AW 5'1 The Atom

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AW 5.1 The Atom

• Early Models of the Atom
• Democritus, 400 B.C. first suggested atoms as the
  smallest indivisible and indestructible
  particles.
• Could not explain chemical behavior
• Lacked experimental support

• John Dalton (1766-1844)
• English school teacher
• Studied combination ratios of elements in
  chemical reactions
• Daltons hypotheses and theories explained
  results of his experiments

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AW 5.1 Atoms

• Daltons Atomic Theory
• All elements composed of tiny invisible
  particles atoms
• Atoms of the same element are identical. They are
  different the atoms of other elements
• Atoms of different elements physically mix
  together or chemically combine in simple
  whole-number ratios to form compounds.
• Chemical reactions occur when atoms are
  separated, joined, or rearranged. Atoms of one
  element, however, are never changed into atoms of
  another element as a result of a chemical
  reaction.

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AW 5.1 Atoms

• How Small is an Atom?
• Atom the smallest particle of an element that
  retains the properties of that element.
• 1 copper penny contains 2.4 x 1022 atoms
• (Earths population 6 x 109, or 6
  billion))
• 100,000,000 copper atoms lined up would form a
  line 1 cm long.
• Atoms can be observed with a scanning tunneling
  microscope.

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AW 5.2 Structure of the Nuclear Atom

• Electrons
• Today we know what Dalton did not atoms can be
  divided into smaller, more fundamental particles.
  Dozens of different sub-atomic particles are
  unleashed when atom smashers fracture atoms.
• Electrons are negatively charged sub-atomic
  particles.
• Discovered in 1897 by J. J. Thomson, English
  physicist (1856-1940)
• Performed experiments by passing electric current
  through gases at low pressure. Used cathode ray
  tube (glass tube sealed at both ends with metal
  electrodes).
• A glowing beam in the tube formed between the
  electrodes when electricity flowed between them.
• One electrode (cathode) became positively charged
  by electricity. The other electrode (anode)
  became negatively charged. The beam that flowed
  between them was the cathode ray.

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AW 5.2 Structure of the Nuclear Atom
Cathode Ray Tubes
Cathode rays are deflected by a magnet and
attracted by a positively charged plate. This
shows that the particles that make up the rays
are negatively charged.
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Holt 3.2 Structure of the Nuclear Atom

• Electrons
• Thompson found that cathode rays are attracted to
  metal plates with a positive charge. Plates that
  carry a negative electrical charge repel the
  rays.
• Opposite charges attract and like charges repel.
• Thompson proposed that a cathode ray is a stream
  of tiny negatively charged particles moving at
  high speed electrons.
• Thompsons conclusion Electrons are parts of the
  atoms of all elements.
• Today we know that an electron carries one
  negative charge and has a mass 1/840 the mass of
  a hydrogen atom.

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Holt 3.2 Structure of the Nuclear Atom

• Protons and Neutrons
• If cathode rays are electrons are electrons given
  off by atoms, what remains of the atoms after
  they have lost their electrons? (Answer A
  particle with one unit of positive charge, a
  proton.)
• E. Goldstein in 1886, observed rays (protons)
  traveling in the opposite direction of cathode
  rays in a cathode ray tube.
• A proton mass is about 1840 times that of an
  electron. This makes the mass of an electron
  negligible in comparison to the mass of a proton.
  
• In 1932 James Chadwick confirmed existence of the
  neutron, a subatomic particle with no charge
• Neutron mass nearly equal to that of a proton.

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AW 5.2 Structure of the Nuclear Atom

• The Atomic Nucleus
• Scientists unsure about structure of the atom
  following discovery of subatomic particles.
• Many thought that electrons were evenly
  distributed throughout an atom filled uniformly
  with positively charged solid material.
• Ernest Rutherford (1871-1937) tested this
  hypothesis in 1911.
• He fired massive alpha particles at thin gold
  foil.
• Alpha particles are positively charged helium
  atoms that have lost two electrons, leaving a
  double positive charged alpha particle.
• According to prevailing theory the alpha
  particles should have passed easily through the
  gold, with only a slight deflection due to the
  positive charge thought to be spread through the
  gold atoms.

