Modern Theories of the Atom

Modern Theories of the Atom Discovery of the Electron through the Neutron

Cathode-Ray Tubes • Sealed glass tube with a small amount of gas inside (low pressure) and metal electrodes at either end. • Developed in the 1870’s – looks like neon lights. Crooke's Tubes

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Victorian Party Novelty

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An electric field or a magneticfield will… …deflect a beam of charged particles.

J.J. Thomson discovered the electron! 1897 source

Thomson’s Cathode Ray Tube source Thomson proposed that cathode rays were streams of particles much smaller than atoms. Found the charge-to-mass ratio of the electron.

Atom is Divisible! • Thomson’s discovery meant that the atom was divisible! • He knew there had to be an equal amount of positive charge because matter is neutral.

Thomson’s Plum-Pudding Model The positive charge is evenly smeared out- like a pudding. The negative charge is in bits – like raisins. source

Robert Millikan - 1909 • Oil-Drop Experiment • determined the charge of the electron: • 1.60 X 10-19 coulomb • Thomson had determined the • charge-to-mass ratio: • 1.76 X 108 coulomb per gram • So Millikan calculated the mass of the electron to be: 9.09 X 10-28 grams. Animation of Oil-Drop Experiment

Proton – Discovered by 1920 • Thomson & Goldstein – 1907 • Discovered a heavy particle with a positive charge in some cathode ray tube experiments. • Rutherford – 1918 • Shot alpha particles at nitrogen gas and got hydrogen. Figured out that the hydrogen had to come from the nitrogen. Suggested that the hydrogen nucleus was an elementary particle. Named it the proton. animation

Rutherford • Famous for many experiments. • Discovered the proton. • Figured out  and  radiation. • Changed our idea of the atom! • One of the most elegant experiments in the history of science!

Rutherford’s Experiment - 1911 source

What did Rutherford conclude from the particles that went straight through? What did Rutherford conclude from the particles that bounced back? Animation source

Rutherford's Results • The atom is mostly empty space! • The size of the nucleus is small compared to the size of the entire atom. • The mass of the atom is concentrated in the nucleus of the atom. • The nucleus has a positive charge.

So how big is the nucleus compared to the entire atom? • Analogy: If the atom was as big as a football stadium, the nucleus would be smaller than a flea on the 50-yard line!

Rutherford proposed the nuclear atom. source Rutherford did not speculate on how the electrons were arranged around the nucleus. source

Neils Bohr - 1913 Planetary Model source

Electrons travel only in specific orbits. • Each orbit has a definite energy (lowest inner). • Atoms emit radiation when an electron jumps from an outerinner orbit. • Outer orbits hold more electrons than inner orbits. • Outer orbits determine atom’s chemical properties. source

Schrodinger – 1926 • Mathematically - treated electrons as waves rather than particles! • Quantum Mechanical Model • Modern Model • Cloud Model

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Modern (Cloud) Model • Electron’s energy has only certain values – it is quantized. (Bohr model had quantization too!) • Electrons are located in “probability regions” or atomic orbitals. These are not circular orbits! • Electrons move around the nucleus at near the speed of light.

Schrodinger’s Model • We talk about the probability of locating an electron at a certain place. • Orbital – a region in an atom in which an electron of a particular amount of energy is most likely to be located. • standing wave patterns with definite energy.

James Chadwick - 1932 • Discovered the neutron in cloud chamber experiments. • About the same mass as a proton. • Electrically neutral. • The neutron changes atomic mass but not the element.

The development of atomic theory represents the work of many scientists over many years. source

Modern Theories of the Atom