Physical Science 9 Chapter 4
Discovery of Subatomic Particle
Democritus was a Greek philosopher who believed that all matter consisted of small particles
that could not be divided. He called these particles atoms.
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He thought that there were different types of atoms with specific sets of properties.
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Example: atoms in solids were rough and prickly.
Aristotle described matter differently. His model was that all substances were built from four
elements- earth, air, fire, and water.
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He thought these elements were combinations of hot, cold, wet, and dry.
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He also did not think there was a limit to the number of times matter could be divided.
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More people accepted Aristotle’s model of the atom over Democritus.
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However, in the 1800’s scientists had enough data from experiments to support what is known
as the atomic model of matter.
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An English teacher named John Dalton developed a theory that all matter is made up of
individual particles called atoms, which cannot be divided.
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There are 4 points to his theory (page 101).
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1. All elements are composed of atoms.
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2. All atoms of the same element have the same mass and atoms of different elements have
different masses.
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3. Compounds contain atoms of more than one element.
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4. In a particular compound, atoms of different elements always combine in the same way.
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However, scientists found that not all of Dalton’s ideas were correct so his theory was revised as
new discoveries were made.
J.J. Thomson used an electric current to learn more about atoms. The tubes he used to perform
his experiments are known as cathode ray tubes (similar to picture tubes in old televisions).
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He thought a glowing beam in the tube was charged particles.
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He noticed that the beam of charged particles was repelled by the negatively charged plate and
attracted to the positively charged plate.
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His experiments provided the first evidence that atoms are made of even smaller particles and
knew that an atom was not charged.
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He is given credit for discovering the electron.
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His model of the atom is known as the plum pudding model.
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A big positive ball with negative electrons placed in the ball.
Rutherford’s Atomic Theory
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Ernest Rutherford discovered that J.J. Thomson’s model of the atom was not correct.
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He discovered that Uranium emits fast-moving particles with a positive charge (alpha particles).
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He asked a student (Ernest Marsden) to find out what happens when alpha particles pass
through gold foil.
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This is known as the Gold Foil Experiment.
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He discovered that not all of the alpha particles passed through the gold foil. Some bounced
back at an angle greater than 90°.
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This made him realize that the positive charge of an atom cannot be spread out.
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The positive charge was concentrated to a small, central area. He called this area the nucleus.
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He came up with a new model of the atom which stated that an atom’s positive charge is
concentrated in its nucleus.
Section 4.2 The Structure of an Atom
There are 3 subatomic particles.
1. Protons (p+)- positively charged (1+)
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relative mass of 1
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actual mass in grams is 1.674 × 10-24.
2. Electrons (e-)- negatively charged (1-)
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relative mass is 1/1836 that of a hydrogen atom
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actual mass is 9.11 × 10-28g.
3. Neutrons (n)- no charge (neutral)
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Relative mass of 1
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Actual mass in grams is 1.675 × 10-24
Subatomic particles can be distinguished by mass, charge, and location in an atom.
Atomic number- is the number of protons in an atom of that element.
Mass number- is the sum of the protons and neutrons in the nucleus of that atom.
Number of neutrons = Mass # - Atomic #
Atoms are neutral so the protons and electrons in an atom are equal.
Isotopes- are atoms of the same elements that have different numbers of neutrons and different mass
numbers.
Ex: Oxygen-16, Oxygen-17, Oxygen-18
(show on board)
The 16, 17, and 18 represent the mass number for that isotope of oxygen.
Section 4.3 Modern Atomic Theory
Niels Bohr
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Agreed with Rutherford’s model of the atom.
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He agreed that there was a nucleus surrounded by a large volume of space.
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His model is often known as the planetary model because it resembled the solar system.
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His model went one step further than Rutherford’s model because it focused on the electrons.
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He stated that the electrons move with constant speed in fixed orbits around the nucleus.
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He thought that electrons had a specific amount of energy.
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This possible energy an electron can have in an atom is called energy levels.
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Similar to the rungs on a ladder or steps on a staircase.
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As you go up the stairs you go to a higher energy level.
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An electron in an atom can move from one energy level to another when the atom gains or
loses energy (Picture of model page 115).
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Evidence of these energy levels is shown when the electrons give off the energy in the form of
light (flame test lab and fireworks)
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Electrons are usually in their normal energy level called the ground state. When they absorb
energy they move to a higher level called the excited state.
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When the electrons fall back to their ground state the energy is released in the form of light.
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Different elements emit different colors.
Doltons 1803 Pictures atoms as tiny indestructible particles with no internal structure.
Thomson 1897 a British scientist discovers the electron, leading to his plum-pudding model.
Hantaro 1904 Japanese physicist suggest that an atom has a central nucleus. Electrons move in
orbits like the rings of Saturn
Rutherford Model 1911 New Zealander Rutherford states that an atom has a dense, positively
charged nucleus. Electrons move randomly in the space around the nucleus.
Bohrs Model 1913 the electrons move in spherical orbits at fixed distances from the nucleus.
Electron Cloud Model 1926 Erwin Schrodinger develops mathematical equation to describe the motion
of electrons in atoms.
1832 James Chadwick a British physicist confirms the existence of neutrons, which have no charge
Atomic nuclei contain neutrons and positively charged protons.