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AW 5.2 Structure of the Nuclear Atom

• The Atomic Nucleus
• Two surprises for Rutherford
• 1. The majority of alpha particles passed
  through the gold foil with no deflection.
• 2. A small fraction of the alpha particles
  bounced off the gold foil at large angles. Some
  even bounced straight back.
• Said Rutherford It was about as credible as if
  you had fired a 15-inch shell at a piece of
  tissue paper, and it came back and hit you.
• Results of Rutherfords experiment A new theory
  of atomic structure. The atom consists mostly of
  empty space with the positive charge and most of
  the mass concentrated in a small, central core
  region - the nucleus, which is composed of
  protons and neutrons.

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AW 5.2 Structure of the Nuclear Atom
Rutherfords Experiment
Most ? (alpha) particles travel straight through
the gold foil but about 18000 were turned
through a large angle. The experiment takes
place in a vacuum to avoid problems of ?
absorption by air.
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AW 5.2 Structure of the Nuclear Atom
Rutherfords Experiment
To learn about the nature of the atom, Rutherford
aimed a beam of alpha particles at a sheet of
gold foil surrounded by a fluorescent screen.
Most of the particles passed through the foil
with no deflection. A few particles were greatly
deflected.
Most ? (alpha) particles travel straight through
the gold foil but about 18000 were turned
through a large angle. The experiment takes
place in a vacuum to avoid problems of ?
absorption by air.
Rutherford concluded that most of the alpha
particles pass through the gold foil because the
atom is mostly empty space. The mass and
positive charge are concentrated in a small
region of the atom. Rutherford called this region
the nucleus. Particles that approach the nucleus
closely are greatly deflected.
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AW 5.2 Structure of the Nuclear Atom
Rutherfords Experiment
Most ? (alpha) particles travel straight through
the gold foil but about 18000 were turned
through a large angle. The experiment takes
place in a vacuum to avoid problems of ?
absorption by air.
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AW 5.3 Distinguishing Between Atoms

• Atomic Number
• Atomic number of an element the number of
  protons in the nucleus of an atom of that
  element.
• An elements atomic number identifies its place
  on the periodic table. Example Oxygen has 8
  protons therefore its atomic number is 8.
• Atoms are electrically neutral. Therefore, for
  each element, the number of protons (positively
  charged particles) equals the number of electrons
  (negatively charged particles).

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AW 5.3 Distinguishing Between Atoms

• Atomic Mass Number
• Atomic mass number the number of protons and
  neutrons in an atom of an element. No. of
  neutrons mass number - atomic no.
• Shorthand notation for gold (Au)

Mass number
Atomic number

• The mass number and name of the element can
  designate atoms Example Hydrogen with mass
  number 1 may be designated hydrogen-1. Gold with
  mass number 197 may be designated Au-197.

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AW 5.3 Distinguishing Between Atoms

• Isotopes
• Atoms that have the same number of protons but
  different numbers of neutrons are called
  isotopes.
• Isotopes of an element have different mass
  numbers because they have different numbers of
  neutrons.
• Despite these differences, isotopes are
  chemically alike because they have identical
  numbers of protons and electrons, which are
  responsible for chemical behavior.

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AW 5.3 Distinguishing Between Atoms
Isotopes of Neon

• Isotopes of Hydrogen
• hydrogen-1 ((hydrogen)
• hydrogen-2 ((deuterium)
• hydrogen-3 ((tritium)

Mass number
Atomic number
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AW 5.3 Distinguishing Between Atoms

• Atomic Mass
• Atomic mass unit (amu) definition1/12 the mass
  of a carbon-12 atom.
• A carbon-12 atom has 6 protons and 6 neutrons,
  and its mass is set at 12 amu.
• Therefore, the mass of a single proton or a
  single neutron is about 1/12 of 12 amu, or about
  1 amu.
• The atomic mass of an element is the weighted
  average of the masses of its isotopes in a
  naturally occurring sample of the element.
• A weighted average mass reflects both the mass
  and relative abundance of the isotopes as they
  occur in nature.

