Atomic Mass



  • Periodic Table, Periodic Properties and Variations of Properties
  • Chemical Bonding
    • Chemical Bond
    • Types of Chemical Bond
    • Electrovalent (or Ionic) Bond
    • Formation of an Electrovalent (or Ionic) Bond
    • The Covalent Bond
    • Types of Covalent Bond
    • Formation of Covalent Bond
    • Properties and Comparison of Electrovalent and Covalent Compounds
    • Effect of Electricity on Electrovalent and Covalent Compounds
    • Coordinate Bond
    • Formation of Coordinate Bond
  • Study of Acids, Bases and Salts
  • Analytical Chemistry
    • Analytical Chemistry
    • Colours of the Salts and Their Solutions
    • Action of Sodium Hydroxide Solution on Certain Metallic Salt Solutions
    • Action of Ammonium Hydroxide on Certain Salt Solutions
    • Action of Alkalis on Certain Metals
    • Action of Alkalis on Metal Oxides
  • Mole Concept and Stoichiometry
    • The Gas Laws
    • Fundamental Laws of Gases
    • Pressure and Volume Relationship or Bolye's Law
    • Temperature - Volume Relationship or Charles's Law
    • Gay Lussac’s Law of Combining Volumes
    • Avogadro’s Law
    • Gas Equation
    • Standard Temperature Pressure (S.T.P.)
    • Absolute Zero
    • Atomic Mass
    • Molecular Mass
    • Mole Concept
    • Relationship Between Vapour Density and Relative Molecular Mass
    • Percentage Composition, Empirical and Molecular Formula
    • Empirical Formula of a Compound
    • Determination of Empirical Formula
    • Determination of Molecular Formula
    • Chemical Equation
    • Balancing Chemical Equation
    • Numerical Problems of Chemical Equation
  • Electrolysis
    • Electrolysis
    • Electrolytes
    • Nonelectrolyte
    • Electrochemical Cells
    • Electrodes
    • Oxidation, Reduction and Redox Reactions
    • Arrhenius Theory of Electrolytic Dissociation
    • Electrochemical Series
    • Preferential Or Selective Discharge of Ions at Electrodes
    • Examples of Electrolysis
    • Electrolysis of Molten Lead Bromid
    • Electrolysis of Acidified Water Using Platinum Electrodes
    • Electrolysis of Copper Sulphate Solution Using Platinum Anode and Copper Or Platinum Cathode
    • Electrolysis of Aqueous Copper Sulphate - Using Copper Electrodes
    • Applications of Electrolysis
  • Metallurgy
  • Study of Compounds
  • Hydrogen Chloride
    • Hydrogen Chloride
    • General Preparation of Hydrogen Chloride Gas
    • Laboratory Preparation of Hydrogen Chloride Gas
    • Physical Properties of Hydrogen Chloride Gas
    • Chemical Properties of Hydrogen Chloride Gas
    • Hydrochloric Acid
    • Laboratory Method of Preparation of Hydrochloric Acid
    • Properties of Hydrochloric Acid
    • Uses of Hydrochloric Acid
    • Tests for Hydrogen Chloride and Hydrochloric Acid
  • Ammonia
    • Ammonia
    • General Methods of Preparation of Ammonia Gas
    • Laboratory Preparation of Ammonia Gas
    • Preparation of Aqueous Ammonia
    • Manufacture of Ammonia (Haber's Process)
    • Physical Properties of Ammonia
    • Chemical Properties of Ammonia
    • Tests for Ammonia Gas and Ammonium Ion
    • Uses of Ammonia
  • Nitric Acid
    • Nitric Acid
    • Laboratory Preparation of Nitric Acid
    • Manufacture of Nitric Acid
    • Physical Properties of Nitric Acid
    • Chemical Properties of Nitric Acid
    • Uses of Nitric Acid
    • Tests for Nitric Acid and Nitrates
    • Effects of Heat on Nitrates
  • Sulphuric Acid
    • Sulphuric Acid
    • Preparation of Sulphuric Acid
    • Manufacture of Sulphuric Acid (Constant Process)
    • Physical Properties of Sulphuric Acid
    • Chemical Properties of Sulphuric Acid
    • Uses of Sulphuric Acid
    • Tests for Sulphuric Acid and Sulphates
  • Organic Chemistry
    • Carbon: a Versatile Element
    • Classification of Compounds of Carbon
    • Organic Compounds
    • Special Features of Carbon
    • Organic Compounds in Daily Life
    • Hydrocarbons
    • Classification of Organic Compounds Based on the Pattern of Carbon Chain
    • Classification of Organic Compound Based on the Kind of Atoms
    • Homologous Series of Carbon Compound
    • Nomenclature of Organic Compounds (IUPAC)
    • IUPAC Nomenclature of Hydrocarbons
    • IUPAC Nomenclature of other classes
    • Alkyl Group
    • Functional Groups in Carbon Compounds
    • Isomers
    • Hydrocarbons: Alkanes
    • Methane
    • Laboratory Preparation of Methane
    • Ethane
    • Laboratory Preparation of Ethane
    • Hydrocarbons: Alkenes
    • Ethene (Ethylene)
    • Preparation of Ethene (Ethylene)
    • Hydrocarbons: Alkynes
    • Ethyne
    • Laboratory Preparation of Ethyne
    • Alcohol
    • Ethanol
    • Laboratory Preparation of Ethanol
    • Carboxylic Acids
    • Ethanoic Acid
  • Practical Work
    • Laboratory Preparation of Hydrogen
    • Laboratory Preparation of Oxygen
    • Laboratory Preparation of Carbon Dioxide
    • Laboratory Preparation of Chlorine
    • Laboratory Preparation of Hydrogen Chloride Gas
    • Laboratory Preparation of Sulphur Dioxide
    • Laboratory Preparation of Hydrogen Sulphide
    • Laboratory Preparation of Ammonia Gas
    • Laboratory Preparation of Water Vapour
    • Laboratory Preparation of Nitrogen Dioxide
    • Action of Heat on a Given Substance
    • Action of Dilute Sulphuric Acid on a Given Substance
    • Dry Test
    • Recognition of Substances by Colour
    • Recognition of Substances by Odour
    • Recognition of Substances by Physical State
    • Recognition of Substances by Action of Heat
    • Flame Test
    • Strength of Acidic or Basic Solutions
    • Indicators
    • Identification of Ions
    • Identification of Cations
    • Identification of Anions
    • Distinction Between Colourless Solutions of Dilute Acids and Alkalis
    • Distinguish Between Black Copper Oxide and Black Manganese Dioxide
  • Atomic mass
  • Gram atomic mass
  • Average atomic mass

