Mass Number
An atom consists of protons, neutrons and electrons. Since the mass of electrons is negligible, the real mass of an atom is determined by the protons and neutrons only.The total number of protons and neutrons present in one atom of an element is known as its mass number.That is,
Mass number = No. of protons + No. of neutrons
For example, one atom of sodium element contains 11 protons and 12 neutrons, so the mass number of sodium is 11 + 12 = 23. Similarly, a normal carbon atom has 6 protons and 6 neutrons, so the mass number of carbon is 6 + 6 = 12. And an ordinary hydrogen atom has 1 proton but 0 neutron (no neutron), therefore,the mass number of ordinary hydrogen is just 1. The mass number of an element is denoted by the letter A.The mass number of sodium is 23,so we can say that for sodium,A = 23.The mass number of carbon is 12, so for carbon, A = 12. And the mass number of ordinary hydrogen is 1, so for ordinary hydrogen, A =1. Please note that protons and neutrons present in a nucleus, taken together, are known as nucleons. So,we can also say that the total number of nucleons present in one atom of an element is known as its mass number.
The mass number of an atom also gives us the atomic mass.This can be explained as follows. We have just seen that:
Mass number = No.of protons + No. of neutrons
Now, Mass of a proton = 1 u
And,Mass of a neutron = 1 u
So,Mass number = No. of protons x 1+No.of neutrons x 1
or Mass number = Mass of protons + Mass of neutrons
But the total mass of protons and neutrons is called atomic mass, so :
Mass number = Atomic mass
Thus, the mass number of an atom gives us the atomic mass of the atom.Actually the atomic mass of an atom is numerically equal to its mass number. For example, if the mass number of an atom -3, the atomic mass will be 23 u.
Notes
Structure of the Atom - Notes
1. Drawback of Rutherford’s Model of the Atom
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2. Rutherford’s Experiment - Discovery of Nucleus
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6. For the symbols H, D and T,tabulate three sub-atomic particles found in each of them.
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9. How to learn naming Chemical Formulae?
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11. Limitations of Rutherford's model of the atom
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13. Arrangement Of Electrons In The Atoms
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14. All about Names of the Chemical Elements
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18. Structure of The Atom - Study Points
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25. Rules for writing of distribution of electrons in various shells for the first 18 elements
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28. The Physical Properties of the Isotopes of an Element are Different
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29. Electronic Configurations of First 20 Elements
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31. Comparison between Proton, Neutron and Electron
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39. All the Isotopes of an Element Have Identical Chemical Properties
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42. Relationship Between Mass Number and Atomic Number
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46. Electronic Configurations of Noble Gases (or Inert Gases)
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47. Applications of Radioactive Isotopes
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48. Electronic Configurations of Elements
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50. Relation Between Valency and Valence Electrons
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