Overview
This lecture covers the differences between atomic number and atomic mass, explains isotopes and radioactive isotopes, and discusses their importance in biological and geological contexts.
Atomic Number and Atomic Mass
- Atomic number is the number of protons in an atom and identifies the element.
- Atomic mass is the sum of protons and neutrons in an atom.
- The number of neutrons can vary between atoms of the same element.
- To find the number of neutrons: atomic mass minus atomic number.
- The periodic table lists the atomic number, which increases left to right across a period.
- Atoms can gain or lose electrons to become ions, but atomic number remains unchanged.
Isotopes
- Isotopes are atoms of the same element with different numbers of neutrons.
- All isotopes of an element have the same number of protons and electrons but different mass numbers.
- Carbon, for example, has isotopes like carbon-12 (6 protons, 6 neutrons) and carbon-14 (6 protons, 8 neutrons).
- Isotopes generally have similar chemical properties but may differ in stability.
Radioactive Isotopes and Radioactive Dating
- Radioactive isotopes (radioisotopes) have unstable nuclei that decay spontaneously, emitting radiation.
- Radiation can damage cellular molecules and is used in scientific dating techniques.
- Carbon-14 decays to nitrogen-14 via beta decay, and this process is used in radiocarbon dating.
- The half-life is the time it takes for half the amount of a radioactive isotope to decay (carbon-14’s half-life ≈ 5,730 years).
- Other isotopes like potassium-40 and uranium-235 have longer half-lives, useful for dating older objects.
- Scientists use ratios of isotopes to date fossils and ancient biological materials.
Atomic Structure Summary
- Atoms are made of protons (positive charge), neutrons (no charge), and electrons (negative charge).
- Protons and neutrons are in the nucleus; electrons orbit the nucleus.
- Neutral atoms have equal numbers of protons and electrons.
Key Terms & Definitions
- Atomic Number — Number of protons in an atom; determines the element.
- Atomic Mass — Total number of protons and neutrons in an atom.
- Isotope — Atoms of the same element with different numbers of neutrons.
- Radioactive Isotope (Radioisotope) — Isotope with an unstable nucleus that decays, emitting radiation.
- Half-life — Time required for half of a radioactive isotope to decay.
- Ion — Atom with a different number of electrons than protons, resulting in a net charge.
Action Items / Next Steps
- Review textbook chapter on atomic structure and isotopes.
- Prepare for a quiz on identifying isotopes and calculating atomic mass.
- Read about the applications of radioactive dating in biology and geology.