Development of Atomic Theory

Ancient Times | Dalton's Postulates | Thomson's

Discovery of Electron Properties | Rutherford's

Nuclear Atom | Bohr's Nuclear Atom | Modern

Atomic Theory

Learning Objectives• Understand the experimental design and

conclusions used in the development of modern atomic theory

Development of Atomic Theory

• Democritus proposed atomos– Indivisible, indestructible particles that

made up matter– Different types possessed different

properties– Changes in matter produced by changes in

the composition of groups containing different types

– Moved through empty space

• Aristotle’s beliefs– Matter composed of earth, air, fire, water– No such thing as empty space

• Idea of atom dismissed for thousands of years– Disagreement– Inability to prove ideas through

experimentation

Ancient Times

• Dalton’s Atomic Theory– Based on experiments

• Elements made of small indivisible particles called atoms

• Atoms of an element are identical to each other and different from atoms of other elements

• Chemical reactions occur when atoms are combined, separated, or rearranged in simplified whole-number ratios

• Atoms not created or destroyed in chemical reactions

Dalton’s Postulates

• Today’s revisions– Atom – the smallest unit of an element that

demonstrates the properties of that element– Atoms of the same element can have different

masses– Atoms can be divided into subatomic particles

• Cathode ray – a stream of electrons that emanate from a negative conductor (http://www.youtube.com/watch_popup?v=XU8nMKkzbT8)

• Electron – a negatively charged subatomic particle– J.J. Thomson proved the existence of electrons

• Mass of electrons is much smaller than the least massive atom– Proved subatomic particles exist

Thomson’s Discovery of Electron Properties

• Plum pudding model postulated by Thomson– Atoms made of a

positively charged mass

– Electrons scattered throughout the mass

– Failed to correctly describe the structure of the atom

Thomson’s Discovery of Electron Properties

• http://www.teachertube.com/video/rutherfords-experiment-8329

Rutherford’s Gold Foil Experiment

• Nucleus – the small, dense, positively charged area in the center of an atom where protons and neutrons are found– Discovered during Ernest Rutherford’s gold-foil

experiment• Nuclei of atoms repelled some positively charged

(alpha) particles, while other alpha particles traveled through empty space

Rutherford’s Gold Foil Experiment

• Nucleus– Small but accounts

for most of an atom’s mass

– Contains protons• Proton – a positively

charged subatomic particle that has a mass of approximately one atomic mass unit

– Contains neutrons • Neutron – a

neutrally charged subatomic particle that has a mass of approximately one atomic mass unit

• Rutherford’s model showed the nucleus surrounded by orbiting electrons

Rutherford’s Gold Foil Experiment

• Discovery that the emission spectrum of hydrogen contains only certain frequencies of light– Prompted Niels Bohr to propose another atomic

theory• Attempted to describe the relationship between energy

and movement of electrons

Bohr’s Nuclear Atom

• Electrons are arranged in circular orbits around the nucleus

• Electrons – In each orbit have a

fixed amount of energy• Nearest nucleus are

in their lowest energy state

– Gain and lose fixed amounts of energy• Release energy as

photons that correspond to specific frequencies of light

Bohr’s Nuclear Atom

• Bohr’s shortcomings– Only explained

emission spectrum of hydrogen

– Did not explain chemical properties

– Electrons do not revolve around nucleus in circular orbits

• Quantum mechanical model– Electrons exhibit

wave and particle properties

– Impossible to know both the speed and location of electrons at the same time• Region around the

nucleus where finding an electron is most probable is called an orbital

Modern Atomic Theory

Learning Objectives• Understand the experimental design and

conclusions used in the development of modern atomic theory

Development of Atomic Theory