or in a nuclear explosion (see Particle Accelerators).

Chemical elements are classified as metals and nonmetals. The atoms of metals are electropositive and combine readily with the electronegative atoms of the nonmetals. A group of elements called metalloids, intermediate in properties between the metals and the nonmetals, are sometimes considered a separate class. When the elements are arranged in the order of their atomic numbers (a number proportional to the net positive charge-the proton-on the nucleus of an atom of an element), elements of similar physical and chemical properties occur at specific intervals (see Periodic Law). These groups of elements with similar physical and chemical properties are called families, examples of which are the Alkaline Earth Metals, Rare Earth Elements, Halogens, and the Noble Gases.

The unit for atomic weight of the elements-the atomic mass unit (amu)-is one-twelfth of the weight of the carbon-12 atom, the standard by which atomic weights or masses of other elements are computed (see Atom And Atomic Theory).

CHEMICAL ELEMENTS

Name

Symbol

Atomic Number

Atomic Weight

Actinium

Ac

89

[227]

Aluminum

Al

13

26.98

Americium

Am

95

[243]

Antimony

Sb

51

121.75

Argon

Ar

18

39.95

Arsenic

As

33

74.92

Astatine

At

85

[210]

Barium

Ba

56

137.34

Berkelium

Bk

97

[247]

Beryllium

Be

4

9.01

Bismuth

Bi

83

208.98

Bohrium (formerly Nielsbohrium)

Bh

107

[264]

Boron

B

5

10.81

Bromine

Br

35

79.90

Cadmium

Cd

48

112.41

Calcium

Ca

20

40.08

Californium

Cf

98

[251]

Carbon

C

6

12.01

Cerium

Ce

58

140.12

Cesium

Cs

55

132.91

Chlorine

Cl

17

35.45

Chromium

Cr

24

51.996

Cobalt

Co

27

58.93

Copper

Cu

29

63.55

Curium

Cm

96

[247]

Dubnium (formerly Hahnium)

Du

105

[260]

Dysprosium

Dy

66

162.50

Einsteinium

Es

99

[254]

Erbium

Er

68

167.26

Europium

Eu

63

151.96

Fermium

Fm

100

[257]

Fluorine

F

9

18.99

Francium

Fr

87

[223]

Gadolinium

Gd

64

157.25

Gallium

Ga

31

69.72

Germanium

Ge

32

72.59

Gold

Au

79

196.97

Hafnium

Hf

72

178.49

Hahnium (see Dubnium)

Ha

   

Hassium

Hs

108

[269]

Helium

He

2

4.0026

Holmium

Ho

67

164.93

Hydrogen

H

1

1.01

Indium

In

49

114.82

Iodine

I

53

126.90

Iridium

Ir

77

192.22

Iron

Fe

26

55.85

Krypton

Kr

36

83.30

Lanthanum

La

57

138.91

Lawrencium

Lr

103

[262]

Lead

Pb

82

207.19

Lithium

Li

3

6.94

Lutetium

Lu

71

174.97

Magnesium

Mg

12

24.31

Manganese

Mn

25

54.94

Meitnerium

Mt

109

[268]

Mendelevium

Md

101

[258]

Mercury

Hg

80

200.59

Molybdenum

Mo

42

95.94

Neodymium

Nd

60

144.24

Neon

Ne

10

20.18

Neptunium

Np

93

[237]

Nickel

Ni

28

58.70

Nielsborhium (see Bohrium)

Ns

   

Niobium

Nb

41

92.91

Nitrogen

N

7

14.01

Nobelium

No

102

[259]

Osmium

Os

76

190.20

Oxygen

O

8

15.999

Palladium

Pd

46

106.40

Phosphorus

P

15

30.97

Platinum

Pt

78

195.09

Plutonium

Pu

94

[244]

Polonium

Po

84

[209]

Potassium

K

19

39.10

Praseodymium

Pr

59

140.91

Promethium

Pm

61

[145]

Protactinium

Pa

91

231.04

Radium

Ra

88

226.03

Radon

Rn

86

[222]

Rhenium

Re

75

186.21

Rhodium

Rh

45

102.91

Rubidium

Rb

37

85.47

Ruthenium

Ru

44

101.07

Rutherfordium

Rf

104

[261]

Samarium

Sm

62

150.35

Scandium

Sc

21

44.96

Seaborgium

Sg

106

[266]

Selenium

Se

34

78.96

Silicon

Si

14

28.09

Silver

Ag

47

107.868

Sodium

Na

11

22.99

Strontium

Sr

38

87.62

Sulfur

S

16

32.06

Tantalum

Ta

73

180.95

Technetium

Tc

43

[99]

Tellurium

Te

52

127.60

Terbium

Tb

65

158.93

Thallium

Tl

81

204.37

Thorium

Th

90

232.04

Thulium

Tm

69

168.93

Tin

Sn

50

118.69

Titanium

Ti

22

47.90

Tungsten

W

74

183.85

Uranium

U

92

238.03

Vanadium

V

23

50.94

Xenon

Xe

54

131.30

Ytterbium

Yb

70

173.04

Yttrium

Y

39

88.91

Zinc

Zn

30

65.37

Zirconium

Zr

40

91.22

*

 

110

[273]

*

 

111

[272]

*

 

112

[277]

**

 

113

 

*

 

114

[285]

**

 

115

 

*

 

116

[173]

**

 

117

 

*

 

118

[175]

[ ] Atomic weight of the longest lived isotope of that element.

* Element not yet named.

** Element yet to be discovered.

When two atoms have the same atomic number, but different atomic weights, they are said to be isotopes. Many natural isotopes are known for some elements, whereas other elements occur in only one isotopic form. Hundreds of synthetic isotopes have been made. Some natural isotopes, and many synthetic ones, are unstable (see Isotope; Radioactivity).

All transuranium elements are radioactive and have very short half-lives. Although physicists theorized as early as the 1960s that a number of superheavy elements with a higher degree of stability may exist, such elements were not produced until the 1990s.        S.Z.L.

For further information on this topic, see the Bibliography, sections General chemistry, Chemical elements.