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Atoms, Sub-Atomic Particles


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Title: Atoms, Sub-Atomic Particles

Unit 3
  • Atoms, Sub-Atomic Particles Nuclear Chemistry

The Particle Theory of Matter
  • In 400 B.C. Democritus, a Greek philosopher,
    first proposed the idea of a basic particle of
    matter that could not be divided any further.
  • He called this particle the atom, based on the
    Greek word atomosmeaning indivisible.
  • This early theory was not backed up by
    experimental evidence and was ignored by the
    scientific community for nearly 2000 years.

Foundations of Atomic Theory
  • By the late 1700s, experiments with chemical
    reactions led to the discovery of 3 basic laws
  • The Law of Conservation of Mass.
  • This law states that mass is neither created nor
    destroyed during ordinarychemical reactions or
    physical changes.
  • Formulated by Antoine Lavoisier in 1789.

Foundations of Atomic Theory
The Law of Conservation of Mass
Foundations of Atomic Theory
  • The Law of Definite Proportions
  • States that a chemical compound contains the same
    elements in exactly the same proportions by mass
    regardless of the size of the sample or source of
    the compound.
  • Discovered by Joseph Proust in 1797.

Foundations of Atomic Theory
  • The Law of Multiple Proportions
  • States that if different compounds are composed
    of the same 2 elements, then the ratio of the
    masses of the elements is always a ratio of
    small whole numbers.
  • Published by John Dalton in 1804.

Daltons Atomic Theory
  • In 1808, John Dalton proposed an explanation for
    the three laws. His atomic theory states
  • All matter is composed of atoms.
  • Atoms of the same element are identical atoms of
    different elements are different.
  • Atoms cannot be subdivided, created, or
  • Atoms of different elements combine in simple
    whole-number ratios to form chemical compounds.
  • In chemical reactions, atoms are combined,
    separated, or rearranged.

Corrections to Daltons Theory
  • Dalton turned Democrituss idea into a scientific
    theory that could be tested by experiment.
  • But not all aspects of Daltons theory have
    proven to be correct. We now know that
  • Atoms are divisible into even smaller particles.
  • A given element can have atoms with different

Daltons Atomic Model
  • An atom is the smallest particle of an element
    that has all the properties of that element.
  • Atoms are too small to seeeven through the
    most powerfulmicroscope!!
  • Dalton thought atoms were solid balls of matter
    and were indivisible.

Discovery of the Electron
  • In 1897, Joseph John (JJ) Thomson showed that
    cathode rays are composed of identical negatively
    charged particles, which were named
  • The electron was the first subatomic particle to
    be discovered.

Thomsons Cathode Ray Tube Experiment
Charge and Mass of the Electron
  • In 1909, Robert Millikan measuredthe charge on
    the electron during his oil drop experiment.
  • Using the charge-to-mass ratio, scientists were
    able to figure out the mass of the electron
    about 1/2000 the mass of a hydrogen atom.

Millikans Oil Drop Experiment
Thomsons Plum Pudding Model
  • After the work of Thomson andMillikan, the
    accepted model of the atom was called the plum
    pudding model.
  • The atom was viewed as a ball of
    positively- charged material with tiny
    negatively-charged electrons spread
    evenly throughout.

Discovery of the Atomic Nucleus
  • More detail of the atoms structure was provided
    in 1911 by Ernest Rutherford and his associates
    Hans Geiger and Ernest Marsden.
  • The results of their gold foil experiment led to
    the discovery of a very densely packed bundle
    of matter with a positive electric charge.
  • Rutherford called this positive bundle of matter
    the nucleus.

The Gold Foil Experiment
Rutherfords Atomic Model
  • After Rutherfords gold foil experiment, the
    accepted model of the atom looked like this
  • A small, positively-charged nucleus with negative
    electrons surrounding it at some distance away.
    Most of the atom is empty space.

Structure of the Atom
  • Rutherford proposed that the nucleus had
    particles with the same amount of charge as an
    electron but the opposite sign, called protons.
  • Relative charge 1
  • Relative mass 1 amu
  • For an atom to be neutral there must be equal
    numbers of protons and electrons.
  • Throughout the 1920s scientists accepted an
    (incorrect) model of the atom composed of protons
    and electrons.

Some Problems
  • How could beryllium have 4 protons stuck together
    in the nucleus?
  • shouldnt they repel each other?
  • If a beryllium atom has 4 protons, then it should
    weigh 4 amu but it actually weighs 9.01 amu!
    Where is the extra mass coming from?
  • Each proton weighs approximately 1 amu.
  • The electrons mass is only about 0.00055 amu and
    Be has only 4 electrons, so they dont account
    for the extra 5 amu of mass.

There Must Be Something Else There!
  • These questions were answered in 1932 by James
    Chadwick (a student of Rutherfords), who
    discovered another particle in the nucleus, which
    he called a neutron.
  • Charge 0 (no charge).
  • Relative mass 1 amu.

Subatomic Particles
  • The nucleus is made up of at least one
    positively charged particle called a proton and
    usually one or more neutral particles called
  • Protons, neutrons, and electrons are often
    referred to as subatomic particles.

