Lecture Notes: Isotopes and Atomic Mass

Element Identification

• Element Type by Protons: An element is defined by the number of protons in its nucleus.
  • E.g., 1 proton = Hydrogen, 6 protons = Carbon, 17 protons = Chlorine.
• Atomic Number: The number of protons, critical in defining the element.

Isotopes

• Definition: Variants of elements that have the same number of protons but different numbers of neutrons.
• Example with Chlorine:
  • Chlorine-35:
    • Atomic structure: 17 protons, 18 neutrons.
    • Notation: Cl-35, mass number of 35 (protons + neutrons).
  • Chlorine-37:
    • Atomic structure: 17 protons, 20 neutrons.
    • Notation: Cl-37, mass number of 37.

Atomic Mass

• Average Atomic Mass on Periodic Table:
  • It is a weighted average of isotopes found in nature.
  • Example: Chlorine
    • 75.77% of Chlorine is Cl-35, 24.23% is Cl-37.
    • Calculate average atomic mass:
      • Weighted calculation involves multiplying percentage abundance by isotopic mass.
      • Result: Average atomic mass of chlorine is 35.45 unified atomic mass units (u).

Mass Considerations

• Atomic Mass Calculation:
  • Approximation: Mass of one proton or neutron ≈ 1 unified atomic mass unit (u).
  • Electrons have negligible mass in atomic mass calculations.
  • Mass Defect:
    • Combined mass of protons and neutrons in nucleus is slightly less due to binding energy effects.
    • Example: Cl-35’s actual atomic mass is slightly less than its nominal mass number of 35 u.

Conclusion

• Understanding isotopes and atomic mass helps in comprehending the nature of elements on the periodic table and their behavior in different chemical contexts.