Chapter 2 Lecture 1: Atoms, Ions and Molecules

Introduction to Atomic Theory

• Overview of atomic theory as the foundational concept in chemistry, describing the building blocks of matter.

Key Subatomic Particles

• Electrons:

  • Discovered in 1897 by J.J. Thomson during experiments with a cathode ray tube.

  • Found to be negatively charged particles.

  • Operating principle of old televisions, where beams of electrons strike phosphor screens.

• Atomic Structure Prior to the 1900s:

  • Initial belief in a "plum pudding model" where electrons are spread across a positive 'sea'.

Major Experiments in Atomic Theory

• Millikan's Oil Drop Experiment:

  • Aimed to determine the charge and mass of electrons, ultimately revealed the charge-to-mass ratio.

• Rutherford's Gold Foil Experiment:

  • Conducted in 1908, involving alpha particles shot at thin gold foil.

  • Observed that most alpha particles passed through, while some were deflected at sharp angles.

  • Led to the discovery of a tiny, dense nucleus where most of the atom's mass is concentrated.

Development of the Nuclear Model

• Nuclear Atom Model:

  • Proposed by Rutherford, highlighting a small, dense, and positively charged nucleus containing protons and neutrons.

  • The electrons then exist in a diffuse 'cloud' around the nucleus.

• Subatomic Particles:

  • Proton: Positively charged particle within the nucleus.

  • Neutron: Neutrally charged particle, discovered in the 1930s.

• Relative Sizes:

  • Nucleus: �.1 picometers in diameter.

  • Atom: About 300 picometers in diameter.

  • Illustrative analogy: If the nucleus is the size of a pinhead on the 50-yard line of a football field, the entire stadium represents the atom's size.

Summary of Subatomic Particles

• Mass and Charges:

  • Proton: +1 charge, approximately 1 atomic mass unit.

  • Neutron: Neutral charge, approximately 1 atomic mass unit.

  • Electron: -1 charge, significantly lighter than protons and neutrons.

• Atomic mass units are commonly based on Carbon-12, where 12 nucleons (6 protons and 6 neutrons) define the mass unit.

Isotopes

• Defined as nuclei with the same number of protons but different numbers of neutrons.

• Symbolizing isotopes:

  • X = element symbol, Z = atomic number (number of protons), A = atomic mass number (protons + neutrons).

• Examples of isotopes:

  • Carbon Isotopes: Carbon-12, Carbon-13, etc.

  • Oxygen Isotopes: Oxygen-16, Oxygen-17, Oxygen-18.

  • Neon Isotopes: Neon-20 (10 protons, 10 neutrons), Neon-22 (10 protons, 12 neutrons).

Next Steps

• Future lesson will focus on the periodic table and the organization of elements.