Atomic Structure, Historical Models, and the Periodic Table

Historical Development of Atomic Theory

• Democritus: Proposed that matter is composed of indivisible particles called atoms ("uncut").

• John Dalton: Formulated atomic theory, modeling atoms as solid, indivisible spheres.

• J.J. Thomson: Discovered negatively charged subatomic particles called electrons ($e^-$) and proposed the "Plum Pudding Model".

• Ernest Rutherford: Discovered the dense, positively charged central nucleus containing protons ($p^+$) via the Gold Foil Experiment.

• Niels Bohr: Proposed quantized energy levels with electrons orbiting the nucleus in planetary paths.

• Electron Cloud Model: Modeled spatial probability density and location of electrons within an electron cloud.

• James Chadwick: Discovered the neutrally charged neutron ($n$) in the nucleus.

Subatomic Particles & Fundamental Atomic Structure

• Proton ($p^+$):

  • Charge: Positive ($+1$).

  • Location: Inside the central nucleus.

  • Mass: $1\,amu$.

• Electron ($e^-$):

  • Charge: Negative ($-1$).

  • Location: Outside the nucleus in orbitals.

  • Mass: Approximately $\frac{1}{2000}\,amu$ (negligible).

• Neutron ($n$):

  • Charge: Neutral ($0$).

  • Location: Inside the central nucleus.

  • Mass: Approximately $1\,amu$.

• Atomic Number ($Z$):

  • Unique number of protons in an atom's nucleus ($\text{Atomic Number} = \text{Protons} = \text{Electrons}$ in neutral atoms).

• Mass Number ($A$):

  • Total sum of protons and neutrons ($\text{Mass Number} = \text{Protons} + \text{Neutrons}$).

  • Calculation: $\text{Neutrons} = \text{Mass Number} - \text{Atomic Number}$.

• Isotopes:

  • Atoms of the same element with the same number of protons but different numbers of neutrons (e.g., Carbon-12 vs. Carbon-14).

Electron Configuration & Atomic Orbitals

• Orbital Shell Capacities:

  • $1\text{st shell} = 2\text{ electrons}$.

  • $2\text{nd shell} = 8\text{ electrons}$.

  • $3\text{rd shell} = 8\text{ electrons}$.

  • $4\text{th shell} = 18\text{ electrons}$.

• Electron Cloud Metaphor: Like a spinning airplane propeller, exact electron coordinates cannot be pinpointed, only their probability density.

• Electron Configuration: Arrangement of electrons in orbitals, most stable in the lowest energy state (ground state).

Reading the Periodic Table & Drawing Bohr Diagrams

• Periodic Table Information: Name, Symbol, Atomic Number, and Weighted Average Atomic Mass.

• Periods: Horizontal rows that indicate the total number of electron energy levels (shells).

• Bohr Diagrams: Step-by-step model drawing placing electrons in concentric shells sequentially around a central nucleus symbol.