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The atomic mass is the mass of an atom, while molecular mass is the mass of a molecule.
- Atoms are the smallest units that constitute matter.
- They are extremely small units, measuring only 10-10 meters or 10-8 centimeters.
- An atom is composed of a nucleus, which consists of neutrons and protons.
- The nucleus is surrounded by shells which consist of nucleus-bound electrons.
- The group of atoms is known as molecules.
- Molecules are the components of matter which are capable of independent existence.
| Table of Content |
Key Terms: Atomic mass, Molecular mass, Atom, Molecule, Mass, Chemical element, Atomic mass unit, Weight, Relative atomic mass
Definition of Atomic Mass and Molecular Mass
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The atomic and molecular mass can be defined as follows:
Atomic Mass
| Atomic mass is defined as the quantity of matter contained in an atom. It is the mass of the single atom which constitutes the element. |
Molecular Mass
| Molecular mass is defined as the mass of a particular molecule. It is the sum of the masses of all the atoms in a molecule. |
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| Unsaturated Hydrocarbons | Cathode Ray Experiment(JJ Thomas) | Difference Between Element and Compound |
| Carbocation Stability | Cis Trans Isomerism | Difference Between Atom and Molecule |
Atomic Mass
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According to the theory proposed by Dalton, every element has a characteristic atomic mass. It is generally measured in Atomic Mass Unit (amu) or Dalton (Da).
- The concept of relative atomic mass came into use due to the difficulty in calculating atomic mass.
- Relative Atomic Mass can be defined as the ratio of the atomic mass to the one-twelfth of the mass of an atom of carbon-12.
- To calculate the relative atomic mass, the mass of hydrogen was assumed to be 1.
- The other elements were compared with hydrogen and the relative atomic masses were obtained.
- The relative atomic mass of an element with isotopes is the weighted average of masses of the atoms of the isotopes.
- Currently, an isotope of carbon-12 is considered as a standard.
- The mass of carbon-12 is 12 amu and all the other elements are assigned their masses accordingly.
- One relative atomic mass unit is equal to 1/12th of the mass of a carbon-12 atom.
Atomic Mass of Different Elements
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The Atomic Masses of the first 10 elements of the periodic table are-
| Atomic Number | Element | Atomic Mass |
|---|---|---|
| 1 | Hydrogen | 1.008 amu |
| 2 | 4.0026 amu | |
| 3 | Lithium | 6.94 amu |
| 4 | Beryllium | 9.0122 amu |
| 5 | Boron | 10.81 amu |
| 6 | Carbon | 12.011 amu |
| 7 | 14.007 amu | |
| 8 | Oxygen | 15.999 amu |
| 9 | Fluorine | 18.998 amu |
| 10 | Neon | 20.18 amu |
Molecular Mass
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Molecular Mass is the total mass of all the elements present in a particular molecule.
- Molecular Mass is calculated by multiplying the atomic mass by the number of atoms present in the molecule and then adding all the masses of all the elements present in the molecule.
- For example, carbon dioxide consists of one atom of carbon and two atoms of oxygen.
- The molecular mass of carbon dioxide is equal to the total atomic mass of one carbon and two oxygen.
- The constituted atoms determine the molecular mass of a molecule.
The molecular mass can be controlled in the following ways-
Mass Spectrometry
Mass spectrometry is useful in determining the molecular mass of monoisotopic atoms. This technique is used in deciding the mass of smaller molecules.
Hydrodynamic Strategy
This strategy is also known as the relative atomic weight determination strategy. The weight is calculated using Mark-Houwink relations. This procedure requires calibration afterward.
Static Light Scattering
In this method, the Zimm method is used to determine the molecular weight from the amount of light dispersed.
Molecular Mass Of Different Compounds
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The Molecular Mass of Different Compounds are given as follows
| Compound | Formula | Molecular Weight |
|---|---|---|
| Ammonia | NH3 | 17.031 amu |
| CO | 28.010 amu | |
| Chlorine | Cl2 | 70.905 amu |
| NO2 | 46.006 amu | |
| Hydrogen Peroxide | H2O2 | 34.015 amu |
| SO2 | 64.065 amu | |
| Ozone | O3 | 47.998 amu |
| Carbon tetrachloride | CCl4 | 153.822 amu |
| Benzene | C6H6 | 78.114 amu |
| Ethylene | C2H4 | 28.054 amu |
Also check:
| Related Concepts | ||
|---|---|---|
| Column Chromatography Principle | Atomic Mass of Elements | Moles to Grams |
| Molecular Weight Formula | Mole Concept | Measurement of Mass and Weight |
Things to Remember
- Atomic Mass refers to the mass of an atom. It is the weight of one atom of an element.
