Chemistry in Focus rd edition Tro by ixieshaofang

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									         Nivaldo J. Tro

         http://academic.cengage.com/chemistry/tro




       Chapter 3
   Atoms and Elements


Deborah Koeck • Texas State University, San Marcos
Understanding the Macroscopic World
• The connection between the microscopic atom and the
  macroscopic element is the key to understanding the
  chemical world.
Atoms
• An atom is the smallest
  identifiable unit of an element.

• Similarities and differences on
  the atomic scale correlate
  with similarities and
  differences on the
  macroscopic scale.
Atoms
Protons Determine the Element
• The atomic number, Z, represents the number of protons in
  the nucleus of an atom.
• The charge of a proton is assigned numerical value of +1.
• Protons constitute a significant part of the mass of an atom.
   – Mass of a proton is 1.0 amu
Chemical Symbols
• Symbols are an abbreviation of the
  chemical name which can be
  based on:

   – The English name for the
     element: hydrogen à H
   – The Greek or Latin name:
     ferrum (iron) à Fe
   – Their place of discovery:
     europium à Eu
   – The scientist honored by the
     naming: curium à Cm
Periodic Table of Elements
Electrons
• A neutral atom has as many
  electrons outside its nucleus
  as protons within its nucleus.
• The opposing charges of
  protons and electrons hold
  electrons within a spherical
  region surrounding the
  nucleus.
• Atoms can lose and gain
  one or more electrons.
Determining Protons and Electrons
Neutrons
• Neutrons are almost as
  massive as protons but carry no
  electrical charge.
• The number of neutrons in the
  atoms of an element can vary
  resulting in ISOTOPES.
   – Can be naturally occurring or
     man-made
• The sum of neutrons and
  protons in an atom is called the
  mass number of the atom.
Atomic Mass
• Atomic masses listed on the periodic table are weighted
  averages of the masses of each naturally occurring isotope
  for that element.
• Silver has two naturally occurring isotopes. One has a
  mass of about 107 amu and the other has a mass of about
  109 amu.
• Silver -107 has an abundance in nature of 51.84%.
• Silver -109 has an abundance in nature of 48.16%.
Atomic Mass
Calculate the average atomic mass of silver.

Silver has two naturally occurring isotopes.
• One has a mass of 106.90509 amu (abundance in nature of
   51.84%.)
• The other has a mass of 108.90476 amu (abundance in
   nature of 48.16%.)

106.90509 amu ( 0.5184) = 55.42 amu

108.90476 amu (0.4816) = 52.45 amu

                                 107.87 amu
Molar Mass
Calculate the Molar Mass of Calcium carbonate

The mass of one mole of a compound.
• Calcium carbonate
• Chemical Formula à CaCO3
Calcium      40.08 grams X 1 mole = 40.08 grams
Carbon      12.01 grams X 1 mole = 12.01 grams
Oxygen      16.00 grams X 3 moles = 48.00 grams
                                       100.09 grams
Periodic Law
• Mendeleev listed the
  known elements in order of
  increasing atomic mass,
  grouping those with similar
  properties.
• He noticed that certain
  similar properties would
  recur in a periodic fashion.
• His tabulation is the
  precursor to our modern
  periodic table.
Mendeleev and the Periodic Table
• Proposed that elements would be discovered to fill in gaps in
  the table.
• Proposed that some measured atomic masses were in error.
• Summarized a large number of observations
   – The underlying reasons for periodic behavior were as yet
     unknown
   – The Bohr model and the quantum mechanical model were
     needed to explain the observations.
The Bohr Model
• This is a MODEL of the atom that links electron behavior
  (microscopic) to the periodic law (macroscopic).
Bohr Diagrams/Electron Configurations
• Based on the behavior of
  electrons as particles
• Electron orbits
   – Specified with n, the orbit’s
     quantum number
• Fixed energies                     • Atoms with full outer orbits
                                       are extremely stable.
• Fixed radii
• Maximum number of electrons        • Atoms with outer orbits
  based on n                           that are not full are
• This model is an                     unstable and will undergo
                                       chemical reactions
  oversimplification (as are most      attempting to fill the outer
  models).                             orbit.
The Quantum Mechanical Model
• Based on the behavior of electrons as waves
• Replace Bohr’s orbits with orbitals, a representation of
  electron location as the probability of finding it in a certain
  region of space.
• Orbitals are grouped into shells and fill similarly to Bohr’s
  orbits.
Determinism and Quantum Mechanics
• Specifics of orbital filling are beyond the scope of this
  text…BUT
   – The statistical nature of the quantum mechanical model
     rocked the scientific establishment.
• An indeterminate universe
• Both models are useful even though Bohr’s, by experiment,
  has been shown to be invalid.
Families of Elements
• Also called groups
• Based on outer electron configurations
• Vertical columns
   – Alkali metals
   – Alkaline earth metals
   – Chalcogens
   – Halogens
   – Noble gases
Element Families
Metals, Nonmetals, and Metalloids
Molecular Elements
• Some elements occur as diatomic molecules.
The Mole Concept
• Counting particles as small as atoms is impractical.
• The mole concept allows us to relate the mass of a sample
  of an element to the number of atoms within it.
• The unit of comparison is called a mole and corresponds to
  6.022 x 1023.
Avogadro’s Number
• Determined such that the
  numerical value of the atomic
  mass of an element in amu is
  equal to the molar mass of
  that element in grams per
  mole.
• These relationships of
  grams, moles, and atoms
  can be used as conversion
  factors.
Chapter Summary
Molecular Concept          Societal Impact
• Atomic Number and Mass   • Chemical processes that
  Number                     cause change are caused by
• Isotopes                   changes in atoms or
• Molar Mass                 molecules.
• Bohr Model               • Pollution results from
                             “misplaced” atoms. Atoms
• Quantum Mechanical         that are not where they are
  Model                      supposed to be located.
• Orbitals                 • Models are used to describe
                             nature.

								
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