From Democritus to Darth Vader: Development of Atomic Theory

Democritus (400 B.C.) Proposed that matter was composed of tiny indivisible particles Greek: atomos, means “not cuttable” OR “uncuttable”

Aristotle (300 B.C.) All matter were mixtures of the four elements of earth, air, fire, and water with associated properties.

Alchemy (next 2000 years) Mix of science, magic, and philosophy. In search of the Philosopher’s Stone and Elixir of Life. Contributed to the beginning of modern chemistry.

Robert Boyle (1691) Re-introduced the idea of the atom Displaced Aristotle’s proposal of the 4 elements

John Dalton (1807) Based his theory on others’ experimental data Billiard ball model : the atom is a uniform, solid sphere

John Dalton 1.All matter is composed of extremely small indivisible particles called atoms. 2.Atoms of the same element are identical in size, mass, and properties. Atoms of different elements are different. 3.Atoms of different elements combine together in simple proportions to create a compound. 4.In a chemical reaction, atoms are combined, separated or rearranged, but not changed.

J. J. Thomson (1897) Cathode Ray Tube Experiments –beam of negative particles Discovered Electrons –negative particles within the atom Plum-pudding Model

J. J. Thomson Passed electric current through a gas in a cathode ray tube. Rays were deflected by electric and magnetic fields. Concluded that rays were made of negatively charged particles.

J. J. Thomson Plum-pudding Model –positive sphere (pudding) with negative electrons (plums) dispersed throughout

Ernest Rutherford (1911) Niels Bohr (1913) Atoms, the Clash of the Titans

Ernest Rutherford (1911) Gold Foil Experiment Discovered the nucleus –dense, positive charge in the center of the atom Nuclear Model

Ernest Rutherford Gold Foil Experiment Bombarded thin gold foil with α particles Observed impact of particles on fluorescent screen

Ernest Rutherford Expected results: α particles would pass through plum-pudding atom Undeflected. Observed results: Some α particles were deflected.

Ernest Rutherford Nuclear Model –dense, positive nucleus surrounded by negative electrons in much empty space

James Chadwick (1932) Discovered neutrons –neutral particles in the nucleus of an atom

James Chadwick (1932) Nuclear Model revision of Rutherford’s Nuclear Model

Flaming Colors Lab Emission Spectra of Gases Flame test of metal salts

Light When you shine a bright beam of light from the sun or an incandescent light through a prism, the incoming white light will be separated into a continuous spectrum of colors.

Line spectra of fluorescent tubes Electrically excited gases of elements is passed through a prism to produce bright line spectra. Energy is given off only at specific wavelengths. Each element has a unique emission spectrum.

Emission Spectra

Niels Bohr (1913) Bright-Line Spectrum –tried to explain presence of specific colors in hydrogen’s spectrum Energy Levels –electrons can only exist in specific energy states Planetary Model

Bohr’s Model Nucleus Electron Orbit Energy Levels

Bohr’s Model Electrons orbiting around the nucleus at fixed distances, corresponding to specific energy levels. Electrons can jump between these special orbits by gaining or giving up energy in the form of light.

Bohr Model Bohr calculated energies matched the spectral lines for the H atom. But Bohr’s calculations only worked for H atom.

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Quantum Mechanics Energy is not continuous, but comes in small, discrete units (quanta) Electrons behave both like particles and like waves The movement of electrons is inherently random It is physically impossible to know both the position and velocity of an electron at the same time

Erwin Schrödinger (1926) Quantum mechanics –electrons can only exist in specified energy states Electron cloud model –orbital: region around the nucleus where e - are likely to be found

A. The Electron Cloud Model Orbital Unlike Bohr’s model in which electrons occupied definite orbits, this model describes orbital clouds as regions in space around the nucleus in which an electron is most likely to be found.

The Electron Cloud Model Electrons do not move about an atom in a definite path, like the planets around the sun. It is impossible to determine the exact location of an electron. Its probable location is based on how much energy it has.

The Electron Cloud Model The atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral. Electron cloud: the space in which electrons are likely to be found. –Electrons with the lowest energy are found in the energy level closest to the nucleus. –Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus.

A. The Electron Cloud Model dots represent probability of finding an e - (not actual electrons)

Electron Cloud Model

Energy Levels in an Atom Energy Level Electrons

Electron Cloud Model

Electron Model - subshells

Electron Cloud Model (Sodium Atom)

What happens when you electrocute a pickle??

Modern Developments Based on the Atom Spectroscopy in Forensic Science LASERS

Spectroscopy Spectroscopy is the measurement of the absorption, scattering, or emission of electromagnetic radiation by atoms or molecules. Qualitative analysis: determine what element is in the sample Quantitative analysis: determine how much of each element is in the sample

Spectroscopy in Drug Testing

NASA’s Deep Impact Mission Material ejected from the impact will be analyzed to determine what is inside a comet and whether the material has remain unchanged through time.

Emission Spectrum from Halley’s Comet

Laser / Atom Connection Light Amplification by Stimulated Emission of Radiation The light released is monochromatic (one color or wavelength) The light released is coherent (all photons are moving in-step) The light is very directional (strong, concentrated, tight beam)

LASER Brainpop scienceandindustry/lasers/