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Atomic Structure

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Title: Atomic Structure


1
Atomic Structure
  • History of Atomic Theory

2
Democritus (460 - 370 BC)
  • Was the first person to come up with the idea of
    atom
  • Believed that all matter was composed of
    indivisible particles he called ATOMS
  • Which is derived from the Greek word Atomos
    meaning indivisible
  • He also believed that different atoms
  • Are different sizes
  • Have different properties
  • Other philosophers of that time did not agree
    with his theories.

3
John Dalton (1766-1844)
  • Dalton is the Father of Atomic Theory
  • Daltons ideas were so brilliant that they have
    remained essentially intact up to the present
    time and has only been slightly corrected.

4
Daltons Atomic Theory (1803)aka 5 Postulates
  1. All matter is composed of extremely small
    particles called atoms. (I agree with Democritus!)

 
2. All atoms of a given element are identical,
having the same - size - mass - chemical
properties.
3. All atoms of different elements are different.
5
Daltons Atomic Theory (1803)aka 5 Postulates
  1. Atoms cannot be created, divided into smaller
    particles, or destroyed.

In a chemical reaction, atoms of different
elements are separated, joined or rearranged.
They are never changed into the atoms of another
element. We will learn more later
5. Atoms combine in definite whole number ratios
to make compounds (you cant have a ½ of a Carbon
bonding with Oxygen its a whole atom or no atom)
6
Daltons Atomic Model
  • Based on Daltons Atomic Theory (5 postulates),
    most scientists in the 1800s believed that the
    atom was like a tiny solid ball that could not be
    broken up into parts.
  • Dalton was credited for the three Atomic Laws
    that were proven after his time.

7
JJ Thomson (1856-1940)
  • Used cathode rays to prove that Daltons
    Solid-ball model could be broken into smaller
    particles
  • Thomson is credited with discovering electrons

8
Cathode Ray Tubes
  • Cathode rays had been used for some time before
    Thompsons experiments.
  • A cathode ray is a tube that has a piece of
    metal, called an electrode, at each end. Each
    electrode is connected to a power source
    (battery).
  • When the power is turned on, the electrodes
    become charged and produce a stream of charged
    particles. They travel from cathode, across the
    tube to the anode.

9
Cathode Ray Tubes
  • Thomson put the tube in a magnetic field. He
    predicted that the stream would travel in a
    straight path.
  • Instead, he found that the path curved away from
    a negatively charged plate and toward a
    positively charged plate
  • Why?
  • Like charges repel each other, and objects with
    unlike charges attract each other, Thomson
    concluded that the stream of charged particles
    had electrons in them.

10
Cathode Ray Tube Experiment
  • Thompson Concluded
  • Cathode rays are made up of invisible, negatively
    charged particles called Electrons.
  • These electrons had to come from the matter
    (atoms) of the negative electrode.
  • Since the electrodes could be made from a variety
    of metals, then all atoms must contain electrons!

11
Thomsons Plum Pudding Model
  • Thomsons Plum Pudding model is a charge sphere
    that has (- )charged electrons scattered inside,
    like raisins in plum pudding.
  • Overall, the atom is neutral atom because the
    atom had the same number of positive and negative
    charges.
  • From Thomsons experiments, scientists concluded
    that atoms were not just neutral spheres, but
    somehow were composed of electrically charged
    particles.
  • The balance of positive and negative charge
    supports the neutral atom.

12
Rutherford (1871-1937)
  • Took Thomsons Plum Pudding Model and added to it
  • Used the Gold Foil Experiment to discover the
    existence of
  • Protons
  • Nucleus
  • You must be able to explain the Gold Foil
    Experimentit will be on the CST

13
Gold Foil Experiment
  • Rutherford directed a narrow beam of alpha
    particles ( charges) at a thin piece of gold
    foil
  • Based on the Plum Pudding Model, he predicted
    that the charges would go through the foil
  • Because the atom overall was neutral, he thought
    the electrons would travel straight through

14
The Gold Foil Experiment
15
Conclusions from Gold Foil
  • Rutherford found that every once and a while, a
    particle was deflected bounced back
  • Why?
  • Because the charge hit a central mass of
    positive charge and was repelled.

16
Conclusions from Rutherfords Gold Foil
Experiment (memorize this!)
  • The atom contains a positively charged nucleus
  • This nucleus contains almost all of the mass of
    the atom, but occupies a very small volume of the
    atom.
  • The negatively charged electrons occupied most of
    the volume of the atom.
  •  The atom is mostly empty space.

17
Rutherfords Planetary Model
  • To explain his observations, Rutherford
    developed a new model
  • The electrons orbit the nucleus like the planets
    revolve around the sun.

18
Bohr (1885-1962)
  • Worked in Rutherfords lab
  • Wondered why electrons are not attracted to the
    nucleus and cluster around it
  • Disproved Rutherfords Planetary Model
  • Designed and proved the way electrons are
    arranged in the atom

19
Bohrs Energy Level Model
  • Energy Level Model Electrons are arranged in
    circles around the nucleus. Each circle has a
    different energy.
  • Electrons are in constant motion, traveling
    around the circle
  • Electrons can jump from one circle to the next
  • But they cant go to the nucleus they traveling
    too fast to be fully attracted.

20
Bohrs Energy Level Model
  He proposed the following
1. Protons and neutrons are in the nucleus
2. Electrons can only be certain distances from
the nucleus.
3. The electrons orbit the nucleus at fixed
energy levels.
4. The electrons must absorb or emit a fixed
amount of energy to travel between these energy
levels
21
Electron Cloud Model
  • As a result of continuing research throughout the
    20th century, scientists today realize that
    energy levels are not neat, planet-like orbits
    around the nucleus of an atom.
  • Instead, they are spherical regions of space
    around the nucleus in which electrons are most
    likely to be found.
  • Electrons travel in lanes like a runner on a
    track. Electrons can be anywhere in their lane.
    The lanes are called electron clouds.

22
Electron Cloud Model
  • Electrons themselves take up little space but
    travel rapidly through the space surrounding the
    nucleus.
  • These spherical regions where electrons travel
    may be depicted as clouds around the nucleus.
  • The space around the nucleus of an atom where the
    atoms electrons are found is called the electron
    cloud.

23
Electron Cloud Model
No electrons here.
Nucleus
Electron clouds
Electrons will be found here 90 of the time.
  • Each cloud is determined by the probability of an
    electrons location. An electron will be found in
    its cloud about 90 of the time.
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