What is an Atom?

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What is an Atom?

• Unit 3 Atomic Structure

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Review from 8th Grade

• OBJECTIVES
• Identify the subatomic particles
• Explain how the atomic number identifies an
  element.
• Use the atomic number and mass number of an
  element to find the numbers of protons,
  electrons, and neutrons
• Review key terms

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Subatomic particles
Actual mass (g)
Relative mass
Name
Symbol
Charge
Electron
e-
-1
1/1840
9.11 x 10-28
Proton
p
1
1
1.67 x 10-24
Neutron
n0
0
1
1.67 x 10-24
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Counting the Pieces

• Atomic Number number of protons in the nucleus
• of protons determines kind of atom (since all
  protons are alike!)
• the same as the number of electrons in the
  neutral atom.
• Mass Number the number of protons neutrons.
• These account for most of mass

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Counting the Pieces

• Protons equal to atomic number
• Neutrons Mass Number Atomic Number
• Electrons In a neutral atom equal to atomic
  number

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Symbols

• Contain the symbol of the element, the mass
  number and the atomic number.

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Symbols

• Contain the symbol of the element, the mass
  number and the atomic number.

Mass number
X
Atomic number
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Symbols

• Find the
• number of protons
• number of neutrons
• number of electrons
• Atomic number
• Mass Number

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F
9
9
Symbols

• Find the
• number of protons
• number of neutrons
• number of electrons
• Atomic number
• Mass Number

80
Br
35
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Symbols

• if an element has an atomic number of 34 and a
  mass number of 78 what is the
• number of protons
• number of neutrons
• number of electrons
• Complete symbol

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Symbols

• if an element has 91 protons and 140 neutrons
  what is the
• Atomic number
• Mass number
• number of electrons
• Complete symbol

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Symbols

• if an element has 78 electrons and 117 neutrons
  what is the
• Atomic number
• Mass number
• Number of protons
• Complete symbol

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What if Atoms Arent Neutral

• Ions charged atoms resulting from the loss or
  gain of electrons

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What if Atoms Arent Neutral

• Anion negatively charged ion result from
  gaining electrons
• Take the number of electrons in a neutral atom
  and add the absolute value of the charge

Identify Number of Protons Number of
Neutrons Number of Electrons
81
1-
Br
35
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What if Atoms Arent Neutral

• Cation positively charged ion result from the
  loss of electrons
• Take the number of electrons in a neutral atom
  and subtract the value of the charge

Identify Number of Protons Number of
Neutrons Number of Electrons
27
Al
3
13
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Distinguishing Between Atoms

• OBJECTIVES
• Explain how isotopes differ, and why the atomic
  masses of elements are not whole numbers.

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Distinguishing Between Atoms

• OBJECTIVES
• Calculate the average atomic mass of an element
  from isotope data.

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Isotopes

• Atoms of the same element can have different
  numbers of neutrons
• Different mass numbers
• Called isotopes

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Naming Isotopes

• We can also put the mass number after the name of
  the element.
• carbon- 12
• carbon -14
• uranium-235

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Atomic Mass

• How heavy is an atom of oxygen?
• There are different kinds of oxygen atoms
• We are more concerned with average atomic mass
• Average atomic mass is based on abundance of each
  element in nature.
• We dont use grams because the numbers would be
  too small

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Measuring Atomic Mass

• Unit is the Atomic Mass Unit (amu)
• It is one twelfth the mass of a carbon-12 atom
• Each isotope has its own atomic mass, thus we
  determine the average from percent abundance

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Calculating averages

• Convert the percent to a decimal (move the
  decimal 2 places to the left or divide by 100)
• Multiply the atomic mass by its percent
  (expressed as a decimal), then add the results.

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Calculating Averages

• Calculating the Average Atomic Mass is similar to
  calculating your grade in this class

Percent of Grade Student's Average
Tests 35 89
Quizzes 25 95
Labs 20 35
Homework 20 100
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Calculating Averages

• If Bromine-79 has an abundance of 50.69 and
  Bromine-81 has an abundance of 49.31 what is the
  average atomic mass of Bromine?
• Why is it better to use the mass of an individual
  isotope for the mass number rather than round the
  number from the periodic table?

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Atomic Mass

• Magnesium has three isotopes. 78.99 magnesium-24
  with a mass of 23.9850 amu, 10.00 magnesium-25
  with a mass of 24.9858 amu, and the rest
  magnesium- 26 with a mass of 25.9826 amu. What is
  the atomic mass of magnesium?
• If not told otherwise, the mass of the isotope is
  the mass number in amu

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Atomic Mass

• Is not a whole number because it is an average.
• are the decimal numbers on the periodic table.

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History of the atom

• Not the history of atom, but the idea of the
  atom.
• Original idea Ancient Greece (400 B.C.)
• Democritus and Leucippus- Greek philosophers.

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Democritus and Leucippus

• Leucippus was the first person to come up with
  the idea of the atom
• Democritus was a student of Leucippus and
  expanded on his idea

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Democritus

• Democritus added
• Matter is composed of atoms which move through
  empty space
• Atoms are solid, homogeneous, indestructible, and
  indivisible
• Different atoms have different shapes and sizes
• The size, shape, and movement of atoms determine
  their properties

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Greek

• Aristotle - Famous philosopher
• All substances are made of 4 elements
• Fire - Hot
• Air - light
• Earth - cool, heavy
• Water - wet
• Blend these in different proportions to get all
  substances

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Aristotle

• There was no scientific evidence to back up
  either Democritus or Aristotle, but people
  believed Aristotle based on reputation
• Aristotles theory persisted for 2,000 years

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Leading to the modern theory

• Late 1700s - John Dalton- England.
• Teacher- summarized results of his experiments
  and those of others.
• Daltons Atomic Theory
• Combined ideas of elements with that of atoms.
• Saw atoms as small solid spheres. Billiard Ball
  Model.

