Overview
This lecture covers the structure of atoms, subatomic particles, isotopes, ions, and how atomic mass is determined as a weighted average of isotopic masses.
Classification of Matter
- Matter is classified as pure substances (elements or compounds) or mixtures.
- Elements are the simplest form of matter and cannot be broken down further.
- Atoms are the fundamental units of matter.
Atomic Structure and Subatomic Particles
- Atoms are extremely small and mostly empty space.
- The nucleus contains protons (positive charge) and neutrons (no charge).
- Electrons (negative charge) are found in the electron cloud outside the nucleus.
- Protons and electrons have equal but opposite charges; neutrons have no charge.
- Opposite charges attract; like charges repel.
- The mass of protons and neutrons is nearly equal and much greater than that of electrons.
- Atomic mass is primarily due to protons and neutrons.
Atomic Number, Mass Number, and Isotopes
- Atomic number (Z) is the number of protons and defines the element.
- Mass number (A) is the total number of protons plus neutrons.
- Number of neutrons = mass number – atomic number (A – Z).
- Isotopes have the same number of protons but different numbers of neutrons.
Atomic Symbols and Isotope Notation
- Isotopes can be named with the element and mass number (e.g., hydrogen-1).
- Atomic symbol: mass number (superscript) and optionally atomic number (subscript) next to the element symbol.
Ions: Cations and Anions
- Atoms are neutral when protons equal electrons.
- Losing electrons creates a positive ion (cation), e.g., Na⁺ or Mg²⁺.
- Gaining electrons creates a negative ion (anion), e.g., Cl⁻ (chloride ion).
- Cations have a positive charge; anions have a negative charge.
Atomic Mass and Weighted Averages
- Atomic mass units (amu) are used to express atomic masses.
- Carbon-12 is defined as exactly 12 amu.
- The atomic mass on the periodic table is a weighted average of all isotopes’ masses.
- Weighted average: sum of (abundance × mass) for each isotope.
- Example: Bromine’s atomic mass (~80) reflects a 50/50 mix of Br-79 and Br-81 isotopes.
Key Terms & Definitions
- Atom — the smallest unit of an element.
- Subatomic Particle — particles (protons, neutrons, electrons) that compose atoms.
- Nucleus — dense center of the atom containing protons and neutrons.
- Electron Cloud — region around the nucleus where electrons are found.
- Atomic Number (Z) — number of protons in the nucleus.
- Mass Number (A) — total number of protons and neutrons.
- Isotope — atoms of the same element with different numbers of neutrons.
- Ion — atom with an unequal number of protons and electrons (charged).
- Cation — positive ion (lost electrons).
- Anion — negative ion (gained electrons).
- Atomic Mass Unit (amu) — standard unit for atomic mass.
- Weighted Average — average considering relative abundances of isotopes.
Action Items / Next Steps
- Watch the assigned textbook video example on calculating protons, electrons, and neutrons in isotopes.
- Review periodic table entries for atomic number and atomic mass.
- Prepare for next class: naming conventions and chemical formulas for ions.