Atomic Theories- Part I Chemistry Mrs. Coyle

A) Early Atomic Theories

Atom The word atom comes from the Greek and means “indivisible”.

Atom The smallest particle in an element that retains its identity in a chemical reaction.

Democritus – Greek philosopher 4th Century BC First to come up with “atom” Matter is composed of tiny particles called atoms These atoms are invisible, indestructible fundamental units of matter

Democritus’s ideas were opposed by Aristotle and Plato.

Antoine Lavoisier (France 1782) Law of Conservation of Mass In a chemical reaction mass is conserved.

Joseph Proust (France 1799) Law of Definite Proportions: The elements that comprise a compound are always in a certain proportion by mass.

John Dalton (England 1766-1844) School teacher Studied the ratios in which elements combine in chemical reactions Formulated first modern Atomic Theory

Dalton’s Atomic Theory All matter is made of atoms. Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

Dalton’s Atomic Theory Atoms of different elements can chemically combine in simple whole number ratios to form compounds. Example: CO2

Dalton’s Atomic Theory Chemical reactions occur when atoms are rearranged. Chemical reactions do not change atoms of one element to another.

Dalton’s Atomic Model

Much of Dalton’s theory still holds today However, we now know that Atomic Theory Much of Dalton’s theory still holds today However, we now know that atoms are not indivisible

Scanning Tunneling Microscope

Scanning Tunneling Microscope

Iron Atom Arrangement - STM

The Size of the Atom If you placed 100,000,000 Cu atoms side by side they would form a line only 1 cm long. Radius of most atoms is about 5x10-11 to 2x10-10m.

B) The Discovery of the Electron and the Proton

History Electron means “amber” in Greek Properties discovered by the Greek Thales of Miletos 600 BC. Rubbed the mineral amber with cat fur and attracted feathers.

Benjamin Franklin (America 1740’s) Law of conservation of charge. Saw electricity as a flowing fluid and called the flow direction positive.

Law of Charges Like charges repel Opposite charges attract

J(oseph) J(ohn) Thomson (England 1897) He discovered the electron while experimenting with cathode rays.

Cathode Ray

Deflection of Cathode Ray

Cathode Ray Tube

JJ Thomson with the CRT

Thomson’s Discovery He determined that the cathode ray was made of negatively charged particles – electrons.

Cathode Rays Thomson also was able to estimate that the mass of the electron was equal to about 1/1840 of the mass of a hydrogen atom. His discovery of the electron won the Nobel Prize in 1906.

Cathode Rays Thompson showed that the production of the cathode ray was not dependent on the type of gas in the tube, or the type of metal used for the electrodes. He concluded that these particles were part of every atom.

Charge of the Electron Charge of Electron 1.6 x 10-19 C (coulombs) Mass of Electron 9.11 x 10-28 g

Atoms have no net electric charge.

Ions Positively charge atom (cation) Negatively charged atom (anion) Atom lost electrons. Negatively charged atom (anion) Object gained electrons.

Electron is the basic quantity of charge. Electric charges always exist in whole number multiples of a single basic unit, the electron.

A particle with a positive charge must be present in the atom to balance each negatively charge electron.

Plum Pudding Model (Thomson)

Application of the CRT

Why is watching television potentially unsafe?

The Discovery of the Proton Discovered by Eugen Goldstein (German) in 1886. He observed “Canal rays” and found that they are composed of positive particles – protons.

Canal Rays Canal Rays passed through holes, or channels, in the reverse direction as the cathode ray. 

Canal Rays

c) Discovery of the Nucleus

Ernest Rutherford (Born in New Zealand 1871-1937) University of Manchester, England Tested Thomson’s theory of atomic structure with the “gold foil” experiment in 1910.

Gold Foil Experiment Bombarded thin gold foil with a beam of ‘alpha’ particles. If the positive charge was evenly spread out, the beam should have easily passed through.

Rutherford's Gold Foil Experiment Rutherford and coworkers aimed a beam of alpha particles at a sheet of gold foil surrounded by a florescent screen.

Rutherford Expected Found

Rutherford's Experiment Most particles passed through with no deflection, while some were highly deflected Rutherford concluded that most particles passed through because the atom is mostly empty space.

Rutherford’s Conclusions All of the positive charge, and most of the mass of an atom are concentrated in a small core, called the nucleus.

Size of Nucleus Compared to the Atom is as a Ball Compares to a Football Field.

The Discovery of the Neutron Discovered in 1932 by James Chadwick (England 1891-1974).

The Discovery of the Neutron Chadwick bombarded alpha particles(helium nuclei) at Beryllium. Neutrons were emitted and in turn hit parafin and ejected protons from the parafin.

Discovery of the Neutron

Neutrons Neutrons have mass similar to protons. No electrical charge.

Properties of Subatomic Particles Symbol Relative Charge Mass Relative to the Proton Mass (g) Electron e- 1- 1/1840 9.11 x 10-28 g Proton p+ 1+ 1 1.67 x 10-24g Neutron n0

Progression of Models

Dalton’s Atomic Model

Plum Pudding Model (Thomson)