Overview
This lecture explains the concept of isotopes, using analogies and examples from carbon, calcium, and iron, and details how to represent isotopes with notation.
What Are Isotopes?
- Isotopes are different versions of the same element, meaning atoms with the same number of protons but different numbers of neutrons.
- Isotopes are like different models of a car: they share core features but have distinct details (protons = core, neutrons = options).
- The identity of an element is determined solely by its number of protons (the atomic number).
Carbon Isotopes Example
- Carbon has three major isotopes: carbon-12, carbon-13, and carbon-14.
- All carbon atoms have 6 protons.
- Carbon-12 has 6 neutrons, carbon-13 has 7 neutrons, and carbon-14 has 8 neutrons.
- The neutrons do not affect the element's identity; they only affect the mass.
Isotope Notation and Mass Number
- The atomic number is the number of protons in an atom and defines the element.
- The mass number is the sum of protons and neutrons in the nucleus.
- Isotope notation is written as:
- Element symbol (C), mass number on upper left, atomic number on lower left (e.g., ¹²₆C).
- Alternatively, isotopes can be written as "Element name - mass number" (e.g., carbon-12).
Other Examples: Calcium and Iron Isotopes
- Calcium always has 20 protons but varies in neutron number to make different isotopes (e.g., calcium-40, calcium-42).
- Iron has an atomic number of 26 and multiple isotopes, each with 26 protons but a different neutron count.
- All isotopes of a given element have the same atomic number but different mass numbers.
Key Terms & Definitions
- Isotope — Atoms of the same element with the same number of protons and different numbers of neutrons.
- Atomic number — The number of protons in an atom; determines the element (e.g., 6 for carbon).
- Mass number — The sum of an atom’s protons and neutrons.
- Isotope notation — A way of writing isotopes showing mass number and atomic number (e.g., ¹²₆C).
Action Items / Next Steps
- Review how to write isotope notation for different elements.
- Practice identifying isotopes by their proton and neutron numbers.