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Atomic weight of elements

We know that an atom is electrically neutral, if it contains negatively charged electrons it must also contain some positively charged particles. This was confirmed by Goldstein in his discharge tube experiment with perforated cathode. Applying high voltage across the electrodes of a discharge tube it was found that some rays were coming from the side of the anode which passed through the holes in the cathode.

These anode rays (canal rays) consisted of positively charged particles formed by ionization of gas molecules by the cathode rays. The charge to mass ratio (e/m value) of positively charge particles was found to be maximum when the discharge tube was filled with hydrogen gas as hydrogen is the lightest element. These positively charged particles are called protons.

e/m varies with the nature of gas taken in the discharge tube. The positive particles are positive residues of the gas left when the gas is ionized. The neutral charge particle, neutron was discovered by James Chadwick by bombarding boron or beryllium with α–particles.

Characteristics of the three fundamental particles

  Electron Proton Neutron
Symbol E or e– P n
Approximate relative mass with respect to proton 1/1836 1 1
Approximate relative charge with respect to proton –1 +1 No charge
Absolute mass in kg 9.109×10–31 1.673×10–27 1.675×10–27
Absolute mass in amu 5.485×10–4 1.007 1.008
Absolute charge (coulomb) 1.602×10–19 1.602×10–19 0
Absolute charge (e.s.u.) 4.8 × 10–10 4.8 × 10–10 0

The atomic mass unit (amu) is 1/12 of the mass of an individual atom of 6C12, i.e. 1.660 × 10–27 kg.

The neutron and proton have approximately equal masses of 1 amu and the electron is about 1836 times lighter, its mass can sometimes be neglected as an approximation.

The electron and proton have equal, but opposite, electric charges while the neutron has no charge.

Table of Rel​ative Atomic Weights

Atomic No. Name of Element


Atomic Weight

89 Actinium Ac 227.0278
13 Aluminium Al 26.981
95 Americium Am 234.0614
51 Antimony Sb 121.7
18 Argon Ar 39.94
33 Arsenic As 74.922
85 Astatine At 209.9871
56 Barium Ba 137.3
97 Berkelium Bk 247.07
4 Beryllium Be 9.012
83 Bismuth Bi 208.981
107 Bohrium Bh 262.12
5 Boron B 10.81
35 Bromine Br 79.904
48 Cadmium Cd 112.40
55 Cesium Cs 132.905
20 Calcium Ca 40.08
98 Californium Cf 251.079
6 Carbon C 12.011
58 Cerium Ce 140.12
17 Chlorine Cl 35.453
24 Chromium Cr 51.996
27 Cobalt Co 58.933
29 Copper Cu 63.54
96 Curium Cm 247.07
66 Dysprosium Dy 162.5
99 Einsteinium Es 252.08
68 Erbium Er 167.27
63 Europium Eu 151.96
100 Fermium Fm 257.09
9 Fluorine F 18.998
87 Francium Fr 223.019
64 Gadolinium Gd 157.2
31 Gallium Ga 69.72
32 Germanium Ge 72.61
79 Gold Au 196.966
72 Hafnium Hf 178.49
105 Hahnium* Ha 262.12
108 Hassium Hs 265
2 Helium He 4.003
67 Holmium Ho 164.930
1 Hydrogen H 1.008
49 Indium In 114.82
53 Iodine I 126.904
77 Iridium Ir 192.2
26 Iron Fe 55.84
36 Krypton Kr 83.80
57 Lanthanum La 138.905
103 Lawrencium Lr 262.11
82 Lead Pb 207.2
3 Lithium Li 6.94
71 Lutetium Lu 174.97
12 Magnesium Mg 24.305
25 Manganese Mn 54.938
109 Meitneium Mt 266
101 Mendelevium Md 258.1
80 Mercury Hg 200.5
42 Molybdenum Mo 95.9
60 Neodymium Nd 144.2
10 Neon Ne 20.17
93 Neptunium Np 237.048
28 Nickel Ni 58.7
41 Niobium Nb 92.906
7 Nitrogen N 14.007
102 Nobelium No 259.1
76 Osmium Os 190.2
8 Oxygen O 15.999
46 Palladium Pd 106.4
15 Phosphorus P 30.974
78 Platinum Pt 195.0
94 Plutonium Pu 244.06
84 Polonium Po 208.98
19 Potassium K 39.10
59 Praseodymium Pr 140.908
61 Promethium Pm 144.91
91 Protactinium Pa 231.036
88 Radium Ra 226.025
86 Radon Rn 222.017
75 Rhenium Re 186.2
45 Rhodium Rh 102.905
37 Rubidium Rb 85.467
44 Ruthenium Ru 101.07
104 Rutherfordium** Rf 261.11
62 Samarium Sm 150.4
21 Scandium Sc 44.956
106 Seaborgium Sg 263.118
34 Selenium Se 78.9
14 Silicon Si 28.08
47 Silver Ag 107.868
11 Sodium Na 22.990
38 Strontium Sr 87.62
16 Sulphur S 32.06
73 Tantalum Ta 180.947
43 Technetium Tc 98.906
52 Tellurium Te 127.6
65 Terbium Tb 158.925
81 Thallium Tl 204.3
90 Thorium Th 232.038
69 Thulium Tm 168.934
50 Tin Sn 118.6
22 Titanium Ti 47.9
74 Tungsten W 183.8
92 Uranium U 238.036
23 Vanadium V 50.941
54 Xenon Xe 131.30
70 Ytterbium Yb 173.04
39 Yttrium Y 88.906
30 Zinc Zn 65.3
40 Zirconium Zr 91.22

