Chapter 2: Atoms, Molecules, and Ions

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Chapter 2 Atoms, Molecules, and Ions
Chemistry 1061 Principles of Chemistry I Andy
Aspaas, Instructor
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Atomic theory of matter

• Daltons atomic theory
• All matter composed of indivisible atoms
• Atom small particle, retains identity in
  reactions
• Element type of matter composed of only one kind
  of atom
• Compound type of matter composed of fixed
  proportion of 2 or more elements
• Chemical reaction rearrangement of atoms to give
  new chemical combinations

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Deductions from Daltons Atomic Theory

• Law of conservation of mass
• Law of definite proportions
• Example 1.00 g C reacted with oxygen
• 2 compounds formed
• 1.3321 g O 1.00 g C
• 2.5542 g O 1.00 g C

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Structure of the Atom

• Dalton said atoms were indivisible, but
  experiments starting around 1900 showed that
  atoms themselves consist of particles
• Nucleus atoms central core
• Positively charged
• Contains most of atoms mass
• Electron
• Very light, negatively charged particles

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Discovery of the electron

• 1897, British physicist J.J. Thomson
• Experiments showed atoms are not indivisible
• Cathode ray tube
• Calculated ratio of electrons mass to its charge
• 1909, American Robert Millikan
• Charge on electron 1.602 x 10-19 coulomb
• Mass 9.109 x 10-34 kg

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Nuclear model of the atom

• 1911, British physicist Ernest Rutherford
• Gold foil experiment
• Most alpha particles pass straight through gold
  foil
• Most of atom empty space
• If a golf ball was the nucleus, the atom would be
  about 3 miles in diameter

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Gold foil experiment
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Nuclear isotopes

• Nucleus comprised of 2 kinds or particles
• Protons
• Have positive charge equal in magnitude to
  electrons negative charge
• More than 1800 times more massive than electron
• Neutrons
• Mass nearly identical to proton, no charge

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Atomic number and mass number

• Atomic number (Z) total number of protons in the
  nucleus of an atom
• New element definition substance whose atoms all
  have the same atomic number
• Mass number (A) total number of protons and
  neutrons in a nucleus
• Nuclide atom characterized by certain mass
  number and certain atomic number
• Nuclide symbol Z subscript, A superscript,
  element

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Isotopes

• Isotopes atoms whose nuclei have the same atomic
  number but different mass numbers
• Some elements have only one naturally occurring
  isotope (sodium-23)
• Some have several naturally occurring isotopes
• Oxygen contains the following isotopes
• 99.769 oxygen-16
• 0.037 oxygen-17
• 0.204 oxygen-18

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Atomic weights

• Dalton an atom of a certain element has a
  characteristic mass
• But, naturally occurring elements may be a
  mixture of isotopes
• Isotope percentages are generally constant
• Dalton actually calculated average atomic masses
• They were relative to the mass of hydrogen, the
  smallest element

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Atomic mass units and atomic weight

• Carbon-12 isotope chosen arbitrarily as standard
• 12 atomic mass units (amu)
• Atomic weight average atomic mass for a
  naturally occuring element, expressed in amu
• Appears on periodic table along with atomic number

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Calculating atomic weight

• Fractional abundance fraction of a total number
  of atoms, which consists of a particular isotope
• Isotopic mass is not exactly equal to mass number
• Neon-20, mass 19.992 amu, abund 0.9051
• Neon-21, mass 20.994 amu, abund 0.0027
• Neon-22, mass 21.991 amu, abund 0.0922
• Multiply isotopic mass by fractional abundance,
  and sum total to get atomic weight
• Gives a weighted average

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Periodic table of the elements

• 1869, Dimitri Mendeleev (and J. Lothar Meyer,
  independently)
• Arranged elements in order of atomic number
• Placed elements in horizontal rows so that
  vertical columns of elements with similar
  properties formed

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Periods

• Period elements in any one horizontal row of the
  periodic table
• First has only 2
• 2nd and 3rd have 8
• 4th and 5th have 18
• 6th has 32
• 7th is incomplete

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Groups

• Group elements in any one vertical column of the
  periodic table
• Are numbered with Roman numeral and letter
• (or just numbered with a number)
• Main-group elements (labeled with A)
• Transition elements (labeled with B)
• Inner transition elements (shown below rest of
  table)

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Groups

• Elements in a single group have similar
  properties
• For example
• IA alkali metals - all react vigorously with
  water (except hydrogen)
• VIIA halogens - all react vigorously with sodium

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Metals, Nonmetals, Metalloids

• Metal substances or mixtures that
• Have a characteristic shine
• Generally are good conductors of heat and
  electricity
• Are usually malleable (can be hammered into
  sheets)
• Are usually ductile (can be drawn into wire)
• Metallic elements are solid at room temp., except
  for mercury

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Metals, nonmetals, metalloids

• Nonmetal element that does not exhibit
  characteristics of a metal
• Most are gases
• Solid nonmetals are usually brittle and hard
• Bromine is only liquid nonmetal
• Metalloid (or semimetal) - both metallic and
  nonmetallic properties
• Many are semiconductors (silicon, germanium)
• Dont conduct when pure, do when doped or at very
  high temperature

