The Structure of the Atom
Chapter 4
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Important Dates for Chapter 4
a. Start: Tuesday October 8
b. End: Thursday October 24
c. Fall Break: October 14-15
d. Vocabulary Quiz: Thursday October 18
e. Test: Thursday October 24
f. Lab: Thursday October 18 and Tuesday October 22
Essential Vocabulary
Alpha particle
Alpha radiation
Atom
Atomic mass
Atomic mass unit
Atomic number
Beta particle
Beta radiation
Cathode ray
Dalton’s atomic theory
electron
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Gamma ray
Isotope
Mass number
Neutron
Nuclear equation
Nuclear reaction
Nucleus
Proton
Radiation
radioactivity
Early Theories of Matter
Section 4.1
1. The Philosophers
2. Democritus (460-370 B.C.)
a. 1st person to propose the idea that matter was not infinitely divisible.
b. Believed atoms matter was made up of tiny individual particles called atomos
c. Believed that atoms could not be created, destroyed, or further divided
d. Atoms are solid, homogeneous, indestructible, and invisible
e. Different kinds of atoms have different sizes and shapes
f. The differing of properties of matter are due to the size, shape, and movement of the atom
g. Apparent changes in matter result from changes in the groupings of atoms and not from changes in the
atoms themselves
h. Most of his ideas do not agree with modern atomic theory, but his belief in the existence of atoms was
amazingly ahead of his time
3. Many Greek philosophers thought matter was formed of air, earth, fire, and water. They also associated
properties with each of the 4 basic components of matter. The pairings of opposite properties, such as hot and
cold, and wet and dry, mirrored the symmetry and balance the philosophers observed in nature. These early
nonscientific and incorrect beliefs were not completely dispelled until the 1800s.
4. The Philosophers
a. Aristotle (384-322 B.C.)
i. One of the most influential philosophers
ii. Wrote extensively on many subjects, including politics, ethics, nature, physics, and astronomy
iii. Most of his writings have been lost through the ages
iv. Criticized Democritus’s ideas of atomic theory
v. He rejected Democritus’s atomic “theory” entirely because it did not agree with his own ideas
on nature
vi. Did not believe that the “nothingness” of empty space could exist
vii. Able to gain wide acceptance for his ideas on nature– ideas that denied the existence of atoms–
for nearly 2000 years!
viii. Concept of the atom was revived in 18th century, but it took another 100 years before significant
progress was made
Many Greek philosophers thought matter was formed of air, earth, fire, and water. They also associated properties with
each of the 4 basic components of matter. The pairings of opposite properties, such as hot and cold, and wet and dry,
mirrored the symmetry and balance the philosophers observed in nature. These early nonscientific and incorrect beliefs
were not completely dispelled until the 1800s.
5. John Dalton (1766-1844)
a. English schoolteacher
b. Marked the beginning of the development of modern atomic theory
c. Revived and revised Democritus’s ideas based upon the results of scientific research he conducted
i. Advancements in science allowed him to perform experiments that allowed him to refine and
verify theories
d. Able to accurately determine mass ratios
e. Proposed his atomic theory in 1803
6. Dalton’s Atomic Theory
a. All matter is composed of extremely small particles called atoms
b. All atoms of a given element are identical, having the same size, mass, and chemical properties; atoms
of a specific element are different from those of any other element
c. Atoms cannot be created, divided into smaller particles, or destroyed
d. Different atoms combine in simple whole number ratios to form compounds
e. In a chemical reaction, atoms are separated, combined, or rearranged
7. Section 4.1 Assessment
a. Why were Democritus’s ideas rejected by other philosophers of his time?
b. Define an atom using your own words.
c. Which statements in Dalton’s original atomic theory are now considered to be incorrect? Describe how
modern atomic theory differs from these statements.
d. Democritus and Dalton both proposed the concept of atoms. Describe the method each of them used to
reach the conclusion that atoms existed. How did Democritus’s method hamper the acceptance of his
ideas?
e. Compare and contrast the atomic theories proposed by Democritus and John Dalton.
