Atomic Theories

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Atomic Theories

• Chapter 4.1

How do we know about atoms when no one has ever
seen inside an atom?
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Greek Philosopher - Democritus

• Believed it is IMPOSSIBLE to divide matter ad
  infinitum.
• Theory Atomos smallest particle of matter.

Lived 460-370 BCE
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Greek Philosopher Aristotle

• Theory All matter consists of 4 elements.
• Air
• Water
• Fire
• Earth
• Theory Matter can be
• divided indefinitely.

Lived 384-322 BCE
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John Dalton

• Math Chemistry teacher
• Member of the Royal Society
• Received many awards and honors.
• Theory (1803) All matter is composed of tiny,
  solid, indestructible particles called atoms.
• Atoms of same elements are alike
• in size and mass.
• Atoms of different elements have different sizes
  and masses.
• Nickname Billiard Ball Theory

Lived 1766-1844 (Two thousand years after the
Greek philosophers.)
Country England
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John Dalton (continued)
Experiments He measured amounts of elements and
masses of compounds.

• Example Electrolysis of water
• electro refers to electricity
• lysis means to cut

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John Dalton (continued)
This experiment shows 2 times the VOLUME of
hydrogen gas compared to the volume of oxygen.
So, there must be twice the proportion of
hydrogen atoms as oxygen in each water molecule.
Therefore the formula for water is H2O.
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John Dalton (continued)
Also, when he measured the MASS of hydrogen and
oxygen, he found out that the ratio of oxygen to
hydrogen 8 1 in every experiment. (Dont
forget there are 2 hydrogen and 1 oxygen atom in
every water molecule) Oxygen has a molar mass of
16 g/mole Hydrogen has a molar mass of 1 g/mole
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John Dalton
Theory Law of Definite and Multiple Proportions

• 1. Chemical compounds are formed by the union of
  2 or more atoms of different elements.
• 2. Atoms combine to form compounds in simple,
  whole number ratios.
• Examples
• CO carbon monoxide is made of 1 carbon and 1
  oxygen atoms
• CO2 carbon dioxide is made of 1 carbon and 2
  oxygen atoms
• H2O water is made of 2 hydrogen and 1 oxygen
  atoms
• H2O2 hydrogen peroxide made of 2 hydrogen and 2
  oxygen atoms

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JJ Thomson

• Won numerous awards and prizes for his work.
• Member of the Royal Society.
• Experimented with gases and electricity in sealed
  glass tubes (called cathode ray tubes) showing a
  glowing stream of particles.

Lived 1856-1940 Country England
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JJ Thomson

• Noticed that when he held a magnet up to the
  tube, the gas was deflected.
• A positive magnet attracted the stream of
  particles.
• A negative magnet repelled the stream of
  particles.
• He discovered electrons!

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JJ Thomson

• Thomsons Theory (1897)
• Atoms consist of negatively charged particles (we
  now call electrons) embedded in positive matter.
  
• Nicknames raisin bun theory or plum pudding
  theory electrons are like the raisins or plums
  in the bread or pudding.

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Ernest Rutherford

• He had many honors, including being President of
  the Royal Society.
• Won many awards, including becoming an English
  knight and baron.
• Famous Experiment (1911) Gold Foil Experiment

Lived 1871-1937 Country England and New Zealand

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Rutherfords Famous Gold Foil Experiment
Theories about Atoms 1. Atoms have a small,
dense core (we now call the nucleus) 2. The
nucleus is positively charged. 3. The nucleus
contains most of the mass of an atom. 4. An
atoms volume is mostly empty space!
Rutherford discovered the NUCLEUS of the atom!
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Niels Bohr

• Studied under both JJ Thomson and Ernest
  Rutherford.
• President of the Royal Danish Academy of Sciences
  
• Foreign member of the Royal Society.
• Won Nobel Prize in Physics in 1922.
• Worked with leading scientists on the Atomic
  Energy Project.

Lived 1885-1962 Country Denmark worked in
England and USA
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Niels Bohr

• Experiment
• Bohr studied the bright-line spectra of
• hydrogen.
• He measured the energy released by
• hydrogens energized electrons when
• they return to ground state.
• He noticed that the released energy was in
  certain packets of light energy (called photons).
  

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Niels Bohr

• Theory (1913)
• Electrons travel in particular orbits around the
  atoms nucleus like planets around the sun.
• Also, the chemical properties of each element is
  determined by the number of electrons in the
  outer orbit of its atoms.

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Modern Theory of the AtomDe Broglie
Shrodingers work led to the develop of the most
current model of the atom called the Electron
Cloud Model

• There is a nucleus containing positively charged
  protons and neutrons which have no charge
• There is a space surrounding the nucleus called
  the Electron Cloud which is a probable location
  of where the electrons are found.

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