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Chemical Elements

by Ron Kurtus (revised 4 January 2016)

A chemical element (usually just called an element) is a class of atoms with a specific number of protons in their nuclei (plural of nucleus in Latin). Each element has its own name and is usually listed according to its atomic number.

Isotopes of an element have different numbers of neutrons. Often the average atomic weight of an element is also stated. This number takes into account the percentages of isotopes, the masses of the particles, and nuclear effects.

The average atomic weight is approximately the number of protons and neutrons of the most common isotope of the element.

Questions you may have include:

This lesson will answer those questions.



Atomic number

The elements are listed according to their atomic number. The atomic number is designated by the number of protons in the nucleus. For example, Hydrogen has one proton, Helium has two protons, Oxygen has eight protons, and so on.

Since the number of electrons equals the number of protons in an electrically stable atom, the atomic number determines many of the chemical characteristics of the element. This is shown in the Periodic Table.

(See Periodic Table for more information.)

Average atomic weight

The atomic weight of an atom was originally defined as a sum of its protons and neutrons. The unit of measurement is the atomic mass unit (amu or u).

Mass defect

Later, it was found that some mass is lost to binding energy required to hold the nucleus together. This is called the mass defect and is the principle behind nuclear energy, according the to the famous equation:

E = mc2

Thus the atomic weight of an individual atom is slightly different than the number of protons and neutrons.

Isotopes

An element has several different number of neutrons in its nucleus. Each is called an isotope of that element. For example, Oxygen typically has 8 protons and 8 neutrons in its nucleus, with an atomic weight of about 16 u. But there is a very small percentage of Oxygen atoms with 9 neutrons in their nuclei and atomic weight of approximately 17 u. There are even some atoms with 10 neutrons.

Thus for the element Oxygen, taking into account for the mass defect and averaging the atomic weight for all its isotopes, you get an average atomic weight of 15.9994 u for Oxygen.

Finding number of neutrons

Looking on the list of elements below, you will see that the Average Atomic Weight is not integer. You can find the number of neutrons in the most common and stable nucleus of an element by simply rounding off the atomic weight and subtracting the atomic number (number of protons).

For example, Magnesium (Mg) is number 12 and has an average atomic weight of 24.3050 u. This rounds off to 24. Thus the number of protons in the most common isotope of Magnesium is 24 - 12 = 12 neutrons.

(See Atomic Weight in Physical Science for more information.)

List of elements

Following is a list of all the elements, according to atomic number.

Elements with the weight in [brackets] are so unstable that scientists have not been able to accurately measure the weight. All of the elements after Uranium (number 92) are artificial and unstable.

An artificial element is one that is so unstable that it does not occur in nature. High energy atomic collisions can manufacture such an element. It immediately decays into a stable element.

Atomic Number

Symbol

Name

Average Atomic Weight
(u)

1

H

Hydrogen

1.00794

2

He

Helium

4.002602

3

Li

Lithium

6.941

4

Be

Beryllium

9.012182

5

B

Boron

10.811

6

C

Carbon

12.0107

7

N

Nitrogen

14.0067

8

O

Oxygen

15.9994

9

F

Fluorine

18.9984032

10

Ne

Neon

20.1797

11

Na

Sodium

22.989770

12

Mg

Magnesium

24.3050

13

Al

Aluminum

26.981538

14

Si

Silicon

28.0855

15

P

Phosphorus

30.973761

16

S

Sulfur

32.065

17

Cl

Chlorine

35.453

18

Ar

Argon

39.948

19

K

Potassium

39.0983

20

Ca

Calcium

40.078

21

Sc

Scandium

44.955910

22

Ti

Titanium

47.867

23

V

Vanadium

50.9415

24

Cr

Chromium

51.9961

25

Mn

Manganese

54.938049

26

Fe

Iron

55.845

27

Co

Cobalt

58.933200

28

Ni

Nickel

58.6934

29

Cu

Copper

63.546

30

Zn

Zinc

65.39

31

Ga

Gallium

69.723

32

Ge

Germanium

72.64

33

As

Arsenic

74.92160

34

Se

Selenium

78.96

35

Br

Bromine

79.904

36

Kr

Krypton

83.80

37

Rb

Rubidium

85.4678

38

Sr

Strontium

87.62

39

Y

Yttrium

88.90585

40

Zr

Zirconium

91.224

41

Nb

Niobium

92.90638

42

Mo

Molybdenum

95.94

43

Tc

Technetium

[98]

