Atomic Mass of Chemical Elements

Periodic Table of Elements - atomic mass
1
H

Hydrogen

1.0079 amu

2
He

Helium

4.0026 amu

3
Li

Lithium

6.941 amu

4
Be

Beryllium

9.0122 amu

5
B

Boron

10.811 amu

6
C

Carbon

12.0107 amu

7
N

Nitrogen

14.0067 amu

8
O

Oxygen

15.9994 amu

9
F

Fluorine

18.9984 amu

10
Ne

Neon

20.1797 amu

11
Na

Sodium

22.9897 amu

12
Mg

Magnesium

24.305 amu

13
Al

Aluminium

26.9815 amu

14
Si

Silicon

28.0855 amu

15
P

Phosphorus

30.9738 amu

16
S

Sulfur

32.065 amu

17
Cl

Chlorine

35.453 amu

18
Ar

Argon

39.948 amu

19
K

Potassium

39.0983 amu

20
Ca

Calcium

40.078 amu

21
Sc

Scandium

44.9559 amu

22
Ti

Titanium

47.867 amu

23
V

Vanadium

50.9415 amu

24
Cr

Chromium

51.9961 amu

25
Mn

Manganese

54.9381 amu

26
Fe

Iron

55.845 amu

27
Co

Cobalt

58.9332 amu

28
Ni

Nickel

58.6934 amu

29
Cu

Copper

63.546 amu

30
Zn

Zinc

65.409 amu

31
Ga

Gallium

69.723 amu

32
Ge

Germanium

72.64 amu

33
As

Arsenic

74.9216 amu

34
Se

Selenium

78.96 amu

35
Br

Bromine

79.904 amu

36
Kr

Krypton

83.798 amu

37
Rb

Rubidium

85.4678 amu

38
Sr

Strontium

87.62 amu

39
Y

Yttrium

88.9059 amu

40
Zr

Zirconium

91.224 amu

41
Nb

Niobium

92.9064 amu

42
Mo

Molybdenum

95.94 amu

43
Tc

Technetium

98 amu

44
Ru

Ruthenium

101.07 amu

45
Rh

Rhodium

102.9055 amu

46
Pd

Palladium

106.42 amu

47
Ag

Silver

107.8682 amu

48
Cd

Cadmium

112.411 amu

49
In

Indium

114.818 amu

50
Sn

Tin

118.71 amu

51
Sb

Antimony

121.76 amu

52
Te

Tellurium

127.6 amu

53
I

Iodine

126.9045 amu

54
Xe

Xenon

131.293 amu

55
Cs

Caesium

132.9055 amu

56
Ba

Barium

137.327 amu

57-71

 

Lanthanoids

 

72
Hf

Hafnium

178.49 amu

73
Ta

Tantalum

180.9479 amu

74
W

Tungsten

183.84 amu

75
Re

Rhenium

186.207 amu

76
Os

Osmium

190.23 amu

77
Ir

Iridium

192.217 amu

78
Pt

Platinum

195.079 amu

79
Au

Gold

196.9665 amu

80
Hg

Mercury

200.59 amu

81
Tl

Thallium

204.3833 amu

82
Pb

Lead

207.2 amu

83
Bi

Bismuth

208.9804 amu

84
Po

Polonium

209 amu

85
At

Astatine

210 amu

86
Rn

Radon

222 amu

87
Fr

Francium

223 amu

88
Ra

Radium

226 amu

89-103

 

Actinoids

 

104
Rf

Rutherfordium

261 amu

105
Db

Dubnium

262 amu

106
Sg

Seaborgium

266 amu

107
Bh

Bohrium

264 amu

108
Hs

Hassium

277 amu

109
Mt

Meitnerium

268 amu

110
Ds

Darmstadtium

281 amu

111
Rg

Roentgenium

272 amu

112
Cn

Copernicium

285 amu

113
Nh

Nihonium

286 amu

114
Fl

Flerovium

289 amu

115
Mc

Moscovium

290 amu

116
Lv

Livermorium

292 amu

117
Ts

Tennessine

294 amu

118
Og

Oganesson

294 amu

57
La

Lanthanum

138.9055 amu

58
Ce

Cerium

140.116 amu

59
Pr

Praseodymium

140.9077 amu

60
Nd

Neodymium

144.24 amu

61
Pm

Promethium

145 amu

62
Sm

Samarium

150.36 amu

63
Eu

Europium

151.964 amu

64
Gd

Gadolinium

157.25 amu

65
Tb

Terbium

158.9253 amu

66
Dy

Dysprosium

162.5 amu

67
Ho

Holmium

164.9303 amu

68
Er

Erbium

167.259 amu

69
Tm

Thulium

168.9342 amu

70
Yb

Ytterbium

173.04 amu

71
Lu

Lutetium

174.967 amu

89
Ac

Actinium

227 amu

90
Th

Thorium

232.0381 amu

91
Pa

Protactinium

231.0359 amu

92
U

Uranium

238.0289 amu

93
Np

Neptunium

237 amu

94
Pu

Plutonium

244 amu

95
Am

Americium

243 amu

96
Cm

Curium

247 amu

97
Bk

Berkelium

247 amu

98
Cf

Californium

251 amu

99
Es

Einsteinium

252 amu

100
Fm

Fermium

257 amu

101
Md

Mendelevium

258 amu

102
No

Nobelium

259 amu

103
Lr

Lawrencium

262 amu

Atomic Mass of Chemical Elements

The atomic mass is the mass of an atom. The atomic mass or relative isotopic mass refers to the mass of a single particle, and therefore is tied to a certain specific isotope of an element. The atomic mass is carried by the atomic nucleus, which occupies only about 10-12 of the total volume of the atom or less, but it contains all the positive charge and at least 99.95% of the total mass of the atom. Note that, each element may contain more isotopes, therefore this resulting atomic mass is calculated from naturally-occuring isotopes and their abundance.

