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.

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.

Table of Content
Characteristics of the three fundamental particles Table of Relative Atomic Weights Isotopes Atomic Mass Difference Between the Atomic Mass and the Atomic Number of Elements Frequently Asked Question (FAQs)

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 Relative Atomic Weights

Atomic No.	Name of Element	Symbol	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)

Isotopes

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:

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%.
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%.
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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