Chemical Bonding

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Chemical Bonding
2
Test Prep

• List and describe using TWO words the 3 types of
  bonding
• Why are ionic compounds brittle
• What large chemical group forms cations?
• Explain cation formation for magnesium.
• What large chemical group forms anions?
• Explain anion formation using arsenic.
• How do you differentiate b/w ionic and covalent
  (molecular) compounds?

3
GPS Standard

• SC3. Students will use the modern atomic theory
  to explain the characteristics of atoms.
• e. Compare and contrast types of chemical bonds
  (i.e. ionic, covalent).

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BIG science concept FORCES

• 2 types of forces of interest to chemists
• These forces explain in LARGE part the
  behavior/properties we observe in nature
• 2 fundamental types of forces studied in
  chemistry
• 1. Intramolecular forces (F inside a single
  molecule. These forces include bonding)
• 2. Intermolecular forces (F between molecules,
  i.e. polarity of molecule)

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Three Types of Chemical Bonding

• Ionic bonding electrons are exchanged between
  atoms (some gain, some lose). We see this type
  of bonding in ionic compounds.
• Covalent bonding electrons are shared between
  atomsno charge on any one atom because no
  exchange of electrons.
• Metallic bonding special bonding between metal
  atoms for pure metals and metal mixtures (alloys).

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Ionic Bonding

• Bonding between metals and non-metals. Metal
  atoms have a low number of valence electrons
  and a low electronegativity. Non-metal atoms
  have numerous valence electrons.

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If the electron clouds overlap (bond)..

• Then Metals
• 1. lose valence electrons
• achieve a stable valence shell (usually 8 e-)
• Lose valence electrons and has a positive charge
  (form a positive ion) based on how many valence
  electrons are lost to achieve noble gas
  configuration.

• Non-metals
• gain valence electrons
• achieve a stable valence shell (usually 8 e-)
• gain electrons and gain a negative charge (form
  negative ion) to achieve noble gas configuration.

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Ionic Lattice is formed

• Positive and negative ions attract each other to
  form a three dimensional continuous lattice
  structure.
• Each ve ion is surrounded by a number of -ves.
• Each -ve ion is surrounded by a number of ves.
• The ratio of ve-ve ions in the lattice is
  determined by the charges of the ions.

• Ions Formula
• Na Cl- NaCl
• 1 -1 11
• Mg2 Cl- MgCl2 2 -1 12
• Na S2- Na2S
• 1 -2 21
• Mg2 S2- MgS
• 2 -2 11

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The Crystal Lattice

• 1-1 Crystal Lattice
• - - - -
• - - - - -
• - - - -
• - - - - -
• The picture above is a 2-D picture. Actual
  crystal is 3-D so have planes above and beneath
  one shown.

• Each ion is surrounded by 6 ions of opposite
  charge (dont forget about plane above and below
  one shown). This is n attractive force between
  these particles. Every ion is surrounded by 6
  nearest neighbors of opposite charge. The
  electrical attraction of ions is what gives the
  crystal its strengthcalled the lattice energy.

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This table shows properties of ionic lattices
(compounds) and explanations of these properties.

• Melting point and boiling point The melting and
  boiling points of ionic compounds are high
  because a large amount of thermal energy is
  required to separate the ions which are bound by
  strong electrical forces. Hence, ionic compounds
  are solids at room temperatures.
• Electrical conductivity Solid ionic compounds
  do not conduct electricity when a potential is
  applied because there are no mobile charged
  particles. No free electrons causes the ions to
  be firmly bound and cannot carry charge by
  moving. This property is very different for the
  liquid state.
• Hardness Most ionic compounds are hard the
  surfaces of their crystals are not easily
  scratched. This is because the ions are bound
  strongly to the lattice and aren't easily
  displaced.
• Brittleness Most ionic compounds are brittle a
  crystal will shatter if we try to distort it.
  This happens because distortion cause ions of
  like charges to come close together then sharply
  repel.

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But I thought ionic compounds conducted
electricity

• Yes and no. To conduct electricity, charge
  carriers must be present to move electrical
  charge from one place to the next.
• In the solid state, there are no mobile charge
  carriersions are fixed in position.
• However, when ionic compounds are heated to their
  melting point (T where they melt), then charge
  carriers become mobile and can conduct
  electricity.
• Another property of ionic compounds is that they
  dissolve (ions separate) in water this will also
  achieve the mobile charge carrying requirement of
  electrical conduction hence SALT water conducts
  electricity, just plain salt does not unless it
  is liquified.

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Covalent Bonding

• Bonding between non-metals and non-metals.
  Therefore all atoms included have fairly high
  electronegativity and few vacancies in valence
  energy levels. When they bond, they share
  electrons to achieve stable configuration.
  Hence, electrons are shared.
• Sharing produces low energy (stable) electron
  arrangements.

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Consider a hydrogen molecule

• In a hydrogen molecule, each hydrogen atom needs
  an electron to achieve noble gas configuration.
  If each atom shares an electron with the other
  atom, each atom can then gain NGCin this case
  like He.
• Diagram on the board of H2 molecule.
• http//www.bcpl.net/kdrews/bonding/bonding3.html
  Well

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This table shows properties of covalent
molecular compounds and explanations of these
properties.

• Do not conduct electricity. No mobile charged
  particles so molecules not charged. Electrons
  tightly bound to atoms or shared by atoms in
  covalent bonds.
• Melting and boiling points low. During
  melting/boiling, molecules become separated.
  Forces of attraction between molecules are weak
  and little thermal energy is required to separate
  them. Many covalently bonded compounds are
  liquids and gases at room temperature.
• Soft. Molecules weakly attracted to each other
  and are easily displaced (also brittlethink
  pencil lead).

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Polar vs. Nonpolar Covalent Bonds

• In covalent bonds, electron pairs are shared
  equally between atoms of equal electronegativity
• If the atoms in a covalent bond have differing
  electronegativities, the atoms with the higher
  electronegativity has gt50 of the shared pairs
  of electrons and the atoms with low
  electronegativity has 50 of the shared pairs of
  electronegativity.
• The atom tending to gain electrons acquires a
  slight negative charge (delta -ve)
• The atom tending to lose electrons acquires a
  sligh positive charge (delta ve)
• When this happens, The bond is POLAR.

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Lewis Structures

• We can use Lewis (electron dot) Structures to
  help us understand the difference between ionic
  and covalent bonding.
• Notes on Board. Page 415-416 437-439.

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Other Web Sites

• http//www.visionlearning.com/library/module_viewe
  r.php?mid55
• http//hyperphysics.phy-astr.gsu.edu/Hbase/chemica
  l/bond2.html
• http//antoine.frostburg.edu/chem/senese/101/compo
  unds/ionicvscovalent.shtml
• http//www.chemtutor.com/compoun.htm
• http//www.okstate.edu/jgelder/bondtable.html
• http//www.bcpl.net/kdrews/bonding/bonding.html
• http//www.quia.com/jq/19617.html
• http//www.chem.uncc.edu/faculty/murphy/1251/slide
  s/C18a/