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Unit 2: Atomic Theory
Chapter 4Chemistry 1KCypress Creek High School
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Table of Contents
Chapter 4: The Structure of the Atom– 4.1: Early Theories of Matter– 4.2: Subatomic Particles & the Nuclear Atom– 4.3: How Atoms Differ– 4.4: Unstable Nuclei & Radioactive Decay
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Democritus
Lived around 400 B.C. Came up with the concept of the atom
4.14.1 Early Theories of MatterEarly Theories of Matter
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Development of the Modern Atomic Theory
In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container.– He observed that the mass of reactants in the
container before a chemical reaction was equal to the mass of the products after the reaction.
– Lavoisier concluded that when a chemical reaction occurs, mass is neither created nor destroyed but only changed.
Lavoisier’s conclusion became known as the law of conservation of mass.
4.14.1 Early Theories of MatterEarly Theories of Matter
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4.14.1 Early Theories of MatterEarly Theories of Matter
Development of the Modern Atomic Theory
In 1799, another French chemist, Joseph Proust, observed that the composition of water is always 11% hydrogen and 89% oxygen by mass.
– Regardless of the source of the water, it always contains these same percentages of hydrogen and oxygen.
Proust studied many other compounds and observed that the elements that composed the compounds were always in a certain proportion by mass. This principle is now referred to as the law of definite proportions.
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4.14.1 Early Theories of MatterEarly Theories of Matter
Dalton’s Atomic Theory
John Dalton (1766-1844), an English schoolteacher and chemist, studied the results of experiments by Lavoisier, Proust, and many other scientists.
Dalton proposed his atomic theory of matter in 1803.
– Although his theory has been modified slightly to accommodate new discoveries, Dalton’s theory was so insightful that it has remained essentially intact up to the present time.
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4.14.1 Early Theories of MatterEarly Theories of Matter
Dalton’s Billiard Ball Model
Called the father of Atomic Theory The following statements are the main
points of Dalton’s atomic theory.1) All matter is made up of atoms.
2) Atoms are indestructible and cannot be divided into smaller particles. (Atoms are indivisible.)
3) All atoms of one element are exactly alike, but are different from atoms of other elements.
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J. J. Thomson - 1903
Discovered the electron (1st subatomic particle) through experiments with cathode ray tube
Plum Pudding model (or Chocolate Chip Cookie model)– “Pudding” or “Cookie” is the
positive charge and most of the mass of the atom
– “Plums” or “Chocolate Chips” are the scattered electrons
POSITIVE CHARGEELECTRONS
4.14.1 Early Theories of MatterEarly Theories of Matter
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Ernest Rutherford - 1911
Nuclear Model (atom contains a nucleus)
Gold foil Experiment Atoms have:
– A nucleus– Protons (positive charge)
in nucleus– Mostly open space– Electrons found
somewhere around the nucleus
4.14.1 Early Theories of MatterEarly Theories of Matter
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Niels Bohr - 1913
Planetary Model– Electrons (e-) have definite
path around the nucleus (orbit)
– e- arranged around the nucleus according to energy level
– e- with lowest energy level are closest to nucleus
4.14.1 Early Theories of MatterEarly Theories of Matter
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Quantum Mechanical Model - 1923
Electron Cloud (modern theory)
Calculates the probability of finding the electron within a given space
Electrons, instead of traveling in defined orbits, travel in diffuse clouds around the nucleus
4.14.1 Early Theories of MatterEarly Theories of Matter
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Stepwise Timeline of Atomic Theory
Dalton 1803
Thomson1897
Rutherford1909
Bohr1913
Modern Theory
4.14.1 Early Theories of MatterEarly Theories of Matter
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TAKS Review
The graph shows the results of a study testing chemical pesticides on a pest species common to cotton plants. Different chemical pesticides were used in five different areas. According to these results, which of the following is the most effective chemical for controlling this pest species?
– a. R– b. S– c. T– d. V
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4.24.2 Subatomic Particles & the Nuclear AtomSubatomic Particles & the Nuclear Atom
The Electron
Because of Dalton’s atomic theory, most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts.
– In 1897, a British physicist, J.J. Thomson, discovered that this solid-ball model was not accurate.
Thomson’s experiments used a vacuum tube.
At each end of the tube is a metal piece called an electrode.
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4.24.2 Subatomic Particles & the Nuclear AtomSubatomic Particles & the Nuclear Atom
Cathode-Ray Tube
When the electrodes are charged, rays travel in the tube from the negative electrode, which is the cathode, to the positive electrode, the anode.