Average Atomic Mass of Chlorine (Cl)
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Holt 3.3

• Relating Mass to Numbers of Atoms
• The Mole
• Amount of a substance that contains as many
  particles as there atoms in exactly 12 g of
  carbon-12.
• The mole is a counting unit.
• Avogadros Number
• Number of particles in exactly one mole of a pure
  substance, or 6.022 x 1023 particles
• Molar Mass
• The mass of one mole of a pure substance.
• Gram/Mole Conversions (Fig. 3-11, p. 82)
• Mole Conversions with Avogadros Number

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Distinguishing Between Atoms
Complete this table (Self Test)
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Distinguishing Between Atoms
Complete this table (Answer Key)
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AW 5.4 The Periodic Table Organizing the
Elements

• Development of the Periodic Table
• 70 elements discovered by the mid-1800s
• No means to relate the elements in a systematic,
  logical way
• Dmitri Mendeleev (Russian chemist, 1834-1907)
  listed elements in columns in order of increasing
  atomic mass.
• He left blank spaces in the table where there
  were no known elements with appropriate
  properties and masses. This enabled prediction of
  the physical and chemical properties of missing
  elements.
• Henry Moseley (British physicist, 1887-1915)
  arranged elements in a table by order of atomic
  number instead of atomic mass. This is the table
  that we use today.

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AW 5.4 The Periodic Table Organizing the
Elements

• The Modern Periodic Table
• Elements listed in order of increasing atomic
  number, from left to right and from top to
  bottom.
• Periods (7 horizontal rows) The properties
  elements in a period change as you move left to
  right. The pattern of properties repeats as you
  move from one period to the next.
• The periodic law When elements are arranged in
  order of increasing atomic number there is a
  periodic repetition of their physical and
  chemical properties.

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AW 5.4 The Periodic Table Organizing the
Elements

• The Modern Periodic Table
• Elements with similar properties end up in the
  same column, called a Group or Family). Each
  group is identified by a number and the letter A
  or B.
• Group A (Groups 1, 2, 13, 14, 15, 16, and 17)
  elements are called representative elements
  because they exhibit a wide range of physical and
  chemical properties. Three classes of
  representative elements 1. Metals - Alkali
  metals (Group 1A, except hydrogen) - Alkali
  earths (Group 2A) - Transition metals (Group
  B) - Inner transition metals2. Nonmetals3.
  Metalloids
• Metals Approximately 80 of all elements- High
  electrical conductivity- High luster (shiny)-
  Ductile (can be drawn into a wire)- Malleable
  (can be flattened into thin sheets)- Solid at
  room temp. (except for mercury)
• Transition Metals- Include copper, silver, gold,
  and iron

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AW 5.4 The Periodic Table Organizing the
Elements

• The Modern Periodic Table
• Nonmetals- Non-lustrous- Poor conductors of
  electricity- Some are gases at room temp.
  (oxygen, chlorine)- Some are brittle solids
  (sulfur)- Halogens Group 7A (chlorine,
  bromine)- Noble gases inert gases
• Metalloids- Elements with properties
  intermediate between metals and nonmetals
  (silicon)

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Chapter 4 Problem Solving in Chemistry -
Summary Key Terms

• Concept Summary
• 5.1 Atoms
• Elements are composed of atoms, which are the
  basic building blocks of matter.
• The atoms of a given element are different from
  the atoms of all other elements.
• 5.2 Structure of the Nuclear Atom
• Atoms contain positively charged protons,
  negatively charged electrons, and electrically
  neutral neutrons.
• The nucleus of an atom is composed of protons and
  neutrons. The electrons surround the nucleus and
  occupy most of the volume of the atom.

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Chapter 4 Problem Solving in Chemistry - Summary

• 5.3 Distinguishing Between Atoms
• The number of protons in an atoms nucleus is
  the atomic number of the element.
• Because atoms are electrically neutral, an atom
  has the same number of protons and electrons.
• The sum of the number of protons and number of
  neutrons is the mass number of an atom.
• Atoms with the same number of protons, but
  different numbers of neutrons are called
  isotopes.
• The atomic mass of an element is expressed in
  atomic mass units (amu).
• The atomic mass of an element is a weighted
  average of all the naturally occurring isotopes
  of that element.
• 5.4 The Periodic Table Organizing the Elements
• In the periodic table, the elements are
  organized into groups (vertical columns) and
  periods (horizontal rows) in order of increasing
  atomic number.
• Elements that have similar chemical properties
  are in the same group.