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ATOMIC THEORY
• Over the course of thousands of years our idea of what matter is made of and what the atom looks like has changed dramatically• There are 6 major contributors, although there
are many many many others who have helped out along the way• The 6 major contributors to the atom were
Democritus, John Dalton, J.J. Thomson, Ernest Rutherford, Neils Bohr, and Erwin Schrodinger
DEMOCRITUS
• Democritus (460 – 370 B.C.) was one of the first people to describe the atom• Unlike all of the other contributors to the atom,
his thoughts were NOT based on scientific evidence and research, but just his own thoughts• Democritus thought that atoms were indivisible
and indestructible
JOHN DALTON
• John Dalton (1766 – 1844) was the first person to describe the atom using scientific evidence and research• He studied the ratios in which elements combine
in chemical reactions to come up with his theory
JOHN DALTON
John Dalton’s Atomic Theory can be explained in 4 parts:1. All elements are composed of tiny indivisible particles
called atoms2. Atoms of the same element are identical. The atoms of
one element are different from the atoms of all other elements
3. Atoms of different elements can physically mix together (mixtures) or chemically combine in whole ratios to form compounds
4. Chemical reactions occur when atoms are separated, joined, or rearranged in different combinations. Atoms of one element will never change into atoms of another element as a result of a chemical reaction.
J. J. THOMSON
• In 1897 J.J. Thomson discovered the electron!• With this discovery we no longer think that the
atom is indivisible, because we know that it is now made up of even smaller parts
J. J. THOMSON
His experiment is the cathode ray experiment:
• He passed an electric current through gases at low pressure in a sealed glass tube with electrodes at both ends, this produced a cathode ray (a glowing beam)
• He then did tests on the cathode ray with electrically charged plates, and he found that the ray went away from the negatively charged plate and towards the positively charged plate
• He knew that like charges repel and opposite charges attract so he reasoned that the ray was negatively charged
J. J. THOMSON
• J.J. Thomson knew that the atom had an overall neutral charge, so he thought the negative electrons were evenly distributed through the positive atom• His model is known as the plum pudding model
ERNEST RUTHERFORD
• In 1911 Rutherford performed the gold foil experiment• He shot alpha particles (Helium atoms that are
super positively charged) at a thin sheet of gold• Because they thought the gold sheet had its
positive charged scattered throughout they expected that the particles should either go through or be slightly deflected • Many did that, but some also bounced off the
gold foil really strongly, which was really surprising and unexplained
ERNEST RUTHERFORD
Based on his experiment Rutherford concluded that:• The atom is mostly empty space• All of the positive charge and all of the mass of the
atom are found in the middle• This positively charged, dense middle region was
called the NUCLEUS• Electrons are found evenly distributed around the
nucleus
His model is known as the “nuclear atom”*When Chadwick found the neutron it was added to Rutherford’s model and put in the nucleus
OTHER IMPORTANT CONTRIBUTIONS
• Robert A. Millikan – found the charge of an electron using the oil drop experiment• Eugen Goldstein – using a similar experiment to
J.J. Thomson he found the positively charged proton, and found that it’s mass was much much larger than an electron (1840 times heavier)• James Chadwick – found the neutron and found
that it has no charge but a mass the same as the proton
OVERVIEW OF THE ATOM
• The atom is the smallest piece possible of any given element while still keeping all of the properties of that element• The atom is made up of 3 subatomic particles: The
proton, neutron and electron• “Subatomic” Sub = under or below• Atomic = of or relating to an atom or atoms
• The atom is TINY!!! One penny (if it were made of pure copper) would contain 2 x 1022 copper atoms• Technology has improved so much that we can now
see individual atoms using an electron microscope