HISTORY OF CHEMISTRY A BRIEF OVERVIEW

1
HISTORY OF CHEMISTRYA BRIEF OVERVIEW

• What have we learned?
• Experimentation?
• How scientists work?
• Some contributions?

2
What Have We Learned?

• Evidence for the existence of atoms is indirect
• Matter can be analyzed and synthesized indicating
  a constant combining property of atoms
• Matter is conserved in analysis and synthesis
• Atoms can not be destroyed
• Crystals of the same substance have the same
  properties and are deposited in a pattern to
  build a crystal

3
EXPERIMENTATION!

• We have been doing a lot of different experiments
  that have mimicked what past scientists have done
• But not all science is done by experimentation

4
HOW SCIENTISTS WORK

• Science is a product of human imagination
• Data must be organized, pondered, and molded into
  theory
• Scientists do not work alone

5
SOME CONTRIBUTORS

• The literature of chemistry holds a wealth of
  information that we can draw upon to help us
  learn
• We will learn how these researchers used
  information from one another to advance their own
  ideas
• We will also see how some of the best ideas came
  from mistakes

6
DEMOCRITUS

• Around 430 BC
• atomos uncuttable
• Ancient Greeks did not prove the existence of
  atoms because they DID NOT DO ANY EXPERIMENTS
• His idea was not really written about until John
  Dalton

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John Dalton

• Model of the atom-with only a few changes his
  atomic theory is still accepted today

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John Dalton

• All elements composed of atoms that can not be
  divided
• Atoms of the same element are exactly the same
  and have the same mass
• Atoms of one element cant be changed into an
  atom of a different elementcant be created or
  destroyed in any chemical change, only rearranged
• Compounds are composed of atoms of different
  elements, combined in ratios
• BILLIARD BALL MODEL

9
JJ THOMSON

• 1st to hypothesize that there are particles
  inside of atoms

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JJ THOMSON

• 1897 Worked with cathode ray tubes and passed
  electricity inside empty glass tubes. He used
  magnets to pull the rays apart. This mysterious
  glow cathode rays in the tube, he theorized
  were made of electrons
• Found that atoms contained negative charged
  particles (electrons), but scientists knew that
  atoms had not electrical charge, so therefore
  atoms must also have a positive charge
• In his model the electrons are scattered
  throughout a ball of positive charge
• PLUM PUDDING OR RAISIN MUFFIN MODEL

11
Ernest Rutherford

• 1911
• Thomsons student
• Wanted to collect more evidence to support
  Thomsons theory
• Famous Gold Foil Experiment

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Earnest Rutherford

• Aimed a beam of charges at a sheet of thin
  gold foil
• If Thomsons theory was correct, the
  particles would pass right through in a straight
  line (the gold atoms would not have a strong
  enough charge in any region to repel the charged
  particles..BUT

13
Earnest Rutherford

• Most particles passed through as expected but a
  few were strongly deflected.WHY?

He hypothesized that an toms charge must be
clustered in a tiny regionNUCLEUS and the the
were being deflected by the nucleus
14
Earnest Rutherford

• They also knew that the electron had little or no
  mass, therefore he reasoned that all of an atoms
  mass is in the positively charged nucleus. He
  called these positively charged particles in the
  nucleus, PROTONS
• PEACH PIT MODEL an atom is mostly empty space
  with a charged center. Electrons move around
  a small nucleus found in the center.

15
Neils Bohr

• 1913
• Danish student of Rutherford and Thomson
• Set out to support Rutherfords theory
• He predicted if Rutherford was correct, the
  electrons should accelerate and give off energy
  as they travel in a circle. As they travel they
  should lose energy and fall into the nucleus, but
  this doesnt happen.

16
Neils Bohr

• Bohr theorized that electrons must have only a
  specific amount of energy. This energy leads
  them to move in certain orbits around the
  nucleus. This is similar to the way planets move
  around the sun.
• They can still circle the nucleus without losing
  energy if they stayed in a certain orbit
• PLANETARY MODEL
• Similar to the lanes on a track. If you want to
  use less energy what lane would you use?
• Bohrs model was essentially correct but he could
  not explain his observations of the actual
  behavior or atoms and electrons.

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Electron Cloud

• Bohr had the right idea but it was not until the
  1920s when Quantum Mechanics came into the
  picture.
• Albert Einstein and Max Planck said that energy
  can be divided into particles

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Electron Cloud

• Werner Heisenberg and Erwin Schrodinger
  developed a theory that matter could also act
  both like waves and particles. They then applied
  this to the motion of electrons

Electrons do not orbit like planets, they can go
anywhere within a cloud-like region, these are
called ENERGY LEVELS. The electrons within a same
energy level all have the same amount of
energy Lowest energy levels are found closest to
the nucleus , these can only hold 2 electrons,
higher energy electrons are in larger energy
levels (they can hold 8 or more)
19
James Chadwick

• 1932, British
• Worked with Ernest Rutherford
• Credited with discovering the Neutron
• His discovery completed the atomic model
• The neutron was very hard to detect because it
  has no charge.
• He was studying atomic mass and atomic number and
  found differences. If the nucleus only had
  and - inside these would be the same but they
  are not, they were double. There must be
  something else inside.
• Even though it is the same mass as a proton it
  is electrically neutral, hence the name neutron