- Molecular Mass refers to the mass of all the atoms present in a molecule.
- The concept of Relative Atomic Mass is being used these days.
- To calculate the relative atomic mass, the 1/12th of the Carbon-12 atom is taken into consideration.
- All the other elements are compared to it and their relative atomic mass is calculated.
- The unit of Atomic Mass is the Atomic Mass unit or amu.
- Molecular Mass can be calculated by adding the sum of all the atoms in a given molecule.
Sample Questions
Ques. What is Atomicity? (3 Marks)
Ans. The number of atoms present in a given molecule is known as its Atomicity. Molecules are categorized as follows based on their atomicity:
- Monatomic Molecules- These molecules are composed of one atom. For Example He, Ne, Ar, Kr, etc.
- Diatomic Molecules- These molecules are composed of two atoms. For Example H2, N2, O2, F2, etc.
- Triatomic Molecules- These molecules are composed of three atoms. For Example O3.
- Polyatomic Molecules- These molecules are composed of more than three atoms. For Example: P4
Ques. The oxygen atom has a relative atomic mass of 16. Explain what it means. (2 Marks)
Ans. The relative mass of an element's atom in comparison to the mass of a carbon-12 atom divided by 12 units is called atomic mass. As a result, the atomic mass of oxygen is 16 u, which means that one oxygen atom is 16 times heavier than one-twelfth of a carbon-12 atom.
Ques. Water is formed when hydrogen and oxygen mix in a mass ratio of 1:8. How much oxygen gas would it take to totally react with 3 g of hydrogen gas? (2 Marks)
Ans. According to the question, 1 gram of Hydrogen reacts with 8 grams of Oxygen
So, 3 grams of Hydrogen will react with 8 x 3 grams = 24 grams
So, 24 grams of oxygen is required to make water.
Ques. What is the Molecular Mass of Methane? (3 Marks)
Ans. The formula for methane is CH4. The molecular mass of Methane can be calculated by the following steps:
Molecular Mass = (1 x 12) + (4 x 1) [The Atomic Mass of Carbon is 12 amu and the Atomic Mass of Hydrogen is amu]
=12 + 4
= 16 amu
Ques. What is the Molecular Mass of Ethyne? (2 Marks)
Ans. The formula for Ethyne is C2H2. The molecular mass of Ethyne can be calculated by the following steps:
Molecular Mass = (2 x 12) + (4 x 1) [The Atomic Mass of Carbon is 12 amu and the Atomic Mass of Hydrogen is 1 amu]
= 24 + 4
= 28 amu
Ques. What is meant by atomic mass? (2 Marks)
Ans. According to the theory proposed by Dalton, every element has a characteristic atomic mass. It is generally measured in unit Atomic Mass Unit (amu) or Dalton (Da). The concept of relative atomic mass came into use due to the difficulty in calculating atomic mass.
Ques. Define relative atomic mass. (5 Marks)
Ans. Relative Atomic Mass can be defined as the ratio of the atomic mass to the one-twelfth of the mass of an atom of carbon-12. To calculate the relative atomic mass, the mass of hydrogen was assumed to be 1. The other elements were compared with hydrogen and the relative atomic masses were obtained. The relative atomic mass of an element with isotopes is the weighted average of masses of the atoms of the isotopes. Currently, an isotope of carbon-12 is considered as a standard. The mass of carbon-12 is 12 amu and all the other elements are assigned their masses accordingly. One relative atomic mass unit is equal to 1/12th of the mass of a carbon-12 atom.
Ques. What is the atomic mass number? (2 Marks)
Ans. The atomic mass number is defined as the total number of nucleons, which are the total number of protons and neutrons together in the atomic nucleus. The atomic mass number is denoted by the symbol A and is also known as the nucleon number.
Ques. Who discovered atomic mass? (2 Marks)
Ans. John Dalton (1803–1805) and Jons Jacoband Berzelius (1808–1826) were the first scientists who measured atomic mass. The English scientist William Prout proposed an early atomic mass hypothesis in a series of papers published in 1815 and 1816.
Ques. What is the basic unit for mass? (2 Marks)
Ans. The weight of an object is calculated using its mass. For example, when you step on a scale, you are weighing your entire body. Grams and kilograms are the most essential mass units in the metric system of measurement.
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