Thomson found that the rays bent toward a positively charged plate and away from a negatively charged plate.
– He knew that objects with like charges repel each other, and objects with unlike charges attract each other.
Click box to view movie clip.
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4.24.2 Subatomic Particles & the Nuclear AtomSubatomic Particles & the Nuclear Atom
Cathode-Ray Tube
Thomson concluded that cathode rays are made up of invisible, negatively charged particles referred to as electrons.
– These electrons had to come from the matter (atoms) of the negative electrode.
Matter is not negatively charged, so atoms can’t be negatively charged either.
– If atoms contained extremely light, negatively charged particles, then they must also contain positively charged particles—probably with a much greater mass than electrons.
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4.24.2 Subatomic Particles & the Nuclear AtomSubatomic Particles & the Nuclear Atom
Protons
In 1886, scientists discovered that a cathode-ray tube emitted rays not only from the cathode but also from the positively charged anode.
– These rays travel in a direction opposite to that of cathode rays.
– Thomson was able to show that these rays had a positive electrical charge.
Years later, scientists determined that the rays were composed of positively charged subatomic particles called protons.
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4.24.2 Subatomic Particles & the Nuclear AtomSubatomic Particles & the Nuclear Atom
Protons
At this point, it seemed that atoms were made up of equal numbers of electrons and protons.
However, in 1910, Thomson discovered that neon consisted of atoms of two different masses.
– Atoms of an element that are chemically alike but differ in mass are called isotopes of the element.
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4.24.2 Subatomic Particles & the Nuclear AtomSubatomic Particles & the Nuclear Atom
Neutrons
Because of the discovery of isotopes, scientists hypothesized that atoms contained still a third type of particle that explained these differences in mass. – Calculations showed that such a particle should
have a mass equal to that of a proton but no electrical charge.
– The existence of this neutral particle, called a neutron, was confirmed in the early 1930s.
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Subatomic Particles
amu – atomic mass unit; based on carbon-12
1 amu = 1/12 mass of C-12 = mass H Impractical to use actual mass of subatomic
particles
Name Symbol Relative Mass
Charge Position
Proton 1H or p+ 1 amu 1 Nucleus
Electron e- 0 amu -1 Outside
Neutron 1n 1 amu 0 Nucleus
4.24.2 Subatomic Particles & the Nuclear AtomSubatomic Particles & the Nuclear Atom
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4.24.2 Subatomic Particles & the Nuclear AtomSubatomic Particles & the Nuclear Atom
Rutherford’s Gold Foil Experiment
In 1909, a team of scientists led by Ernest Rutherford in England carried out the first of several important experiments that revealed an arrangement far different from the cookie-dough model of the atom.
– The experimenters set up a lead-shielded box containing radioactive polonium, which emitted a beam of positively charged subatomic particles through a small hole.
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4.24.2 Subatomic Particles & the Nuclear AtomSubatomic Particles & the Nuclear Atom
Rutherford’s Gold Foil Experiment
Today, we know that the particles of the beam consisted of clusters containing two protons and two neutrons and are called alpha particles.
– The sheet of gold foil was surrounded by a screen coated with zinc sulfide, which glows when struck by the positively charged particles of the beam.
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4.24.2 Subatomic Particles & the Nuclear AtomSubatomic Particles & the Nuclear Atom
The Nuclear Model of the Atom
To explain the results of the experiment, Rutherford’s team proposed a new model of the atom.
Because most of the particles passed through the foil, they concluded that the atom is nearly all empty space.
– Because so few particles were deflected, they proposed that the atom has a small, dense, positively charged central core, called a nucleus.
– The new model of the atom as pictured by Rutherford’s group in 1911 is shown below.
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TAKS Review
Trade winds blow from east to west across the Pacific Ocean. The winds move surface waters westward across the ocean. This causes deeper, colder water to rise to the surface along the coast. This upwelling of deep ocean waters brings with it nutrients that would otherwise lie near the bottom of the ocean. Which of the following conclusions is supported by the information above?
– a. Trade winds help maintain some food chains.– b. Trade winds can reverse parts of the water cycle.– c. Trade winds produce useful minerals in some oceans.– d. Trade winds may be able to reduce greenhouse gases
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4.34.3 How Atoms DifferHow Atoms Differ
Atomic Numbers
The atomic number of an element is the number of protons in the nucleus of an atom of that element.
– It is the number of protons that determines the identity of an element, as well as many of its chemical and physical properties.
– Because atoms have no overall electrical charge, an atom must have as many electrons as there are protons in its nucleus.
