Overview
This lecture explains isotopes, how to interpret and write isotope symbols, and how to calculate and extract atomic, mass, and neutron numbers from isotope data, using examples from elements like carbon, hydrogen, oxygen, and others.
What Are Isotopes?
- Isotopes are atoms of the same element with different numbers of neutrons.
- All isotopes of an element have the same number of protons (atomic number).
- Different isotopes have different mass numbers due to varying neutrons.
Atomic Number, Mass Number, and Neutrons
- The atomic number (Z) is the number of protons in an atom.
- The mass number (A) is the total number of protons and neutrons (A = Z + N).
- The number of neutrons (N) is calculated as N = A - Z.
Examples with Carbon Isotopes
- Carbon-12: 6 protons, 6 neutrons (12 = 6 + 6), makes up ~99% of Earth's carbon.
- Carbon-13: 6 protons, 7 neutrons (13 = 6 + 7), about 1% of Earth's carbon.
- Carbon-14: 6 protons, 8 neutrons (14 = 6 + 8), rare and radioactive, used in radiometric dating.
Isotope Symbols and Notation
- Isotope symbols use the format: mass number (A) at top left, atomic number (Z) at bottom left, element symbol to the right (e.g., ¹²₆C).
- The element symbol alone tells you the number of protons (Z).
- Sometimes only the mass number and element symbol are shown (e.g., ¹²C).
Practice Examples
- Uranium-235: Z = 92, A = 235, symbol = ²³⁵₉₂U.
- Cesium-137: Z = 55, A = 137, symbol = ¹³⁷₅₅Cs.
- Fluorine-18: Z = 9, A = 18, symbol = ¹⁸₉F.
- Hydrogen isotopes: Hydrogen-1 (protium, N=0), Hydrogen-2 (deuterium, N=1), Hydrogen-3 (tritium, N=2).
More on Isotopes and Related Terms
- Isotopes: Same element (same Z), different N and thus different A.
- Isobars: Atoms with the same mass number (A) but different elements (different Z).
Key Terms & Definitions
- Isotope — Atoms of the same element with different numbers of neutrons.
- Atomic Number (Z) — Number of protons in an atom.
- Mass Number (A) — Sum of protons and neutrons in an atom (A = Z + N).
- Neutron Number (N) — Number of neutrons in an atom (N = A - Z).
- Weighted Average — The average atomic mass reflecting the abundance of each isotope.
- Isobar — Atoms of different elements but with the same mass number.
Action Items / Next Steps
- Practice writing isotope symbols for various elements.
- Use the formula N = A - Z to calculate neutron numbers for example isotopes.
- Review periodic table to quickly find atomic numbers (Z) for elements.