Atomic Number
  • The atomic number (Z) of an element is the number
    of protons of each atom of that element.
  • Atoms of the same element all have the same
    number of protons.

Mass Number
  • The mass number is the total number of protons
    and neutrons in the nucleus of an atom.
  • Atoms of the same element can
    havedifferentmass numbers.

  • Isotopes are atoms of the same element that have
    different masses.
  • Isotopes have the same number of protons and
    electrons but different numbers of neutrons.
  • Most of theelements consist of mixtures

Designating Isotopes
  • Hyphen notation The mass number is written with
    a hyphen after the name of the element.
  • uranium-235
  • Nuclear symbol The superscript indicates the
    mass number and the subscript indicates the
    atomic number.

Mass number
Atomic number
Calculating Neutrons
  • The number of neutrons is found by subtracting
    the atomic number from the mass number.
  • mass number - atomic number number of neutrons
  • Nuclide is a general term for a specific isotope
    of an element.

Calculating Subatomic ParticlesSample Problem
  • How many protons, electrons, and neutrons are
    there in an atom of chlorine-37?
  • Solution
  • Number of protons
  • Number of electrons
  • Number of neutrons

atomic number (on periodic table)
number of protons
mass number - protons
The Atomic Mass Unit
  • The standard used by scientists to compare units
    of atomic mass is the carbon-12 atom.
  • One atomic mass unit, or 1 amu, is exactly 1/12
    the mass of a carbon-12 atom.
  • The atomic mass of any atom is determined by
    comparing it with the mass of the carbon-12

Average Atomic Mass
  • Average atomic mass is the weighted average of
    the atomic masses of the naturally occurring
    isotopes of an element.
  • The average atomic mass of an element
    depends on both the mass and the
    relative abundance of each of the
    elements isotopes.

Calculating a Weighted Average
  • If you have the following grades, what would your
    marking period average be?
  • First, change percents to decimals.
  • Next, multiply each grade by its decimal percent.
  • Finally, add up all the products.

Category Percent of Grade Grade
Tests 45 72
Quizzes 10 78
Labs 25 84
HW/CW 10 98
Project 10 94
100 0.45
x 32.4
x 7.8
100 0.10
100 0.25
x 21.0
x 9.8
100 0.10
x 9.4
100 0.10

Calculating Average Atomic MassSample Problem 1
  • Copper consists of 69.15 copper-63, which has an
    atomic mass of 62.929 601 amu, and 30.85
    copper-65, which has an atomic mass of 64.927 794
    amu. What is the Average Atomic Mass of Copper?
  • Solution
  • Change percents to decimals.
  • Multiply the atomic mass of each isotope by its
    relative abundance.
  • Add up all of the products.

Relative Abundance Mass
Cu-63 0.6915 62.93
Cu-65 0.3085 64.93
x 43.52
x 20.03

Calculating Average Atomic MassSample Problem 2
  • A student believed that she had discovered a new
    element and named it mythium. Analysis found it
    contained two isotopes. The composition of the
    isotopes was 19.9 of atomic mass 10.013 and
    80.1 of atomic mass 11.009. What is the average
    atomic mass, and do you think mythium was a new
  • Solution
  • Average Atomic Mass(.199 x 10.013) (.801 x
    11.009) 10.811
  • Because the atomic mass is the same as the atomic
    mass of boron, mythium was not a new element.

Round off to
Forces in the Atom
  • Electrons and protons attract because of opposite
    electrical charges, but protons and protons repel
    since they have the same charge.
  • The nucleus is heldtogether by a mysterious
    force called the strong nuclear force which
    only exists between nucleons (protons and
    neutrons) whichare very close together.

Valley of Stability
for Z 1 ? 20, stable N/Z 1
for Z 20 ? 40, stable N/Z approaches 1.25
for Z 40 ? 80, stable N/Z approaches 1.5
for Z gt 83, there are no stable nuclei
Naturally Radioactive Elements
  • All of the elements beyond atomic number 83 are
    unstable and thus radioactive.

Nuclear Reactions
  • Large, unstable nuclei spontaneously break apart
    to form smaller, more stable nuclei.
  • A nuclear reaction is a reaction that affects the
    nucleus of an atom.Example
  • A transmutation is a change in the identity of a
    nucleus as a result of a change in the number of
    its protons.

Nuclear ReactionsSample Problem
  • Identify the products that balance the following
    nuclear reactions
  • a.
  • b.
  • Solution
  • Atomic Mass 212 4 _____
  • Atomic Number 84 2 _____
  • Atomic Mass 22 ____ 22
  • Atomic Number 11 ____ 10

Radioactive Decay
  • Radioactive decay is the spontaneous
    disintegration of a nucleus into a lighter
    nucleus, accompanied by nuclear radiation.
  • Nuclear radiation is particles and/or
    electromagnetic radiation emitted from the
    nucleus during radioactive decay.

Types of Radioactive Decay
  • Alpha Emission
  • An alpha particle (a) is two protons and two
    neutrons bound together and is emitted from the
    nucleus during some kinds of radioactive decay.
  • The atomic number decreases by two and the mass
    number decreases by 4.