44

Ru

Ruthenium

101.07

45

Rh

Rhodium

102.90550

46

Pd

Palladium

106.42

47

Ag

Silver

107.8682

48

Cd

Cadmium

112.411

49

In

Indium

114.818

50

Sn

Tin

118.710

51

Sb

Antimony

121.760

52

Te

Tellurium

127.60

53

I

Iodine

126.90447

54

Xe

Xenon

131.293

55

Cs

Cesium

132.90545

56

Ba

Barium

137.327

57

La

Lanthanum

138.9055

58

Ce

Cerium

140.116

59

Pr

Praseodymium

140.90765

60

Nd

Neodymium

144.24

61

Pm

Promethium

[145]

62

Sm

Samarium

150.36

63

Eu

Europium

151.964

64

Gd

Gadolinium

157.25

65

Tb

Terbium

158.92534

66

Dy

Dysprosium

162.50

67

Ho

Holmium

164.93032

68

Er

Erbium

167.259

69

Tm

Thulium

168.93421

70

Yb

Ytterbium

173.04

71

Lu

Lutetium

174.967

72

Hf

Hafnium

178.49

73

Ta

Tantalum

180.9479

74

W

Tungsten

183.84

75

Re

Rhenium

186.207

76

Os

Osmium

190.23

77

Ir

Iridium

192.217

78

Pt

Platinum

195.078

79

Au

Gold

196.96655

80

Hg

Mercury

200.59

81

Tl

Thallium

204.3833

82

Pb

Lead

207.2

83

Bi

Bismuth

208.98038

84

Po

Polonium

[209]

85

At

Astatine

[210]

86

Rn

Radon

[222]

87

Fr

Francium

[223]

88

Ra

Radium

[226]

89

Ac

Actinium

[227]

90

Th

Thorium

232.0381

91

Pa

Protactinium

231.03588

92

U

Uranium

238.02891

93

Np

Neptunium

[237]

94

Pu

Plutonium

[244]

95

Am

Americium

[243]

96

Cm

Curium

[247]

97

Bk

Berkelium

[247]

98

Cf

Californium

[251]

99

Es

Einsteinium

[252]

100

Fm

Fermium

[257]

101

Md

Mendelevium

[258]

102

No

Nobelium

[259]

103

Lr

Lawrencium

[262]

104

Rf

Rutherfordium

[261]

105

Db

Dubnium

[262]

106

Sg

Seaborgium

[266]

107

Bh

Bohrium

[264]

108

Hs

Hassium

[277]

109

Mt

Meitnerium

[268]

110

Uun

Ununnilium

[281]

111

Uuu

Unununium

[272]

112

Uub

Ununbium

[285]

113

Uut

ununtrium

new - 2016

114

Uuq

Ununquadium

[289]

115

Uup

Ununpentium

new - 2016

116

Uuh

Ununhexium

unknown

117

Uus

Ununseptium

new - 2016

118

Uuo

Ununoctium

unknown

Summary

An element is a basic chemical unit. Elements have an atomic number and atomic weight assigned to them. There are 92 natural elements, plus some that have been artificially created. Artificial elements are highly unstable and usually exist for only a fraction of a second.


Be observant


Resources and references

Ron Kurtus' Credentials

Websites

Chemical Elements - Site with details on Periodic Table

Chemistry Resources

Books

Top-rated books on Chemistry

Top-rated books on Physical Science


Questions and comments

Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. I will try to get back to you as soon as possible.


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