Journey Inside | Class 9 Science Notes

8.6 Mass Number

The mass number (symbol A) of an atom is the total number of nucleons (protons and neutrons) in its nucleus. Since protons and neutrons have approximately equal mass, the mass number approximates the atomic mass. For example, helium has 2 protons and 2 neutrons, so its mass number is 4. Hydrogen's most common isotope has 1 proton and 0 neutrons, mass number 1. Table 8.3 lists mass numbers of various elements. The mass number helps distinguish isotopes of the same element, which have the same atomic number but different mass numbers due to varying neutron counts. The standard notation for an atom includes the mass number as a superscript and atomic number as a subscript before the element symbol (e.g., 12/6 C for carbon-12). Electrons have negligible mass and are not included in mass number calculations. Understanding mass number is crucial for studying isotopes, atomic mass, and nuclear reactions.

📊 Diagram: Table 8.3: Mass number of different elements.

🧪 Activity: Pause and Ponder questions calculating neutrons, protons, and electrons from atomic and mass numbers.

🔗 Connection: Leads to electron distribution in energy levels and electronic configuration.