Ha (Z = 105) is also called as Neisbonium (Ns)

Rf (Z = 104) is also called as Kurchutovium (Ku)


Before moving on to atomic mass, it is essential to learn about isotopes. Atoms of the same element that have different numbers of neutrons are known as isotopes. A isotope of a given element is defined by the sum of the number of protons and neutrons in its nucleus. An element (which has the same atomic number) can have multiple isotopes with different numbers of neutrons. The most common examples are the carbon isotopes 12C and 13C, which have 6 and 7 neutrons, respectively. Isotopes vary in their abundance in nature depending on the source of the materials.

Atomic Mass

A atomic mass is the average mass of atoms of an element measured in the atomic mass unit (amu) or Daltons, D. Atomic mass is determined by averaging the mass of all isotopes of an element. The averaging procedure also involves taking into account the amount of each isotope and multiplying it by the given mass.

Let's consider the gas neon, which has three isotopes:

  1. neon-20: Consists of 10 protons and 10 neutrons. The atomic mass unit of the gas is 19.992, and the amount of the gas is 90.48%.
  2. neon-21: Consists of 10 protons and 11 neutrons. The atomic mass unit of the gas is 20.994, and the amount of the gas is 0.27%.
  3. neon-22: Consists of 10 protons and 12 neutrons. The atomic mass unit of the gas is 21.991, and the amount of the gas is 9.25%.

To determine the atomic mass of neon then, the average of the isotopes must be extracted:

0.9048 × 19.992 = 18.09 amu

0.0027 × 20.994 = 0.057 amu

0.0925 × 21.991 = 2.03 amu

The average atomic weight is, therefore: 20.18 amu

The atomic weight of elements is directly related to the term "mole," which is an important way of measuring the amount of a substance. The relationship between atomic weight and the number of moles is such that when measured, amu directly indicates the amount in grams of an element present in 1 mole.

For example, 55.847 amu is the atomic mass of iron. Therefore, one mole of iron weights 55.847 grams. Thus, the same concept is also used to explain the molar amounts of compounds and ionic molecules. E.g., one mole of sodium chloride (NaCl) has a molecular weight of 58.44 amu (Na: 22.989 amu and Cl: 35.453 amu), giving a molar mass of salt of 58.44 grams. Generally, the molecular weight of water is 18.02 amu, giving a molar mass of 18.02 grams.

Difference Between the Atomic Mass and the Atomic Number of Elements

Atomic Mass Atomic Number
Atomic mass is similar to the number of neutrons and protons which are present in the nucleus of an element. Atomic number is called as number of protons that are present in the nucleus of an element.
It is the average weight of a specific element. It is the total number of nucleons that are present in the atom’s nucleus.
Atomic mass is denoted by the letter A The letter Z is used for representing the atomic number.
Atomic mass cannot be used for defining the type of element. Atomic numbers usually help to classify and identify an element.
Atomic mass is also used in the classification of different isotopes of the same element Only isotopes of an element share the same atomic number.
Atomic mass is always measured using the atomic mass unit (amu). The atomic number is simply a digit which is used for placing the elements in the periodic table.

Frequently Asked Question (FAQs)

Q1. What is Atomic Mass?

Ans. An atomic mass is the quantity of matter contained in an atom of an element. It is expressed as a multiple of one-twelfth the mass of the carbon-12 atom, 1.992646547 × 10−23 gram, which is assigned an atomic mass of 12 units. In this scale, 1 atomic mass unit (amu) corresponds to 1.660539040 × 10−24 gram.

Q2. What is Atomic Mass of Element?

Ans. An atomic mass of an element is the average mass of the atoms of an element that is measured in the atomic mass unit (amu, also known as daltons, D). An atomic mass is a weighted average of all the isotopes of that particular element, where the mass of isotope is multiplied by the amount of that particular isotope.

Q3. How do you find the atomic mass number of an element?

Ans. The number of protons and the number of neutrons together determine the elements mass number i.e, mass number = protons + neutrons. If you want to find how many neutrons an atom has, you can simply subtract the number of protons or atomic number from the mass number.

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