Concept of Atomic Mass:

The most remarkable concept that Dalton’s atomic theory proposed was that of atomic mass. According to him, each element had a characteristic atomic mass. The theory could explain the law of constant proportions so well that scientists were prompted to measure the atomic mass of an atom. Since determining the mass of an individual atom was a relatively difficult task, relative atomic masses were determined using the laws of chemical combinations and the compounds formed.

Let us take the example of a compound, carbon monoxide (CO), formed by carbon and oxygen. It was observed experimentally that 3 g of carbon combines with 4 g of oxygen to form CO. In other words, carbon combines with 4/3 times its mass of oxygen. Suppose we define the atomic mass unit (earlier abbreviated as ‘amu’, but according to the latest IUPAC recommendations, it is now written as ‘u’ – unified mass) as equal to the mass of one carbon atom, then we would assign carbon an atomic mass of 1.0 u and oxygen an atomic mass of 1.33 u. However, having these numbers as whole numbers or as near to whole numbers as possible is more convenient. While searching for various atomic mass units, scientists initially took 1/ 16 of the mass of an atom of naturally occurring oxygen as the unit. This was considered relevant due to two reasons:

Imagine a fruit seller selling fruits without any standard weight with him. He takes a watermelon and says, “This has a mass equal to 12 units” (12 watermelon units or 12 fruit mass units). He makes twelve equal pieces of watermelon and finds the mass of each fruit he is selling relative to the mass of one piece of the watermelon. Now he sells his fruits by relative fruit mass unit (fmu), as in Fig.

(a) Watermelon, (b) 12 pieces, (c) 1/12 of watermelon, (d) how the fruit seller can weigh the fruits using pieces of watermelon


Similarly, the relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12th the mass of one carbon-12 atom.

Atomic masses of a few elements : 


Atomic mass (u)




















Atoms of most elements are not able to exist independently. Atoms form molecules and ions. These molecules or ions aggregate in large numbers to form the matter we can see, feel, or touch.

Mass Number

Mass Number denoted by ‘A’. Protons and neutrons are also called nucleons. The mass number is defined as the total number of protons and neutrons present in the nucleus of an atom.

In the notation for an atom, the atomic number, mass number and symbol of the element are to be written as:

Properties of mass number:

The sum of protons and neutrons provides this number of a certain element.

  • It is represented by the letter A.

  • Protons and Neutrons are together termed nucleons.

  • Example: Atoms of carbon consist of 6 protons and 6 neutrons. Therefore, the mass number of Carbon is 12.

  • The number of neutrons may vary in an element. However, the total number of protons is the same in all atoms of an element. Therefore, the atoms of the same element with the same atomic number but a different mass number are termed isotopes.

  • Generally, atomic mass and mass numbers are different terms and may vary slightly. In most cases, they are not the same. However, the weight of an electron is almost negligible, so we can consider the atomic mass of an atom to be almost equal to its mass number.

Concept of Atomic Mass:

The mass of an atom is concentrated in its nucleus, and it is due to the protons (p) and neutrons (n) in it. (mass of electrons is negligible as compared to that of neutrons and protons)

The number (p + n) in the atomic nucleus is called the atomic mass number. Protons and neutrons are together called nucleons (mass of electrons is negligible as compared to that of neutrons and protons)

  • This is commonly expressed in terms of a unified atomic mass unit (AMU).

  • It can be best defined as 1/12 of the mass of a carbon-12 atom in its ground state.

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