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Chemical formulas

• Chemical formula atomic symbols with numeric
  subscripts that show relative proportions of
  atoms of different elements in a substance
• Al2O3 ratio of Al atoms to O atoms is 2 3
• No subscript ratio of 1 (NaCl, 11)

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Molecular substances

• Molecule group of atoms that are chemically
  bonded together - tightly connected by attractive
  forces
• Molecular substance composed of all the same
  molecules
• Molecular formula shows number of different
  atoms of an element in a molecule
• Structural formula shows how atoms are bonded
  together, a line indicates a chemical bond

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Molecular substances

• Some elements are molecular substances
• Cl2, O2, N2, S8
• Some exist as individual atoms
• He, Ne
• Some exist as a very large but indefinite number
  of atoms bonded together
• C
• Polymer extremely large molecules made of small
  molecules repeatedly bonded together (monomers)
• Natural (wool, cotton), synthetic (plastics,
  Nylon, polyester, Kevlar)

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Ionic substances

• Unlike molecular substances, ionic substances are
  formed from charged atoms or groups of atoms
  called ions
• Metal atoms tend to lose electrons result is a
  positive charge (cation)
• Nonmetals tend to gain electrons result is a
  negative charge (anion)
• Sodium loses one electron to become Na
• Calcium loses two electrons to become Ca2
• May consist of a group of bonded atoms that have
  a deficiency or surplus of electrons (SO42)

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Ionic compounds

• Ionic compounds are composed of cations and
  anions
• Ionic formula given by smallest possible integer
  number of different ions in substance
• Charges are omitted
• Ionic compounds are uncharged as a whole
• Ex. Na and Cl- NaCl
• Fe3 and SO42- Fe2(SO4)3

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Naming simple compounds

• Chemical nomenclature systematic naming of
  chemical compounds
• Organic compounds carbon-containing molecular
  substances
• Inorganic compounds composed of elements other
  than carbon (except CO, CO2, cyanides, and
  carbonates)

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Ionic compounds

• Named given by name of cation followed by name of
  anion
• Potassium sulfate
• Monatomic ion formed from a single atom

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Predicting a monatomic ions charge

• Most main-group metallic ions have one monatomic
  cation with charge equal to group number
• Aluminum, Group IIIA, Al3
• Many high-atomic-number metals have a cation
  equal to group number AND one equal to the group
  number minus 2
• Thallium, Group IIIA, Tl3, Tl
• Most transition elements can have several
  monatomic cations. 2 is very common
• A nonmetal main-group ion forms an anion
• Charge 8 - group
• Oxygen, Group VIA, O2-

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Naming monatomic ions

• Monatomic cations are named after element if
  theres only one possible cation for that element
• If more than one possible cation, use charge as
  roman numeral after name
• Fe2 iron(II), Fe3 iron(III)
• Or use suffixes with latin name (-ous for lower
  charge, -ic for higher)
• Cu2 cuprous, Cu3 cupric
• Monatomic anions use -ide suffix (bromide,
  sulfide)

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Polyatomic ions

• Polyatomic ion ion consisting of 2 or more atoms
  chemically bonded together, which carry a net
  electric charge
• Oxoanions central element some oxygens
• Suffixes indicate relative amount of oxygens
• Fewer oxygens -ite
• More oxygens -ate
• Ex. SO32- is sulfite, SO42- is sulfate

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Polyatomic anions

• If more than 2 oxoanions, use prefixes
• hypo- and per-
• ClO- hypochlorite ion
• ClO2- chlorite ion
• ClO3- chlorate ion
• ClO4- perchlorate ion

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Binary molecular compounds

• Name first element with its exact name
• Suffix second element with -ide
• Use Greek prefixes to indicate number
• Dont use mono unless there are more than one
  possible amount
• Ex. CO vs CO2

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Acids and corresponding anions

• Acids have H as the cation
• Oxoacids contain an oxoanion
• -ate becomes -ic
• -ite becomes -ous
• End name with acid
• Binary compounds of hydrogen and nonmetal
• Prefix hydro-
• Suffix -ic acid

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Hydrates

• Simply indicate how many water molecules are
  present by using the Greek numerical prefix and
  -hydrate at the end
• MgSO4 7H2O magnesium sulfate heptahydrate

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Writing chemical reactions

• Reactant ----gt Product
• Arrow means reacts to form or yields
• Useful to indicate state or phase
• (g) gas, (l) liquid, (s) solid,
• (aq) aqueous solution
• Use a coefficient to indicate relative number of
  particles involved
• ? over arrow means heat is applied
• A compound written over the arrow is usually a
  catalyst

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Balancing chemical equations

• Mass must be conserved so use coefficients to
  make sure the same number of each atom occurs on
  each side of the equation
• Start by balancing atoms for elements that occur
  in only one substance on each side
• Ex H3PO3 ---gt H3PO4 PH3
• Start by balancing oxygen
• Practice!