What is mass number?

Short Answer

The mass number is the total number of protons and neutrons present in the nucleus of an atom. It is usually represented by the symbol A.

The mass number determines the mass of an atom in atomic mass units (amu) and helps distinguish isotopes of the same element, which have the same number of protons but different numbers of neutrons.

Detailed Explanation :

Definition of Mass Number

The mass number (A) is the sum of the number of protons (Z) and number of neutrons (N) in an atom:

A = Z + N

  • Z: Atomic number, number of protons
  • N: Number of neutrons

Unlike atomic number, which identifies the element, mass number gives information about the total mass of the nucleus.

Properties of Mass Number

  1. Whole Number: Mass number is always a whole number, as it represents countable particles.
  2. Not in Periodic Table: Mass number is not listed in the periodic table; it is specific to a particular isotope.
  3. Determines Isotopes: Isotopes have the same atomic number but different mass numbers. For example:
    • Carbon-12 → 6 protons, 6 neutrons (A = 12)
    • Carbon-14 → 6 protons, 8 neutrons (A = 14)

Difference Between Mass Number and Atomic Mass

  • Mass Number: Count of protons + neutrons, always whole.
  • Atomic Mass: Weighted average of all isotopes, may be decimal.
  • Example: Carbon atomic mass = 12.01 amu, taking into account C-12, C-13, and C-14 isotopes.

Calculating Number of Neutrons

The number of neutrons in an atom can be calculated as:

N = A – Z

  • Example: Oxygen-17 → A = 17, Z = 8 → N = 17 – 8 = 9 neutrons

Significance of Mass Number

  1. Identifying Isotopes: Different isotopes of the same element have different mass numbers.
  2. Nuclear Reactions: Mass number is crucial in nuclear fission and fusion calculations.
  3. Radioactivity: Determines stability of nuclei; some mass numbers result in radioactive isotopes.
  4. Calculating Atomic Mass: Mass numbers of isotopes contribute to average atomic mass of an element.
  5. Chemistry and Physics Applications: Important in stoichiometry, molar mass calculations, and nuclear chemistry.

Examples of Mass Numbers

  1. Hydrogen:
    • Protium: A = 1 (1 proton, 0 neutrons)
    • Deuterium: A = 2 (1 proton, 1 neutron)
    • Tritium: A = 3 (1 proton, 2 neutrons)
  2. Chlorine:
    • Chlorine-35 → 17 protons, 18 neutrons → A = 35
    • Chlorine-37 → 17 protons, 20 neutrons → A = 37

These examples illustrate how mass number distinguishes isotopes.

Historical Background

  • Mass number was introduced when scientists realized atomic weight alone could not explain isotopic differences.
  • It provides a simple count of particles in the nucleus, helping in nuclear chemistry and atomic studies.

Relation to Atomic Number

  • Atomic number (Z): Determines identity of element
  • Mass number (A): Determines nuclear mass and isotope
  • Example: Both C-12 and C-14 have Z = 6, but A = 12 and 14 respectively.
Conclusion

Mass number (A) is the sum of protons and neutrons in an atom’s nucleus. It helps in identifying isotopes, calculating neutrons, and understanding nuclear properties. While atomic number defines the element, mass number determines its nuclear mass and isotope type, playing a vital role in atomic structure, chemistry, and nuclear science.