Combinations of Atoms

1
Combinations of Atoms
2
Chemical Bonds

• Forces that hold groups of atoms together and
  make them function as a unit.

3
Ionic Configuration and Size

• Ions are formed when electrons are gained or lost
  from an atom. The gain or loss follows the
  pattern called the octet rule, that an atom
  forms an ion in which it attains the same
  electron configuration as the nearest noble gas.
  Most metals therefore lose electrons, and as a
  result get smaller.
• Likewise, nonmetals gain electrons to form ions,
  thus increasing in size by the opposite rule to
  metals.

4
The Octet Rule

• Noble gases are the least reactive of elements
  primarily due to their very stable valence
  electron structure of Ns2 Np6 eight electrons.
  This is the basis of what is called the octet
  rule Elements tend to react by transferring or
  sharing electrons until all have attained the
  same valence configuration as the nearest noble
  gas
• This rule is very useful to predict the behavior
  of the lighter elements (1-22) primarily, and
  representative elements as well

5
Ionic Bonds

• Electron transfer from one atom to another is the
  basis for forming positive and negative ions.
  When a positive and negative are formed, they
  attract to each other, forming a strong ionic
  bond. These bonds are so strong that all ionic
  compounds are solids at room temperature.

6
Ionic Bonds

• Elements with few valence electrons (1 to 3) will
  lose them to form positive ions. Elements with 5
  to 7 valence electrons will gain until they have
  8.
• The ions formed arrange themselves in a giant
  array called a crystal lattice. This means that
  ionic compounds do not contain specific
  molecules, and that their formula only indicates
  the ratio of to ions.

7
Essential Ions in the Body

• Calcium, Ca2--needed for bones and teeth
  sources are dairy and grains deficiency results
  in weak bones and teeth
• Sodium, Na--needed for water and acid balance
  sources are salt, meat, vegetables deficiency
  results in imbalance of blood chemistry
• Potassium, K--needed for nerve and muscle
  function found in meat and grains deficiency
  brings heart and muscle problems
• Magnesium, Mg2--necessary for nerve and muscle
  function found in dairy, grains, flour
  deficiency can lead to muscle tremor and
  convulsions

8
Electron Dot Diagrams

• The valence electrons of an atom can be
  represented by dots placed around the element
  symbol
• Na C Xe
• Compounds can also be shown with ions having
  proper number of electrons
• Phosphorus Sodium chloride MgCl2

9
Ionic properties

• Ionic substances are held together by very strong
  forces, so all ionic compounds are solids at room
  temperature, and some have very high melting and
  boiling points. Those compounds are used to line
  the ladles for molten iron and steel.
• Ionic compounds exist as large crystals of
  alternating positive and negative charges, rather
  than specific molecules. These can be separated
  by melting or by dissolving in water to allow
  electricity to flow. This conductivity is why
  they are called electrolytes. Not all ionic
  compounds dissolve in water, especially if the
  ionic forces between ions are very large.

10
Binary Ionic--Lattice Energy

• The change in energy when separated gaseous ions
  are packed together to form an ionic solid is
  called lattice energy.
• M(g) X?(g) ? MX(s)
• Lattice energy is negative (exothermic) from the
  point of view of the system.

11
Ions

• Cation A positive ion
• Mg2, NH4
• Anion A negative ion
• Cl?, SO42?
• Ionic Bonding Force of attraction between
  oppositely charged ions.

12
Polyatomic Ions

• Colvalent bonding creates some special ions made
  up of two or more elements. Such ions are
  important in many substances in nature and living
  organisms.
• CO3-2 NO3- PO4-3

13
Formation of an Ionic Solid

• 1. Sublimation of the solid metal
• M(s) ? M(g) endothermic
• 2. Ionization of the metal atoms
• M(g) ? M(g) e? endothermic
• 3. Dissociation of the nonmetal
• 1/2X2(g) ? X(g) endothermic
• 4. Formation of X? ions in the gas phase
• X(g) e? ? X?(g) exothermic
• 5. Formation of the solid MX
• M(g) X?(g) ? MX(s) quite exothermic

