Monday, November 16, 2009

The Atomic Theory

Atomic Theories

The idea of an atom -- the smallest particle of matter -- has intrigued mankind since the beginning of civilization.  Throughout the centuries the "view" of the atom has changed.  New ideas, and new technologies have influenced the model of the  atom.  This view of the atom is still a Theory and therefore it is still subject to change.  The  modern model of the atom is called the Quantum Model. The chart below summarizes the various atomic models that have been developed during the course of history.

Scientist & approximate Date

Name of Model, Sketch
and main idea of theory

Importance and
Improvement on previous model

Shortcomings - Problems
or why was it changed


Democritus 
 c.300 BC

 Atom the indivisible particle

Atomos (in ancient Greek) means "that which cannot be further broken down into smaller pieces".

  • Talks about the atom as the smallest particle of matter.
  • Defines the atom as an indivisible particle
  • Explains certain natural occurrences such as the existence of elements
  • Does not give a scientific view of the atom only a conceptual definition
  • Does not talk about subatomic particles
    (Electrons, Protons, Neutrons)

  


Dalton
  c.1800

 The solid sphere model

Atoms are seen as solid, indestructible spheres (like billiard balls)

  •  Explains a lot of chemical properties such as how atoms combine to form molecules
  • Explains chemical change better than the  Particle Theory
  • Confirms the basic Laws of Chemistry: Conservation of Mass & definite Proportions
  •  Does not include the existence of the nucleus
  • Does not explain the existence of ions or isotopes
  • Does not talk about subatomic particles
    (Electrons, Protons, Neutrons)


J.J. Thomson
 c.1850

 The raisin bun Model or the
chocolate chip cookie model :
 Atoms are solid spheres made-up of a solid positive mass (or core) with tiny negative particles embedded in the positive core.

  •  Infers on the existence of electrons and protons
  • Introduces the concept of the nucleus
  • Infers on the relative nuclear density and atom mass of different atoms
  •  Does not explain the existence of electrons outside the nucleus does not explain the role of electrons in bonding
  • Does not talk about neutrons therefore can't explain radioactivity and the existence of isotopes


Rutherford
c. 1905

 The Planetary Model

Famous Gold Leaf Experiment proves that the nucleus is positive and the electrons are outside the nucleus.

 

  •  Does not place electrons in definite energy levels around the nucleus
  • Doesn't include neutrons in the nucleus
  • Does Not relate the valence electrons atomic charge


(Neils Bohr)

Bohr- Rutherford
c. 1920

 Electrons in Definite energy Levels around  the nucleus

Used atomic spectra to prove that electrons are placed in definite orbitals (called shells) around the nucleus.

 

  •  Explains the role of valence electrons in bonding
  • Relegates the number of valence electrons to the Periods of a periodic table
  • Fully explains ionic and covalent bonding
  • Places electrons in definite energy levels
  • 2 e- in the first
  • 8 e- in the second
  • 8 e- in the third


  •  It does not explain the shapes of molecules or other abnormalities that result form unevenly shared pairs of electrons (such as the abnormal behaviour of water, the difference in Carbon-Carbon Bonds between diamond and graphite etc..)

Modern Theory

Many Scientists Contributed.  Some of the more famous  are:

  • Schroedinger
  • Einstein
  • Luis De Broglie
  • Max Planck
  • Frank Hertz
  • Maxwell
  • Fermi

Quantum Mechanical Model

or Electron Cloud Model

The analogy here is that of a "beehive" where the bees are the electrons moving around the nucleus in a "cloud" of energy levels.

Advanced Theories will explain bonding and other facts about the behaviour of atoms and their chemical and physical properties in forming new compounds.

  

 

Other important facts about the particles of an atom:

Subatomic Particle

Symbol

Charge

Relative Mass*

Location

Proton

p+

positive

2000

nucleus

Electron

e-

negative

1

orbits around nucleus

Neutron

n0

neutral (zero)

2000

nucleus

*Relative mass means that is the electron has a mass of 1 unit, the proton and neutron will have a mass 2000 times that of the electron.

The mass number (also known as atomic mass or atomic weight) and the atomic number from the Periodic Table are very important numbers because they tell us how many subatomic particles are contained in a given atom.

The atomic number tells us the number of electrons and the number of protons., i.e. Atomic Number = Number of electrons = Number of Protons.  The atomic mass tells the total number of particles in the nucleus, i.e. Atomic Mass = # of protons + number of neutrons.

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