Atomic Theory Chapter 4

Atomic Theory  Science is based off of observations.  A Scientific Law is a summary of what is seen in observations.  A Scientific Theory is an explanation of why these observations are occurring.  Both laws and theories are tested by using them to predict what would happen in certain situations.

Atomic Theory  -Explains why all matter acts as it does because it is composed of tiny particles called atoms, the basic building blocks of all matter  First theorized by Democritus ~400 BC in ancient Greece.  He thought that matter could NOT be divided infinitely. You had to reach a smallest possible piece. He named this piece an atom, which meant indivisible or can’t be cut.

More Democritus  He and his followers, atomists, said atoms were small hard particles all made of the same material, but different sizes and shapes.  They were also always moving and capable of joining together.  Since this was ancient Greece, He and the atomists had no way of ever proving his theories and few people believed it.

Law of Conservation of Mass  A chemical reaction does not change the mass of the matter present.  The mass of the reactants is equal to the mass of the products.  This is true even if a different phase of matter is created.

Law of definite proportions  1790’s  All pure compounds have exactly the same proportions by mass of elements regardless of size  Water is always 2 hydrogen atoms to every 1 oxygen atom  By mass that is 2 (g) H : 16 (g) O

Dalton’s Model (1803)  people started to accept the idea of atoms because of his experiments  He worked with gases and found that they acted as though they were made of solid microscopic particles  all elements are made of atoms (indivisible and indestructible)  atoms of the same element are exactly alike  atoms of different elements are different  compounds are formed by joining two or more elements

John Dalton

Thomson’s Cathode Ray

Thomson’s Model (1897)  found negative particles could come from neutral elements  atom is made of smaller things (+ & -), and is divisible  successfully separated negative particles (electrons) but could not separate the positive particle (protons)  “plum pudding model” negative particles floating in a positively charged gel like material

Plum Pudding Model- Thomson Positive Gel Negative Particles

Sir J. J. Thomson The sir means he was knighted

Rutherford’s Model (1911)  fired protons at a sheet of gold foil most went through unaffected, some bounced away  there is a small dense area of positive particles at the center of the atom- the nucleus  electrons are scattered near the outside of the atom with mostly empty space between the nucleus and the electrons

Gold Foil Experiment Radioactive source Gold foil

Rutherford’s Model nucleus (small dense positive area) electrons Empty Space

Ernest Rutherford

Bohr Model (1913)  electrons move in definite orbits around the nucleus  these orbits or energy levels are located at certain distances from the nucleus

Bohr’s Model nucleus Electrons

Neils Bohr

Wave Model (present day)  based on complex math equations  orbits are more complex than originally thought  de Broglie stated that electrons (particles) have wave properties, and he viewed these as standing waves, like those produce when a guitar string is plucked (classical physics.)  Schrodinger assumed that the electron in Hydrogen behaves as a standing wave.

Wave Model (continued)  When Schrodinger’s equation is analyzed, many solutions are found.  Each solution represents an atomic orbital.  An atomic orbital is the most probable location for finding an electron.

What is an orbital?  It is not a Bohr orbit (not moving in a circular path.)  How is the electron moving?  We don’t know!  There is a fundamental limitation to just how precisely we can know both the position and momentum of a particle at a given time

This is kind of how we assume an electron travels e-e-

Heisenberg Uncertainty Principle  The more accurately we know the particle’s position, the less accurately we can know it momentum and vice versa.  We can’t know the exact motion of the electron around the nucleus.  The area that an electron orbits is called an “electron cloud”

Louis de Broglie Erwin Schrodinger

Werner Heisenberg