Overview
This lecture explains the concept of neutrons, isotopes, and how atomic mass is a weighted average based on the percent abundance of isotopes.
Neutrons: Basics
- Neutrons are neutral particles found in the nucleus of an atom.
- Neutrons have almost the same mass as protons (about 1 atomic mass unit, amu).
Atomic Mass and the Periodic Table
- The periodic table lists the element's symbol, atomic number (number of protons), and atomic mass (in amu).
- Atomic mass shown is a weighted average of all naturally occurring isotopes of the element.
Averages: Normal vs. Weighted
- A normal (arithmetic) average treats all values equally: sum the values and divide by the count.
- A weighted average assigns different weights (percent abundances) to values: multiply each value by its percent (in decimal), then sum.
Isotopes: Definition and Representation
- Isotopes are atoms of the same element (same number of protons, same atomic number) with different numbers of neutrons.
- Each additional neutron increases atomic mass by roughly 1 amu.
- The mass number (A) is the sum of protons and neutrons in the nucleus.
- Isotopes can be represented by element symbol with mass and atomic number (e.g., ¹⁴₆C) or with dash notation (e.g., C-14).
Example: Carbon Isotopes and Abundance
- Carbon has three main isotopes: carbon-12, carbon-13, and carbon-14.
- Carbon-12: 98.89% abundance, mass = 12 amu (standard).
- Carbon-13: ~1.1% abundance.
- Carbon-14: present in trace amounts (very low abundance).
- Natural abundance is always expressed as a percentage.
Key Terms & Definitions
- Neutron — a neutral particle in the atomic nucleus with mass ≈ 1 amu.
- Isotope — atoms of the same element with different numbers of neutrons.
- Atomic Number (Z) — number of protons, defines the element.
- Mass Number (A) — sum of protons and neutrons in an atom.
- Atomic Mass — weighted average mass of all naturally occurring isotopes, shown on the periodic table.
- Weighted Average — average where each value is multiplied by its percent abundance before summing.
- Natural Abundance — the relative proportion (percentage) of an isotope in a natural sample.
Action Items / Next Steps
- Review how to calculate atomic mass using weighted averages and percent abundances.
- Practice identifying isotopes and calculating numbers of protons, neutrons, and electrons.
- Read about other examples of isotopes and their role in determining atomic mass.