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Homework 8a

• p.214 1, 2, 4, 5
• p. 217 7, 8, 9, 10, 11
• p.220 12, 13, 14

16
The Chemists ShorthandFormulas

• Chemical Formula
• Symbols types of atoms
• Subscripts relative numbers of atoms
• CO2
• Structural Formula
• Individual bonds are shown by lines.
• OCO

17
Writing Formulas From Names

• Ionic of any type
• Write the ions represented with their charges
• Calculate the least common multiple
• Multiply to get balanced charges
• iron(III) oxide sodium phosphate

18
Examples

• sodium nitrate calcium hydroxide
• iron(II) chlorate silver chromate

19
Naming Compounds
Binary Ionic Compounds

• 1. Cation first, then anion
• 2. Monatomic cation name of the element
• Ca2 calcium ion
• 3. Monatomic anion root -ide
• Cl? chloride
• CaCl2 calcium chloride

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Naming Compounds(continued)
Binary Ionic Compounds (Type II)

• metal forms more than one cation
• use Roman numeral in name
• PbCl2
• Pb2 is cation
• PbCl2 lead (II) chloride

21
Naming Compounds(continued)

• Polyatomic Ionic
• When a cation is combined with a polyatomic ion
• Name the cation first, then the polyatomic ion

K2CrO4--- Potassium chromate
22
Examples
Ca3(PO4)2 NH4MnO4 Fe3(AsO4)2
23
HOMEWORK 8b

• p. 224 19- 23 all
• p. 225 24-28 all
• p. 226 29-33 all
• p. 227 34, 35, 39

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Bonding Models for Metals

• Electron Sea Model A regular array of metals in
  a sea of electrons.
• Band (Molecular Orbital) Model Electrons
  assumed to travel around metal crystal in MOs
  formed from valence atomic orbitals of metal
  atoms.

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Electron Sea Model
26
Band theory M.O. model
27
Metal Alloys
Substances that have a mixture of elements and
metallic properties. Alloys are made to increase
strength or to resist corrosion (rusting).

• 1. Substitutional Alloy some metal atoms
  replaced by others of similar size.
• brass Cu/Zn
• 2. Interstitial Alloy Interstices (holes) in
  closest packed metal structure are occupied by
  small atoms.
• steel iron carbon
• 3. Both types Alloy steels contain a mix of
  substitutional (Cr, Mo) and interstitial (carbon)
  alloys.

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Copper Zinc
Brass
Brass
Iron Carbon
Steel
Steel
30
Homework 8c

• p. 231 40, 41, 43
• p. 236ff 61, 72, 74, 75, 76, 78, 88, 95, 96

31
Covalent Bonding

• Nonmetals cannot form ionic bonds with each other
  since both would attract electrons. So to attain
  an octet, they must share to make a covalent
  bond. When this happens, a specific unit, called
  a molecule, is formed.
• Seven elements exist naturally as diatomic
  covalent molecules H2, N2, O2, F2, Cl2, Br2, I2.
  These are always found in this manner, and in
  any reaction must be represented this way.

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Lewis Structure

• Shows how valence electrons are arranged among
  atoms in a molecule.
• Reflects central idea that stability of a
  compound relates to noble gas electron
  configuration.

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Rules for Drawing Lewis Structures

• Add up all of the valence electrons for the atoms
  involved in the molecule. In polyatomic ions,
  subtract electrons for a charge, add for a -
  charge
• Select a most likely central atom and arrange
  other atoms around it. Place pairs of electrons
  between atoms.
• Arrange the remaining electrons around external
  atoms first. If the central atom is not
  satisfied, form double or triple bonds to make
  the molecule work.

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Example 8.6
35
Multiple Bonds

• The octet rule tells us to make sure all elements
  get a noble gas configuration. The Lewis dot
  structures can show us how pairs of electrons are
  shared to be sure all atoms are satisified. If
  more than one pair is needed between atoms, a
  double or triple bond will be formed.
• Examples H2O CO2 N2

36

• A sigma (?) bond centers along the internuclear
  axis.
• A pi (?) bond occupies the space above and below
  the internuclear axis.