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Daltons Atomic Theory

• All matter is made of tiny indivisible particles
  called atoms.
• Atoms of the same element are identical, those of
  atoms of different elements are different.
• Atoms of different elements combine in whole
  number ratios to form compounds.
• Chemical reactions involve the rearrangement of
  atoms. No new atoms are created or destroyed.

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Laws Related to Atomic Theory

• Law of Conservation of Mass- Antoine Lavoisier
  states that matter is neither created nor
  destroyed in chemical reactions
• Law of Definite Proportions- Joseph Proust
  states that all compounds contain the same
  elements in the same ratio
• Law of Multiple Proportions- John Dalton
  elements combine in small whole number ratios

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Just How Small Is an Atom?

• Think of cutting a piece of lead into smaller and
  smaller pieces
• How far can it be cut?
• An atom is the smallest particle of an element
  that retains the properties of that element
• Atoms-very small

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Structure of the Nuclear Atom

• OBJECTIVES
• Distinguish among protons, electrons, and
  neutrons in terms of relative mass and charge.
• Describe the structure of an atom, including the
  location of the protons, electrons, and neutrons
  with respect to the nucleus.

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Parts of Atoms

• J. J. Thomson - English physicist. 1897
• Made a piece of equipment called a cathode ray
  tube.
• It is a vacuum tube - all the air has been pumped
  out.

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Thomsons Experiment

-
Vacuum tube
Metal Disks
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Thomsons Experiment

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Thomsons Experiment

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Thomsons Experiment

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Thomsons Experiment

-

• Passing an electric current makes a beam appear
  to move from the negative to the positive end

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Thomsons Experiment

-

• Passing an electric current makes a beam appear
  to move from the negative to the positive end

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Thomsons Experiment

-

• Passing an electric current makes a beam appear
  to move from the negative to the positive end

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Thomsons Experiment

-

• Passing an electric current makes a beam appear
  to move from the negative to the positive end

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Thomsons Experiment

• By adding an electric field

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Thomsons Experiment

-

• By adding an electric field

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Thomsons Experiment

-

• By adding an electric field

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Thomsons Experiment

-

• By adding an electric field

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Thomsons Experiment

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• By adding an electric field

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Thomsons Experiment

-

• By adding an electric field

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Thomsons Experiment

-

• By adding an electric field he found that the
  moving pieces were negative

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Plum Pudding Model

• Proposed by JJ Thomson
• Said the atom had a uniform positive charge in
  which the negatively charged electrons resided

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Other particles

• Proton - positively charged pieces 1840 times
  heavier than the electron by E. Goldstein
• Neutron - no charge but the same mass as a proton
  by J. Chadwick
• Where are the pieces?

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Millikans Oil Drop

• JJ Thomson used the cathode ray to find the mass
  to charge ratio of an electron.
• He did not know the mass or charge, only the
  ratio
• Millikans Oil Drop experiment determined the
  charge of an electron

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Millikans Experiment
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Millikans Experiment
X-rays
X-rays give some electrons a charge.
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Millikans Experiment
From the mass of the drop and the charge on the
plates, he calculated the mass of an electron
Some drops would hover
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Rutherfords experiment

• Ernest Rutherford -English physicist. (1910)
• Believed in the plum pudding model of the atom
• Wanted to see how big they are.
• Used radioactivity.
• Alpha particles - positively charged pieces-
  helium atoms minus electrons
• Shot them at gold foil which can be made a few
  atoms thick.

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Rutherfords experiment

• When an alpha particle hits a fluorescent screen,
  it glows.
• Heres what it looked like

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Fluorescent Screen
Lead block
Uranium
Gold Foil
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He Expected

• The alpha particles to pass through without
  changing direction very much.
• Because?
• the positive charges were thought to be spread
  out evenly. Alone they were not enough to stop
  the alpha particles.

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What he expected
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Because
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He thought the mass was evenly distributed in the
atom
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Since he thought the mass was evenly distributed
in the atom
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What he got
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How he explained it

• Atom is mostly empty.
• Small dense, positive piece at center.
• Alpha particles are deflected by it if
  they get close enough.

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Density and the Atom

• Since most of the particles went through, it was
  mostly empty space.
• Because the pieces turned so much, the positive
  pieces were heavy.
• Small volume, big mass, big density.
• This small dense positive area is the nucleus.

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Bohr Model

• Bohr worked with the concepts of energy,
  wavelength and frequency
• Each color of light is associated with a
  different energy
• Each atom gives off its own unique color so..
• Electrons of diiferent atoms have different
  energies

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Bohr Model

• Each atom has its own specific electron
  arrangement
• Electrons are in Energy levels in the atom
• When an electron goes from an excited state back
  down to its ground state the atom emits light

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The Bohr Ring Atom
n 4
n 3
n 2
n 1
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The Bohr Model

• Doesnt work.
• Only works for hydrogen atoms.
• Electrons dont move in circles.
• The quantization of energy is right, but not
  because they are circling like planets.
• Back to the drawing board

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The Quantum Mechanical Model

• A totally new approach.
• De Broglie said matter could be like a wave, like
  standing waves.
• The vibrations of a stringed instrument.
• More on this in Unit 4

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Models of the Atom to Know

• Daltons
• Thomsons
• Rutherford
• Bohr
• Quantum