DISCOVERY OF THE ATOM

Atoms, Molecules and Ions

Learning Outcomes

• Learn the basic postulates of Dalton’s atomic theory.

• Describe the key experiments that led to the discovery of electrons and the nuclear model of the atom.
• Describe the structure of the atom in terms of protons, neutrons, and electrons and express the relative

Learning Content

Atomic Theory of Matter

Radioactivity

Dalton's Atomic Theory

Atomic Models

Discovery of the Atomic Structure

Law of Constant Composition

Disovery of the Nucleus

J.J Thomson's Cathode Rays

Law of Conservation of Matter

Discovery of Sub-atomic particles

Milikan's Oil Drop Experiment

Law of Multiple Proportions

Atomic Theory of Matter

Democritus (Greek philosopher) believed that there was a smallest particle— “atomos” (uncuttable, indivisible)—that made up all of nature

Experiments in the 18th and 19th centuries led to an organized atomic theory by John Dalton in the early 1800s, which explained several laws known at that time: – (1) Law of constant composition – (2) Law of conservation of mass – (3) Law of multiple proportions

Law of Conservation of Matter

• The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place.
• Basis of Dalton’s Postulate #3
• Can’t create matter in a chemical reaction!

Law of Constant Composition

• Also known as the Law of Definite Proportions.
• The elemental composition of a pure substance never varies
• In a given compound, the relative numbers and kinds of atoms are constant. • Basis of Dalton’s Postulate #4

+info

Law of Multiple Proportions

+info

Dalton's Atomic Theory

Discovery of Atomic Theory

1800

Mid 1800’s – scientists studied electrical discharge through partially evacuated tubes.

+info

J.J. THOMSON'S CATHODE RAYS

• JJ Thomson proposed in 1897 that Cathode Rays were particles (negatively charged) that we now know are electrons.
• Thomson measured the charge/mass ratio of the electron to be 1.76x10^8 Coulombs/gram (C/g).
• Once the charge/mass ratio of the electron was known, determination of either the charge or the mass of an electron would yield the other

Millikan Oil Drop Experiment

• X-ray irradiation causes oil drops to pick up one or more electrons.
• Gravitational force (Fg ) pulls the oil droplets downward, opposed by electric force (Fe ) that pulls droplets upward.
• Robert Millikan (1909) determined the charge on the electron, equal to 1.60x10^-19 C.

+info

Radioactivity

The spontaneous emission of radiation by an atom

First observed by Henri Becquerel. (1852-1908)

Also studied by Marie (1867-1934) and Pierre Curie (1857-1906). Discovered Po and Ra. Suggested that atoms of certain substances can disintegrate.

Atomic Models

PLANTERARY MODEL

NUCLEAR MODEL

SOLID SPHERE MODEL

John Dalton

PLUM PUDDING MODEL

Neils Bohr

Ernest Rutherford

QUANTUM MODEL

J.J Thomson

Erwin Schrodinger

Discovery of the Nucleus

Ernest Rutherford shot alpha particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.

Sub-atomic particles