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Writing Formulas From Names

• Binary Covalent
• Observe the prefixes
• Place first element first with correct number
• Place second element second with correct number
• dinitrogen tetroxide silicon dioxide

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Naming Compounds(continued)
Binary covalent (Type III)

• Compounds between two nonmetals
• First element in the formula is named first.
• Second element is named as if it were an anion.
• Use prefixes to tell how many
• Never use mono- on the first element
• P2O5 diphosphorus pentoxide

40
Common Names

• Some compounds are only known by their common
  names and NEVER called by systematic names
• H2O NH3 CH4
• www.dmho.org www.armory.com/crisper/DMHO
• www.lhup.edu/dsimanek/dmho.htm

41
Homework 9a

• p. 244 1-5 all
• p. 247 6, 7, 8, 9, 12
• p. 249 13-17 all
• p. 251 24, 27 a,c,e

42
Localized Electron Model

• A molecule is composed of atoms that are bound
  together by sharing pairs of electrons using the
  atomic orbitals of the bound atoms.
• Two types of electron pairs bonding pairs and
  lone pairs. Bonding pairs are linkages between
  atoms, lone pairs are electrons solely owned by
  an atom.

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Localized Electron Model

• Elements of the Model
• 1. Description of valence electron arrangement
  (Lewis structure).
• 2. Prediction of geometry (VSEPR model).
• 3. Description of atomic orbital types used to
  share electrons or hold lone pairs.

44
Resonance

• Occurs when more than one valid Lewis structure
  can be written for a particular molecule.
• These are resonance structures. The actual
  structure is an average of the resonance
  structures.

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Example 8.9
46
Comments About the Octet Rule

• 2nd row elements C, N, O, F observe the octet
  rule.
• 2nd row elements B and Be often have fewer than 8
  electrons around themselves - they are very
  reactive.
• 3rd row and heavier elements CAN exceed the octet
  rule using empty valence d orbitals.
• When writing Lewis structures, satisfy octets
  first, then place electrons around elements
  having available d orbitals.

47
Homework 9b

• p. 255 30-34 all
• p. 256 35-38 all
• p. 258 39-41, 42, 48

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Shape of Molecules

• The distribution of bonding and non-bonding
  electron pairs helps to shape a molecule. These
  pairs spread out as far away from each other as
  possible and create various shapes.
• It is the combination of shape and bond polarity
  which determine if the molecule is polar.

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Molecular Architecture

• The structure of a molecule is important in how
  it reacts and to its physical properties
• Once the Lewis structure of a molecule is
  determined, the shape of the molecule then can be
  predicted according to the VSEPR model.

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VSEPR Model

• The structure around a given atom is determined
  principally by minimizing electron pair
  repulsions.

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Predicting a VSEPR Structure

• 1. Draw Lewis structure.
• 2. Count pairs, both bonding and lone pairs
  around the central atom.
• 3. Determine positions of atoms from the way
  electron pairs are shared.
• 4. Determine the name of molecular structure from
  the number of bonding and lone pairs and their
  necessary arrangements. Remember that lone pairs
  prefer to be at 120º or greater from each other.

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Sample 8.12
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Hybridization

• The mixing of atomic orbitals to form special
  orbitals for bonding.
• The atoms are responding as needed to give the
  minimum energy for the molecule.
• To determine hybridization, count lone and
  bonding pairs, but count multiple bonds only
  once.

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Sample 9.3,4,5
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Electronegativity

• Covalent bonding calls for another measurement,
  the attraction of an atom for the electrons it is
  sharing with another atom, called
  electronegativity.
• Covalent bonds are classified as to whether they
  are balanced, or non-polar, or unbalanced, or
  polar. This can be calculated from the
  electronegativities of the elements in the bond
  if on subtracting the less from the greater,
  there is a difference of 0.5 or more, the bond is
  polar, if less it is non-polar.

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Polarity of Molecules

• Molecules will align in an electric field if both
  has polar bonds and a polar shape. When this
  happens, the molecule will have partially
  separated charges, on the less electronegative
  elements and on the more electronegative.
  Polarity of molecules affects the behavior of
  molecules, especially in making solutions.

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Homework9c

• p. 262 49-53, 56, 59
• p. 266 60-63 all
• p. 267 64, 66, 69, 70

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Homework 9d

• Group in-class assignment
• p. 272ff 81, 89, 90, 92,
• 95-100, 102, 104-107,
• 110, 111, 113