Subatomic Particles and the Nuclear Atom
Section 4.2
Has your hair ever clung to your comb? Have you ever gotten shocked by a doorknob after walking across carpet?
Observations such as these led scientists in the 1800s to look for some sort of relationship between matter and electric
charge.
1. Discovering the Electron
2. Using a recently invented vacuum pump, electricity was passed through glass tubes from which most of the air
and matter had been removed
a. Electrode connected to negative end is called anode; electrode connected to positive end is called
cathode
3. Sir William Crookes
a. English physicist noticed a flash of light within one of the tubes while working in a darkened laboratory
b. Flash was produced by some form of radiation striking a light-producing coating that had been applied
to the end of the tube
c. This led to the discovery of radiation traveling from the cathode to the anode within the tube; became
known as a cathode ray
d. Now you have TVs and computer screens
4. Scientists continued their research using cathode ray tubes, and by the end of the 1800s, they were fairly
convinced of the following:
a. Cathode rays were actually a stream of charged particles
b. Particles carried a negative charge (the exact value of the charge remained unknown)
c. Because changes in type of electrode or varying the gas did not affect the ray produced, it was
concluded that negative particles were present in all forms of matter
i. Called these negative particles electrons
5. In spite of the progress made from all of the cathode ray tube experiments, no one had succeeded in
determining the mass of a single cathode ray particle
a. English physicist J.J. Thomson began a series of experiments in the late 1890s to determine the ratio of
its charge to its mass
i. His conclusion was shocking because it meant that there were particles smaller than an atom; in
other words, Dalton was wrong
b. The next significant discovery came in 1909 when American physicist Robert Millikan determined the
charge of an electron to be 9.1x10-28g
c. Thomson then proposed a model called the plum pudding model which didn’t last long
6. The Nuclear Atom
a. Ernest Rutherford became interested in studying how positively charged alpha particles interacted with
solid matter
b. Aware of Thomson’s plum pudding model, Rutherford expected only minor deflections through the gold
foil
c. After a few days of testing, Rutherford were amazed to discover that a few of the alpha particles were
deflected at very large angles; some even deflected straight back
d. This meant the plum pudding model was incorrect; Rutherford concluded there must be a nucleus
centrally located within the atom that contained the positive charge and most of its mass
e. Gold Foil Experiment
7. Completing the Atom– The Discovery of Protons and Neutrons
a. In 1932, Rutherford’s coworker, English physicist James Chadwick showed that the nucleus also
contained another subatomic particle, a neutral particle called the neutron.
b. A neutron has a mass nearly equal to that of a proton but carries no electrical charge
c. By 1920, eight years after his revolutionary gold foil experiment, Rutherford had refined the concept of
the nucleus. He concluded that the nucleus contained positively charged particles called protons. A
proton is a subatomic particle carrying a charge equal but opposite that of an electron.
8. Section 4.2 Assessment
a. Briefly evaluate the experiments that led to the conclusion that electrons were negatively charged
particles found in all matter.
b. Describe the structure of a typical atom. Be sure to identify where each subatomic particle is located.
c. Make a table comparing the relative charge and mass of each of the subatomic particles.
d. Compare and contrast Thomson’s plum pudding atomic model with Rutherford’s nuclear atomic model.
e. Make a timeline of the development of modern atomic theory. Be sure to include the discovery of each
subatomic particle.
