4. Atomic Theory 2.1

Atomic Theory Overview

Historical Context

• Leucippus (450 BCE): Proposed the idea of indivisible matter.

• Democritus: Expanded on Leucippus' idea, named the indivisible particle "atomos."

  • Believed different atoms caused different properties.

• Aristotle's Influence: His four-element theory (earth, air, fire, water) overshadowed atomic theory for centuries.

John Dalton's Contribution (1808)

• Authored A New System of Chemical Philosophy.

• Proposed:

  • All matter is made of indivisible atoms.

  • Atoms of different elements have different weights.

  • Compounds form from atoms of two or more elements in simple ratios.

  • Atoms of a given element are identical in size and mass.

• Impact: Laid the foundation for modern atomic theory even as some of his ideas were later disproven.

Structure of Atoms

The Transition to Subatomic Particles

• By the late 1800s, evidence suggested atoms consist of smaller particles.

J.J. Thomson's Experiments

• Used a cathode ray tube to discover electrons in 1897.

  • Particles flowed from a negatively charged electrode to a positively charged one.

  • Concluded particles were negatively charged (electrons).

• Developed the 'plum pudding' model: Electrons scattered in a positive charge.

Ernest Rutherford's Contributions (1910)

• Conducted the gold foil experiment:

  • Most alpha particles passed through foil; some were deflected.

  • Concluded most of an atom is empty space; proposed a tiny, dense nucleus.

• Discovered protons in 1920 by observing nitrogen gas.

• Proposed existence of neutrons, later confirmed by James Chadwick.

Electron Behavior and Nucleus

Electrons

• Negatively charged particles in a cloud around the nucleus; mass is ~1800 times less than protons/neutrons.

• Cloud of electrons defines the atom's size.

• Attracts protons via electrostatic attraction.

• Common experiences (e.g., static electricity from balloons) caused by electron transfer.

• Electrons in current form the basis of electricity.

Nucleus Overview

• Nucleus is extremely small; comprises about 99.97% of the atom's mass.

• Contains protons (positive) and neutrons (neutral).

• Size analogy: If an atom were the size of Perth Stadium, the nucleus would be about the size of a pea.

Summary of Subatomic Particles

###Properties of Subatomic Particles

Advanced Concept: Quarks

• Discovered in the early 1960s as components of protons and neutrons.

• Six types of quarks: up, down, strange, charm, top, and bottom.

• Contributed to a broader understanding of elementary particles with the Higgs Boson confirmed in 2012.

Review Section

Summary Points

• Atoms: Basic units of all substances.

• Comprised of a positively charged nucleus and a negatively charged electron cloud.

• Nucleons: Refers collectively to protons and neutrons in the nucleus.

• Size and Mass: Protons/neutrons have similar sizes; electrons are significantly smaller. Neutrons are uncharged.

• Electrostatic Attraction: Governs the interactions between electrons and protons.

Key Questions

1. Name the three subatomic particles that atoms are composed of.

2. What subatomic particles make up the most mass of an atom and where are they found?

3. How are electrons held within the cloud surrounding the nucleus?

Answers

1. Proton, neutron, and electron.

2. Protons and neutrons; they are found in the nucleus.

3. Electrostatic attraction binds negative electrons to positive protons in the nucleus.