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CBSEScience (English Medium) Class 11
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Atomic Mass

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Topics

  • Introduction of Atomic Mass
  • Properties of Mass Number
  • Determination of Relative Atomic Mass

Introduction of Atomic Mass:

The protons (p) and neutrons in an atom concentrate their mass in its nucleus. The 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. This is commonly expressed in terms of a unified atomic mass unit (AMU).

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

Symbol Representation:

An element’s atomic symbol can be written to show both its atomic number (Z) and mass number (A).

  • Element Symbol: Represents the chemical element.
  • Z (Atomic Number): Indicates the number of protons.
  • A (mass number): represents the total number of protons and neutrons.

`"_Z^Asymbol`

For example, the conventional symbol

`"_6^12C`

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

Properties of Mass Number:

The total of protons and neutrons determines the number of a specific element. It is represented by the letter A. Protons and neutrons are together termed nucleons.

  • The number of neutrons may vary in an element. However, all element atoms have the same number of protons. 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.

Determination of Relative Atomic Mass:

Atoms are extremely small, making it difficult to measure their actual mass. To solve this, scientists introduced the concept of relative atomic mass. In order to express the relative mass of other atoms, scientists initially chose the hydrogen atom as the reference.

  • A hydrogen atom with one proton in its nucleus has a relative atomic mass of 1.
  • The relative atomic mass of other atoms was determined by comparing their mass to that of hydrogen. For example, one nitrogen atom was found to be 14 times heavier than a hydrogen atom, so its relative atomic mass was determined to be 14.
  • On this scale, the relative atomic masses of several elements were fractional. Over time, the reference atom was changed to ensure greater consistency.
  • In 1961, the carbon atom (12C) was selected as the standard reference atom. Its relative atomic mass was fixed at 12. Using the carbon scale, the relative atomic mass of hydrogen was calculated as 12×`"1"/"12"`=1
  • On this scale, the relative mass of one proton and one neutron is approximately equal to one unit. This approach allowed the relative atomic masses of various elements to be determined accurately and consistently.

Modern technology allows for highly accurate measurements of atomic masses. As a result, the unified mass (u) is now the standard unit for atomic mass. The value of 1 unified mass unit is 1 u = 1.66053904 × 10⁻²⁷ kg, also referred to as Dalton (Da).

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Atoms and Molecules(Atomic Mass) [00:17:33]
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Atoms and Molecules (Average Atomic Mass)
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