Evolution of the Atomic Model From Dalton to Quantum Mechanics

Who are Gerd Binning and Heinrich Roher ? Invented the scanning tunneling microscope. Produces an image of individual atoms.

Gold Atoms STM Image STM Image

STM image, 7 nm x 7 nm, of a single zig-zag chain of Cs atoms (red) on the GaAs(110) surface (blue). STM image, 35 nm x 35 nm, of single substitutional Cr impurities (small bumps) in the Fe(001) surface.

The Four Elements Ancient thought. Aristotle Alchemy The basis for western thinking about the natural (especially inanimate) world right up until the rise of chemistry in the 18th century

Democritus of Abdera First suggested the existence of tiny fundamental particles that make up matter which he called atoms. Atoms were indivisible and indestructible. Lacked experimental evidence.

John Dalton English schoolteacher Performed experiments to test and correct his theories.

Dalton’s Atomic Theory All elements are composed of tiny individual particles called atoms. Atoms of the same element are identical. Atoms of an element are unique to that element. Atoms of different elements can physically mix together or can chemically combine to form compounds. Atoms of one element can never be changed into atoms of another element.

J.J. Thompson Discovered electrons in 1897

Thompson’s Experiment

Thompson’s Atom The Plum pudding model of the atom was made after the discovery of the electron but before the discovery of the proton or neutron. In it, the atom is envisioned as electrons surrounded by a soup of positive charge

The Gold Foil Experiment Ernest Rutherford The Gold Foil Experiment

The Gold Foil Experiment

What Happened?

Why It Happened

Rutherford’s Atom Most of the atom is made up of empty space. Each atom has a small, dense, positively charged nucleus. Electrons are located around the nucleus.

Rutherford’s Model of the Atom

Niels Bohr Danish physicist and student of Rutherford. 1885 to 1962

Bohr’s Model of the Atom Electrons are arranged in concentric circular paths, or orbits, around the nucleus. Electrons orbit the nucleus in much the same way planets orbit the sun. Electrons in a particular path have a fixed energy.

Quantum Mechanical Model The electron cloud model. During the 1930’s and 1940’s experiments provided additional information that resulted in modifications to Bohr’s model. Primarily a mathematical model.

What’s Different About This Model? Shows the most probable location of an electron. Electrons are shown as a dense cloud. Higher Density=Higher Probability Shows the shape of the space in which electrons are found 90% of the time.

What about the nucleus? The concept of the nucleus remains unchanged. It is small, dense and positively charged. It contains both protons and neutrons.