Lecture Notes on Elements, Isotopes, and Atomic Mass

Introduction to Atomic Number

• Atomic Number: Defined by the number of protons in an atom's nucleus.
  • Example:
    • Hydrogen: 1 proton
    • Carbon: 6 protons
    • Chlorine: 17 protons
• The atomic number determines the type of element.

Isotopes

• Isotopes: Different versions of the same element with the same number of protons but different numbers of neutrons.
• Example: Chlorine
  • Chlorine-35:
    • Mass Number: 35
    • Protons: 17
    • Neutrons: 18 (Mass Number - Protons)
  • Chlorine-37:
    • Mass Number: 37
    • Protons: 17
    • Neutrons: 20 (Mass Number - Protons)

Atomic Mass

• Average Atomic Mass: Weighted average of the masses of the stable isotopes of an element found in nature.
• Chlorine's average atomic mass calculation:
  • Chlorine-35: 75.77% abundance
  • Chlorine-37: 24.23% abundance
  • Formula:
    • (75.77% * Atomic Mass of Cl-35) + (24.23% * Atomic Mass of Cl-37)
    • Result: 35.45 unified atomic mass units (u)

Mass Defect

• Proton and neutron masses are roughly 1 unified atomic mass unit each.
• The combined mass of protons and neutrons in a nucleus is less than their individual masses summed up, known as Mass Defect.
• Actual atomic mass of Chlorine-35 is slightly under 35 unified atomic mass units due to mass defect.

Important Points

• Atomic number = Number of protons = Element type
• Isotopes vary in neutron number, not proton number.
• Average atomic mass is a weighted average, not a simple sum.
• Mass defect impacts the exact atomic mass of isotopes.