The Story of the Atom History and Other Early Stories

Early Philosophers  Science didn’t always exist as we know it today – no controlled experiments, no known information about atoms.  Curiosity and observations sparked these early philosophers to think about the mysteries of life.

Greek Philosophers  Observations led to the thought that matter was composed of earth, water, air and fire.  Matter could endlessly be divided into smaller pieces.  No way to test the validity of these claims.

Democritus (more than 2000 Years ago!!)  Atomos are tiny individual particles  Matter is composed of empty space through which atoms move  Atoms are solid, homogeneous, indestructible, and indivisible  Different kinds of atoms have different sizes and shapes  The differing properties of matter are due to the size, shape, and movement of atoms  Changes in matter result from changes in the groupings of atoms; not from the changes in atoms themselves

Aristotle… traitor to Democritus’ thoughts And not nearly as handsome as Democritus  Influential Greek philosopher  Criticized Democritus – atoms can’t move through empty space since the “nothingness” of empty space doesn’t exist.  Primitive science and the great influence of Aristotle led to the rejection of Democritus’ ideas about 2000 years later.

John Dalton  English schoolteacher who revised Democritus’ ideas based on scientific research which led to Dalton’s Atomic Theory All matter is composed of extremely small particles called atoms All atoms of an element are identical, having the same size, mass, and chemical properties. Atoms of a specific element are different from those of any other element

Dalton’s Atomic Theory Continued  Atoms cannot be created, divided into smaller particles or destroyed  Different atoms combine in simple whole- number ratios to form compounds  In a chemical reaction, atoms are separated, combined, or rearranged

Advancements in Dalton’s Theory (Why is Dalton so successful?)  Ability to perform experiments and refine theories  Chemical reactions explained law of conservation of mass  All evidence led to the acceptance of Dalton’s Theory until a few pieces of missing info…

Problems with Dalton’s Theory?  Are atoms divisible?  Proton  Neutron  Electron  Do atoms of a given element have identical properties?  Differing masses

Subatomic Particles  Proton (p+): positively charged Found in the nucleus  Neutron (n 0 ): neutral charge Found in the nucleus  Electron (e-): negative charge Found surrounding the nucleus

Definition of an atom (Current)  The smallest particle of an element that retains the properties of the element  Size of atom World population = Atoms in a penny = So, how did we learn about the subatomic particles that make up an atom?

Discovery of the Electron  Charles Goodyear American automobile tires Mixture of rubber and sulfur  Cathode Ray Tube  Tube filled with low pressure gas  Cathode (+) disk  Anode (-) disk  Vacuum pump

Evidence of the Electron  Anode produces a thin beam of e-. Altering of different gases has no effect on the cathode ray. The particles in the ray are all matter.  Cathode ray is deflected in a magnetic field indicating it is composed of charged particles.  Cathode ray deflected toward the (+) charged plate by an electric field, which means the ray must have a (-) charge.

J.J. Thomson – identified 1 st subatomic particle!!  Used cathode ray experiments to determine the ratio of its charge to its mass  Smaller mass than any element  Proved Dalton was wrong: Atoms are divisible into smaller subatomic particles (e-)

JJ Thomson  Plum Pudding Model (or Chocolate Chip Ice Cream Model)  Atoms are neutral  Chips = (-) charged particles  Vanilla = (+) charged matter

Nuclear Atom and Rutherford  Gold foil experiment  (+) charged alpha particles (Helium atoms with 2 missing electrons; double positive charge) deflected at large angles  Proof of the nucleus  Rutherford New and Improved Nuclear model:  Positive charge and all the mass are concentrated in a small region in the center of the atom.  He called this region the nucleus and suggested it was composed of the protons and neutrons.  This region is extremely dense.

Robert Millikan and the e-  Another American  Determined the charge of an electron (1-)  The e- is found in the space surrounding the nucleus  Mass of an e- is _ 1 _ of a hydrogen atom 1840

Chadwick  Nucleus also contained a particle called a neutron. Neutrons have a mass of 1 amu and a neutral charge.  Atoms consist of a “cloud” of fast moving, (-) charged e- surrounding a tiny, dense nucleus containing virtually all mass, but occupying only about 1/10 th of the volume of the atom.