Types of Radioactive Decay (continued)
  • Beta Emission
  • A beta particle (ß) is an electron emitted from
    the nucleus during some kinds of radioactive
    decay (a neutron can be converted into a proton
    and an electron.)
  • The atomic number increases by one and the mass
    number stays the same.

Types of Radioactive Decay (continued)
  • Positron Emission
  • A positron (ß) is a particle that has the same
    mass as an electron, but has a positive charge
    (to decrease the number of protons, a proton can
    be converted into a neutron by emitting a
  • The atomic number decreases by one and the mass
    number stays the same.

Types of Radioactive Decay (continued)
  • Electron Capture
  • In electron capture, an inner orbital electron is
    captured by the nucleus of its own atom. (An
    inner orbital electron combines with a proton to
    form a neutron.)
  • The atomic number decreases by one and the mass
    number stays the same.

Types of Radioactive Decay (continued)
  • Gamma Emission
  • Gamma rays (?) are high-energy electromagnetic
    waves emitted from an unstable nucleus.
  • Atomic number and mass number both stay the
    same because gamma rays have no charge and no
  • They are pure energy, and very dangerous to
    living things.

Comparing Alpha, Beta and Gamma
  • Alpha particles are big andslow. They cant
    penetrate skin or paper.
  • Beta particles have about 100 x the penetrating
    power of alpha. They can be stopped by clothing,
    wood, or aluminum foil.
  • Gamma rays have the greatest penetrating ability.
    They can only be stopped by a thick layer of
    lead or concrete. They cause a lot of damage to
    living cells.

  • Half-life is the time required for half the
    atoms of a radioactive nuclide to decay.
  • Each radioactive nuclide has its own half-life.
  • More-stable nuclides decay slowly and have
    longer half-lives.

Half-LifeSample Problem 1
  • The half-life of radon-222 is 4 days. After what
    time will ¼ of a given amount of radon remain?
  • Solution
  • Determine the number of half-lives that it takes
    to cut a sample to ¼ of the original amount.
  • Then multiply that number by the half-life.

8 days
4 days

Half-LifeSample Problem 2
  • Phosphorus-32 has a half-life of 14.3 days. How
  • milligrams of phosphorus-32 remain after 57.2
    days if
  • you start with 4.0 mg of the isotope?
  • Solution
  • Determine the number of half-lives in 57.2 days.
  • For each half-life, multiply the original amount
    by ½

57.2 days
4 half-lives

14.3 days

0.25 mg
4.0 mg

Uses of Radiation
  • Radioactive dating scientists can determine the
    approximate age of an object based on the amount
    of certain radioactive nuclides present.

Uses of Radiation (continued)
  • Radioactive tracers are radioactive atoms that
    are incorporated into substances so that movement
    of the substances can be followed by radiation
  • Radioactive tracers can be used by doctors to
    diagnose diseases.
  • Radioactive tracers are also used in agriculture
    to determine the effectiveness of fertilizers.

Uses of Radiation (continued)
  • Irradiated Food nuclear radiation is used to
    prolong the shelf life of food.

Uses of Radiation (continued)
  • Nuclear Power Plants use energy as heat from
    nuclear reactors to produce electrical energy.
    They have five main components
  • 1. Shielding radiation-absorbing material used
    to decrease exposure to radiation from nuclear
  • Fuel usually Uranium-235.
  • 3. Coolant usually water, it absorbs excess
    heat energy.
  • 4. Control rods neutron-absorbing rods that
    limit the number of free neutrons
  • 5. Moderator used to slow down the fast
    neutrons produced by fission.

Nuclear Power Plant
Whole-Body Radiation Exposure
  • Rem The unit used to measure the biological
    effects of absorbed radiation in humans.

Radiation Detection
  • Film badges - use exposure of film tomeasure the
    approximate exposure of people working with
  • Geiger-Müller counters - instruments that detect
    radiation by counting electric pulses carried by
    gas ionized by radiation.
  • Scintillation counters - instruments that convert
    scintillating light to an electric signal for
    detecting radiation.

Fission vs. Fusion
  • Nuclear Fission very heavy nuclei split into
    smaller, more stable nuclei.
  • Can occur spontaneously or when nuclei are
    bombarded by particles.
  • Controlled fission chain reactions are used in
    nuclear power plants. Uncontrolled fission
    chain reactions are used in nuclear bombs.

Fission vs. Fusion (continued)
  • Nuclear Fusion low-mass nuclei combine to form
    a heavier, more stable nucleus.
  • Fusion releases even more energy per gram of
    fuel than fission.
  • Scientists are not yet able to control fusion
    reactions, so we cant use them in power plants.
  • Fusion is the primary process that fuels our sun
    and the stars.

Nuclear Waste
  • Radioactive waste produced in nuclear reactors
    can take hundreds of thousands of years to decay.
  • Disposal of nuclear waste is done with the
    intentionof never retrieving it.
  • There are 77 disposal sites around the country.
    A new one (Yucca Mountain) is being developed for
    the permanent disposal of much of our nuclear
    waste beginning in 2017.