Therefore, the atomic number of an element also tells the number of electrons in a neutral atom of that element.
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4.34.3 How Atoms DifferHow Atoms Differ
Masses
The mass of a neutron is almost the same as the mass of a proton. The sum of the protons and neutrons in the nucleus is the mass
number of that particular atom. – Isotopes of an element have different mass numbers because they
have different numbers of neutrons, but they all have the same atomic number.
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4.34.3 How Atoms DifferHow Atoms Differ
Atomic Mass
In order to have a simpler way of comparing the masses of individual atoms, chemists have devised a different unit of mass called an atomic mass unit, which is given the symbol .
– An atom of the carbon-12 isotope contains six protons and six neutrons and has a mass number of 12.
– Chemists have defined the carbon-12 atom as having a mass of 12 atomic mass units.
Therefore, 1 = 1/12 the mass of a carbon-12 atom. 1 is approximately the mass of a single proton or neutron.
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4.34.3 How Atoms DifferHow Atoms Differ
Information in the Periodic Table
The number at the bottom of each box is the average atomic mass of that element.
This number is the weighted average mass of all the naturally occurring isotopes of that element.
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Isotope Notation
Element Symbol with mass number and atomic number
Can also be the element name dash mass number
XMass
Number
Atomic Number
or Element- Mass
4.34.3 How Atoms DifferHow Atoms Differ
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Practice
Symbol # Protons # neutrons # electrons Atomic Number
Mass Number
9 10 9 9 19
28 59
150 94
30 65
F199
C146
4.34.3 How Atoms DifferHow Atoms Differ
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Isotopes
Atoms of the same element with different numbers of neutrons– Think of it as different sized shirts!
6 neutrons Carbon-12 7 neutrons Carbon-13 8 neutrons Carbon-14
4.34.3 How Atoms DifferHow Atoms Differ
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4.34.3 How Atoms DifferHow Atoms Differ
Calculating Atomic Mass
Copper exists as a mixture of two isotopes. – The lighter isotope (Cu-63), with 29 protons and 34 neutrons,
makes up 69.17% of copper atoms. – The heavier isotope (Cu-65), with 29 protons and 36
neutrons, constitutes the remaining 30.83% of copper atoms.
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4.34.3 How Atoms DifferHow Atoms Differ
Calculating Atomic Mass The atomic mass of Cu-63 is 62.930 amu, and the atomic mass
of Cu-65 is 64.928 amu. Use the data above to compute the atomic mass of copper.
First, calculate the contribution of each isotope to the average atomic mass, being sure to convert each percent to a fractional abundance.
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4.34.3 How Atoms DifferHow Atoms Differ
Calculating Atomic Mass
The average atomic mass of the element is the sum of the mass contributions of each isotope.
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Percent Abundance
If you are given information about an elements isotope you can estimate the most abundant isotope!
Example: Carbon-12, Carbon-13, Carbon-14. Look at the atomic mass on the periodic table. Which isotope is the mass closest to?
4.34.3 How Atoms DifferHow Atoms Differ
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4.34.3 How Atoms DifferHow Atoms Differ
Question 1
Calculate the atomic mass of germanium. Answer
– 72.59 amu
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Radioactivity
Reactions which involve a change in an atom’s nucleus are called nuclear reactions.– In the late 1890’s scientists noticed that some
substances spontaneously emitted radiation in a process they called radioactivity.
– The rays and particles emitted by the radioactive material were called radiation.
Unstable nuclei lose energy by emitting radiation in a spontaneous process called radioactive decay.
4.44.4 Unstable Nuclei & Radioactive DecayUnstable Nuclei & Radioactive Decay
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Types of Radiation
Scientists named the radiation that was deflected toward the negatively charged plates alpha radiation.
– This radiation is made up of alpha particles contains 2 protons & 2 neutrons.
Scientists named the radiation that was deflected toward the positively charged plate beta radiation.
– This radiation consists of fast moving electrons called beta particles containing an electron.
The third type of radiation is called gamma radiation or gamma rays, high energy radiation that has no mass.
4.44.4 Unstable Nuclei & Radioactive DecayUnstable Nuclei & Radioactive Decay
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TAKS Review
A medical researcher hypothesizes that a newly developed medication can reduce high blood pressure. Which of these would most likely be the dependent variable in a study involving this medication?
– a. The number of participants in the study– b. The ages of people treated for high blood pressure with
other medications– c. The blood pressure of the participants in the study– d. The number of people treated for high blood pressure with
other medications
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End of Unit 2
Be Prepared for Unit 2 Test.