Lecture Notes: Atomic Number, Mass Number, and Isotopes

Introduction

• Presenter: Dr. Kent
• Topic: Atomic number, mass number, and isotopes in chemistry

Atomic Number (Z)

• Defined as the number of protons in an element's nucleus.
• Key to an element's identity; changing protons changes the element.
• Found above the elemental symbol on the periodic table.
  • Example:
    • Hydrogen: Atomic number 1 (1 proton)
    • Vanadium: Atomic number 23 (23 protons)
    • Sulfur: Atomic number 16 (16 protons)
• Cannot change in chemical reactions; requires nuclear chemistry to alter.

Mass Number

• Sum of protons and neutrons in an atom's nucleus.
• Different from atomic mass (which includes isotopic averages and may not be a whole number).
• Important in atomic notation:
  • Atomic symbol with mass number as a superscript.
  • Example of Carbon isotopes:
    • Carbon-13: 6 protons and 7 neutrons (mass number 13).

Isotopes

• Atoms of the same element with different numbers of neutrons.
• Same atomic number, different mass numbers.
• Example with Hydrogen:
  • Hydrogen-1: 1 proton, 0 neutrons
  • Hydrogen-2 (Deuterium): 1 proton, 1 neutron
  • Hydrogen-3 (Tritium): 1 proton, 2 neutrons
• Naming: Element name followed by mass number (e.g., Cobalt-60).
• Determining neutrons: Subtract atomic number from mass number.

Example Calculations

• Lead-206:
  • Element symbol: Pb
  • Atomic number: 82 protons
  • Mass number: 206
  • Neutrons: 206 - 82 = 124 neutrons
• Neon Atom (Neutral):
  • Atomic number: 10
  • Electrons: 10 (to be neutral)
• Sodium Atom:
  • Atomic number: 11
  • Protons: 11
  • Neutrons: 15 - 7 = 8 (for Nitrogen-15)
• Nickel-61:
  • Atomic number: 28
  • Neutrons: 61 - 28 = 33 neutrons
• Uranium-238: Symbolized as U-238

Conclusion

• Recap on understanding atomic numbers, mass numbers, and isotopes.
• Encouragement to visit website for more lessons and practice.
• Website: getchemistryhelp.com
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