Formation of Interstellar Polyatomic Substances

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Formation of Interstellar Polyatomic Substances

• Lewis Structures, Chemical Bonding, Formulas,
  Equations

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The only neutral atoms that do not undergo
reactions are those of inert gases which all have
8 valence electrons (octet), except He (duet)
Sienko/Plane, Chemistry, 4th,McGraw-Hill, NY,
1971, 47 and http//EnvironmentalChemistry.com/yog
i/periodic/index.html
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Stability of Atoms

• The neutral atom is the stable form for only the
  inert gases
• All other atoms undergo chemical reactions so as
  to attain a more stable electron structure
  (usually that of the inert gas nearest in atomic
  number, Z)

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Lewis Symbols

• Method of bookkeeping for valence electrons
• Lewis symbol -valence electrons represented by
  dots around the chemical symbol for an element
• For A-families (Main Group elements) the number
  of valence electrons Group (or family) number

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Staley, Essentials of Chemistry, Benjamin
Cummings, CA, 1984, 318
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Chemical Bonding

• Chemical bonding - Rearrangement of electron
  structures of 2 or more atoms
• Bonding occurs so that each atom attains a more
  stable electron structure
• Most common stable electron structure for Main
  Group elements is that of an inert gas, usually
  8e (Octet Rule), except 2e for H, He, and Li

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Ionic Bonding

• Occurs when a metal combines with a non-metal
• Sufficient valence electrons are transferred from
  the metal to the non-metal so that each atom
  attains an octet (or duet for H, Li, and Be)

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Electrovalence
He structure (Duet)
Ne structure (Octet)
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Structure of Salt
Ionic bond -attraction of oppositely charged ions
http//www.geog.ouc.bc.ca/physgeog/contents/6a.htm
l
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Covalent Bonding

• Occurs when non-metal atoms combine
• Pairs of electrons are shared between non-metal
  atoms so that each atom attains an octet (or duet
  for H)
• A shared pair of electrons is called a bonding
  pair (see hydrogen molecule)

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Lone pair of electrons - pair of electrons not
shared between two atoms
Central atom - an atom which is bonded to more
than one other atom
Terminal atom - atom which is bonded to only one
other atom
Multiple bond - bond consisting of more than one
shared-pair of electrons between two atoms - two
pairs shared is a double bond and three shared
pairs constitute a triple bond
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Examples of Covalent Bonding
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Draw NO
http//wine1.sb.fsu.edu/chm1045/notes/Bonding/Draw
ing/Bond06.htm
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Metallic Bonding

• Simplest model is Electron Sea Model - metal
  atoms lose valence electrons to the crystal
  producing cations of the metal surrounded by
  delocalized electrons - electrons free to move
• Delocalized electrons account for electrical and
  thermal conductivity
• Represented by empirical formula

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Electron - Sea Model of a Metal
Cations of the metal are surrounded by mobile
electrons. Attractions between opposite charges
hold the system together and constitute the
metallic bond.
http//www.wpbschoolhouse.btinternet.co.uk/page04/
4_72bond.htm
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Network Covalent or Atomic Substances

• It is possible for many atoms to link up to form
  a giant covalent structure or lattice
• The atoms are usually non-metals
• This produces a very strong 3-dimensional
  covalent bond network or lattice
• The diamond is an example - shown on next slide

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Network Covalent or Atomic Substance - Diamond
Carbon can form four single bonds so each carbon
bonds to four others etc.
http//www.wpbschoolhouse.btinternet.co.uk/page04/
4_72bond.htm
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Four Types of Bonding
X represents a non-metal atom
Straight lines between atoms are covalent bonds
M represents a cation of a metal
X- represents an anion
e- represents an electron
Masterton/Hurley, Chemistry Principles and
Reactions, 3rd, Saunders, PA, 1989, 246
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Chemical Formulas

• Ionic, atomic, and metallic substances are
  represented by an empirical formula -subscripts
  indicate the ratio in which the atoms have
  combined
• Molecular covalent substances are represented by
  a molecular formula - subscripts indicate the
  number of atoms of each type in one molecule of
  the compound

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Ionic Compound
NaCl is an empirical formula for an ionic
compound (metal non-metal)
Indicates that for every Na there is a Cl in the
compound
Covalent Compound
H2O is a molecular formula for a covalent
compound (two non-metals)
Indicates that there are 2 H atoms and 1 O atom
in a molecule of the compound
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Molecular Weight and Mole

• The sum of the atomic weights of all the atoms in
  a formula is called the molecular weight (MW).
• MW of H2O 2(1.008 amu) 16.00 amu 18.02 amu
• MW expressed in grams,18.02 g, is the mass of one
  mole of water or 6.02 x 1023 molecules of water

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Chemical Equations

• A chemical equation is a method of representing a
  chemical change in terms of formulas for stable
  forms of the substances (all atoms have attained
  octets or duets of electrons)
• General form A B gt C D, where A and B are
  formulas for reactants and C and D are formulas
  for products

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Writing Chemical Equations

• Write formulas for stable reactants and products
  in equation in the general form - called skeleton
  equation
• Balance skeleton equation by inserting
  coefficients so as to obtain the same number of
  atoms of each element on both sides of the
  equation using fractions if necessary

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Writing Equations (cont)

• Multiply coefficients by appropriate factor to
  clear fractions and obtain the lowest set of
  integers as coefficients
• Omit coefficients of 1 in final equation
• Balance H2 O2 gt H2O
• 1 H2 1/2 O2 gt 1 H2O
• 2 H2 O2 gt 2 H2O

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Balance C3H8 O2 gt CO2 H2O
http//www.emc.maricopa.edu/faculty/farabee/BIOBK/
BioBookCHEM1.htmlChemical Bonding
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Comparison of Chemical and Nuclear Reactions
http//www.lcc.edu/science/nuclearchem/index.html
- source no longer available
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Interstellar Substances

• Atoms in space combined to form both ionic and
  covalent substances
• Many of the substances formed in space do not
  follow the Octet Rule observed on earth due to
  the relatively small numbers of collisions of
  atoms possible in space

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http//www.science.gmu.edu/jmirick/csi904/project
.html
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http//www.science.gmu.edu/jmirick/csi904/project
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Continued...
http//www.science.gmu.edu/jmirick/csi904/project
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