How Atoms Differ
Section 4.3
1. Henry Moseley discovered atoms of each element contain a unique positive charge in their nuclei.
a. Thus, the number of protons in an atom identifies it as an atom of a particular atom and that number is
referred to as its atomic number
2. Isotopes and Mass Number
3. Most elements are found as a mixture of isotopes found in a constant relative abundance
a. Those that contain more neutrons have a greater mass
b. Have essentially the same chemical behavior regardless of differing number of neutrons
c. To make it easy to identify each of the various isotopes of an element, chemists add a number after the
element’s name
i. This number is known as the mass number and represents the number of protons and neutrons
in the nucleus
a. # of neutrons = mass # - atomic #
Earlier you learned that Dalton’s atomic theory was wrong about atoms being indivisible. It was also incorrect in stating
that all atoms of a particular element are identical. While it is true that all atoms of particular element have the same
number of protons and electrons, the number of neutrons on their nuclei may differ. Atoms with the same number of
protons but different numbers of neutrons are called isotopes.
4. Mass of Individual Atoms
a. The masses of protons and neutrons are about 1.67x10-24g each.
b. Electrons are very small– about 1/1840 that of one proton or neutron.
c. Because these extremely small masses expressed in scientific notation are difficult to work with,
chemists developed a method of measuring based on a chosen atomic standard- a carbon-12 atom.
d. Thus, one atomic mass unit (amu) is defined as 1/12 the mass of a carbon-12 atom.
e. Because an atom’s mass depends on the number of protons and neutrons it contains, and because
protons and neutrons have masses close to 1 amu, you might expect the atomic masses to be very near
a whole number.
f. This does not hold true when the definition of atomic mass is taken into account.
i. Atomic mass of an element is the weighted average mass of the isotopes of that element.
5. Section 4.3 Assessment
a. Which subatomic particle identifies an atom as that of a particular element? How is this particle related
to the atom’s atomic number?
b. What is an isotope? Give an example of an element with isotopes.
c. Explain how the existence of isotopes is related to atomic masses not being whole numbers.
d. Nitrogen has two naturally occurring isotopes, N-14 and N-15. The atomic mass of nitrogen is 14.007
amu. Which isotope is more abundant in nature? Explain.
e. List the steps in the process of calculating average atomic mass given data about the isotopes of an
element.
Unstable Nuclei and Radioactive Decay
Section 4.4
1. Recall that chemical reactions involve the change of one more substances into new substances. Although atoms
may be rearranged, their identities do not change during the reaction. You may be wondering why atoms of one
element do not change into another element during a chemical reaction. The reason lies in the fact that the
chemical reaction involves only an atom’s electrons- the nucleus remains unchanged.
2. Radioactivity
a. Nuclear reactions are reactions that do involve an atom of one element changing into an atom of
another element.
b. In the late 1890s, scientists noticed that some substances spontaneously emitted radiation in a process
they termed radioactivity.
c. The rays and particles emitted by the radioactive material were called radiation.
d. Unstable nuclei lose energy by emitting radiation in a spontaneous process called radioactive decay.
i. Instability is caused by either too many or too few neutrons.
3. Types of Radiation
a. Alpha Radiation
i. Deflected toward the negatively charged plate
ii. Made up of alpha particles
1. 2 protons, 2 neutrons, +2 charge
b. Beta Radiation
i. Deflected toward the positively charged plate
ii. Made of one fast moving electrons called beta particles
1. 1 electron, -1 charge
c. Gamma Radiation or Gamma Rays
i. High-energy radiation that possess no mass and no charge
4. Scientists began researching radioactivity in the late 1800s. By directing radiation from a radioactive source
between two electrically charged plates, scientists were able to identify three different types of radiation. Some
was deflected toward the negatively charged plate, some was deflected toward the positively charged plate, and
some was not deflected at all.
5. Section 4.4 Assessment
a. Explain how unstable atoms gain stability. What determines whether or not an atom is stable?
b. Create a table comparing the mass and charge of alpha, beta, and gamma radiation.
c. In writing a balanced nuclear equation, what must be conserved?
d. Explain how a nuclear reaction differs from a chemical reaction.
e. Classify each of the following as a chemical reaction, a nuclear reaction, or neither.
i. Thorium emits a beta particle.
ii. Two atoms share electrons to form a bond.
iii. A sample of pure sulfur emits heat energy as it slowly cools.
iv. A piece of iron rusts.