Atomic Structure and Chemical Bonds

Chemistry 101 Bio 121 Lecture 03 CH 02 Notes

Announcements

  • Bio 131 (lab) starts this week.
  • SENG 331 - Qwickly details were mentioned, though not elaborated.

Main Points Overview

  1. Overview of atoms: What they are made of and their fundamental properties.
  2. How do we know what atoms are? A historical perspective on atomic models.
  3. Neutrons don't change chemical reactions: Focus on their role in isotopes and dating.
  4. Protons give atoms their identity, electrons "do chemistry": The specific roles of these subatomic particles.

1. What is Matter Made Of?

  • Subatomic Particles:
    • Protons: Positively charged particles, found in the nucleus.
    • Neutrons: Neutrally charged particles, found in the nucleus. They act like glue, shielding protons from each other.
    • Electrons: Negatively charged particles, orbit the nucleus.
  • Atomic Structure: A "textbook" atom typically has a 1:1:11:1:1 ratio of protons:electrons:neutrons.
  • Definition of an Atom: An atom is the smallest piece of matter that retains the properties of an element.

Atoms are Forged by Nuclear Fusion in Stars

  • Mass-Energy Equivalence: Explained by Einstein's famous equation, E=MC2E = MC^2, which states that mass and energy are interchangeable.
  • Stellar Nucleosynthesis:
    • Stars initially fuse Hydrogen atoms into Helium, converting some mass into energy in the process.
    • When a star exhausts its Hydrogen supply, Helium atoms are fused together, and this process continues with larger and larger atoms being formed.
    • Eventually, massive stars explode in a supernova, spreading these newly forged elements throughout the universe.
  • **Elemental Abundance: **
    • In Living Things: Hydrogen, Oxygen, Carbon, and Nitrogen are the most common/abundant elements.
    • In the Universe: Hydrogen, Helium, Oxygen, Carbon, and Nitrogen are the most common elements.
    • Trace Elements: Other elements are used in biology in very small quantities.

2. How Do We Know What Atoms Are? – A Brief History of Atomic Models

  • Thomson (Late 1800s - Early 1900s):
    • Initially proposed a "nebular atom model" where atoms were thought not to be made of particles but of immaterial vortices.
    • Discovery of Electrons (1897): This pivotal discovery proved that atoms do have particles within them.
    • "Plum Pudding" Atomic Model (1904): Thomson then proposed that atoms consist of a diffuse positive cloud embedded with negative electrons, much like plums in a pudding. This model suggested a uniform distribution of mass and charge.

Refutation of the Plum Pudding Model: The Gold Foil Experiment

  • 1909 - Hans Geiger and Ernest Marsden: Performed the "gold foil" experiment to test Thomson's model.
  • Experiment Setup: Positively charged alpha particles were directed at a thin sheet of gold foil.
  • Prediction (based on Plum Pudding Model): Due to the diffuse nature of the positive charges in the plum pudding model, alpha particles were predicted to pass through the foil with very little deflection or scattering.
  • Observation:
    • Most alpha particles did fly straight through, just as predicted.
    • However, some particles were highly scattered (deflected at large angles), and a very small percentage even bounced back. This observation directly contradicted the plum pudding model's prediction.
  • Conclusion & New Model:
    • The experiment demonstrated that atoms must have a dense, positively charged nucleus at their center, with electrons orbiting it (similar to a solar system model, though actual electron orbits are not simple circles).
    • Later, it was predicted that the positively charged protons within the nucleus are held together by neutrons, which act like a