The size and mass of atoms are so small that the use of normal measuring units, while possible, is often inconvenient. Units of measure have been defined for mass and energy on the atomic scale to make measurements more convenient to express. The unit of measure for mass is the atomic mass unit (amu). One atomic mass unit is equal to 1.66 x 10-24 grams. One unified atomic mass unit is approximately the mass of one nucleon (either a single proton or neutron) and is numerically equivalent to 1 g/mol.

For 12C the atomic mass is exactly 12u, since the atomic mass unit is defined from it. For other isotopes, the isotopic mass usually differs and is usually within 0.1 u of the mass number. For example, 63Cu (29 protons and 34 neutrons) has a mass number of 63 and an isotopic mass in its nuclear ground state is 62.91367 u.

There are two reasons for the difference between mass number and isotopic mass, known as the mass defect:

  1. The neutron is slightly heavier than the proton. This increases the mass of nuclei with more neutrons than protons relative to the atomic mass unit scale based on 12C with equal numbers of protons and neutrons.
  2. The nuclear binding energy varies between nuclei. A nucleus with greater binding energy has a lower total energy, and therefore a lower mass according to Einstein’s mass-energy equivalence relation E = mc2. For 63Cu the atomic mass is less than 63 so this must be the dominant factor.

Note that, it was found the rest mass of an atomic nucleus is measurably smaller than the sum of the rest masses of its constituent protons, neutrons and electrons. Mass was no longer considered unchangeable in the closed system. The difference is a measure of the nuclear binding energy which holds the nucleus together. According to the Einstein relationship (E=mc2), this binding energy is proportional to this mass difference and it is known as the mass defect.

See also: Atomic Mass Number – Does it conserve in a nuclear reaction?

Mass Number of Chemical Elements

The total number of neutrons in the nucleus of an atom is called the neutron number of the atom and is given the symbol N. Neutron number plus atomic number equals atomic mass number: N+Z=A. The difference between the neutron number and the atomic number is known as the neutron excess: D = N – Z = A – 2Z.

Neutron number is rarely written explicitly in nuclide symbol notation, but appears as a subscript to the right of the element symbol. Nuclides that have the same neutron number but a different proton number are called isotones. The various species of atoms whose nuclei contain particular numbers of protons and neutrons are called nuclides. Each nuclide is denoted by chemical symbol of the element (this specifies Z) with tha atomic mass number as supescript. Therefore, we cannot determine the neutron number of uranium, for example. We can determine the neutron number of certain isotope. For example, the neutron number of uranium-238 is 238-92=146.

Density of Chemical Elements

Typical densities of various substances are at atmospheric pressure.

Density is defined as the mass per unit volume. It is an intensive property, which is mathematically defined as mass divided by volume:

ρ = m/V

In words, the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance. The standard SI unit is kilograms per cubic meter (kg/m3). The Standard English unit is pounds mass per cubic foot (lbm/ft3).

Density – Atomic Mass and Atomic Number Density

Since the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance, it is obvious, the density of a substance strongly depends on its atomic mass and also on the atomic number density (N; atoms/cm3),

  • Atomic Weight. The atomic mass is carried by the atomic nucleus, which occupies only about 10-12 of the total volume of the atom or less, but it contains all the positive charge and at least 99.95% of the total mass of the atom. Therefore it is determined by the mass number (number of protons and neutrons).
  • Atomic Number Density. The atomic number density (N; atoms/cm3), which is associated with atomic radii, is the number of atoms of a given type per unit volume (V; cm3) of the material. The atomic number density (N; atoms/cm3) of a pure material having atomic or molecular weight (M; grams/mol) and the material density (⍴; gram/cm3) is easily computed from the following equation using Avogadro’s number (NA = 6.022×1023 atoms or molecules per mole):Atomic-Number-Density

Since nucleons (protons and neutrons) make up most of the mass of ordinary atoms, the density of normal matter tends to be limited by how closely we can pack these nucleons and depends on the internal atomic structure of a substance. The densest material found on earth is the metal osmium, but its density pales by comparison to the densities of exotic astronomical objects such as white dwarf stars and neutron stars.

If we include man made elements, the densest so far is HassiumHassium is a chemical element with symbol Hs and atomic number 108.  It is a synthetic element (first synthesised at Hasse in Germany) and radioactive. The most stable known isotope, 269Hs, has a half-life of approximately 9.7 seconds. It has an estimated density of 40.7 x 103 kg/m3.  The density of Hassium results from its high atomic weight and from the significant decrease in ionic radii of the elements in the lanthanide series, known as lanthanide and actinide contraction.

Properties of other elements