Molecular Compounds

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Molecular Compounds
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Bonding Review - Compounds

• Which statement(s) is(are) true about compounds?
• A) They have to contain at least 2 different
  kinds of elements (like Na, S)
• B) The combination is of a specific ratio of
  atoms. (as in Na2S)
• C) The combination of atoms in a compound can be
  predicted knowing of valence electrons.(Na has
  1, S has 6)

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Answer

• All 3 are correct.
• A compound is composed of 2 or more elements
  bonded together in a certain ratio.
• The ratio IS able to be figured out knowing how
  many valence electrons each type of atom has.

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Element types and bonding

• MgCl2 - Mg is a metal, Cl is a non-metal (ionic)
• SO2 - S and O are both nonmetals, molecular
  compound (using covalent bonds to hold atoms
  together).

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Practice

• A) LiBr
• B) SBr2
• C) Li2SO4
• A) ionic
• B) molecular
• C) ionic (polyatomic SO4 is an ion silly)

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Molecular Compounds

• Substances consisting of atoms that are
  covalently bonded
• Covalent bonds
• Made through electron sharing among atoms
• Electrons are not transferred
• Sharing allows substance to achieve stability
  (Noble gas configuration)
• Sharing forms molecules

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Molecules

• Are held together by the attraction of electrons
  of one atom and the nucleus of a second atom
• A single bond forms from a single pair of shared
  electrons
• Two pairs of electrons form a double bond
• Paired electrons have opposite spins and occupy
  less space than a pair of electrons surrounding
  only one atom
• Their bonds are flexible, somewhat like springs

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Ionic compounds
Covalent compounds
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Covalent Compound Properties

• Lower melting and boiling points vs. ionic
• Molecules have no charge, hence do not conduct
  heat or electricity in any state.
• Melting molecular compounds
• Does NOT separate the clusters of atoms within a
  molecule,
• Separates just molecules from each other

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Naming Binary Molecular Compounds

• Molecular compound names include prefixes that
  indicate the number of atoms in the molecule
• They are composed of nonmetal elements

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To name molecular compounds

• Write the name of the least electronegative
  element with a prefix to indicate the number of
  atoms of the element that are present
• Write the name of the most electronegative
  element second with a prefix to indicate the
  number of atoms of the element that are present
• Suffix ide is added to the name of the last
  element

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More rules . . .

• The prefix mono- is NOT written with the first
  word of a compounds name
• Some prefixes are sometimes shortened to make a
  name easier to say

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Number of atoms Prefix Number of atoms Prefix
1 Mono- 6 Hexa-
2 Di- 7 Hepta-
3 Tri- 8 Octa-
4 Tetra- 9 Nona-
5 Penta- 10 Deca-
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Examples

• H2O is dihydrogen monoxide
• NH3
• N2H4
• phosphorus pentachloride
• nitrogen monoxide
• carbon tetrachloride

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What is an Acid?

• An acid is a molecular substance that dissolves
  in water to produce hydrogen ions
• They are molecular compounds that separate into a
  cation and an anion in water (ionic tendency)

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Naming Acids

• Binary acids
• The name begins with the prefix hydro-
• The name is derived from the anion
• The suffix ide should be changed to ic
• Polyatomic acids
• Do not use prefix hydro-
• The suffix ate should be changed to ic
• The suffix ite should be changed to ous

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Examples

• H2Se is hydroselenic acid
• HCl
• HNO3
• H2SO4
• nitric acid
• phosphoric acid
• oxalic acid
• hydroiodic acid

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What are Lewis Dot Structures?

• Use the chemical symbol to represent the nucleus
  and inner energy levels
• Uses dots to represent valence electrons
• Types of bonds
• single bonds share 1 pair of electrons (also
  known as sigma)
• double bonds share 2 pairs of electrons (also
  known as pi)
• triple bonds share 3 pairs of electrons (one
  sigma and two pi bonds)

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Rules for Drawing Lewis Dot Structures

• See handout

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Molecular Shapes

• VSEPR Valence Shell Electron Pair Repulsion
  theory system for predicting molecular shape
  based on the idea that pairs of electrons orient
  themselves as far apart as possible
• Can only really be used with simple molecules

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Terms

• Structural formula indicates the spatial
  arrangement of atoms and bonds within a molecule
• Ligand an atom attached to the central atom
• Unshared pairs pairs of electrons that are not
  involved in covalent bonding, but instead belong
  exclusively to central atom

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ligands unshared pairs Molecular shape
2 0,3 Linear
1-2 Bent
3 0 Trigonal planar

1 Trigonal pyramidal
2 T shaped
4 0 Tetrahedral
1 See-saw
2 Square planar
5 0 Trigonal bipyramidal
1 Square pyramidal
6 0 octahedral
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Linear
Bent
Trigonal pyramidal
Trigonal Planar
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T-shaped
Tetrahedral
see-saw
Square planar
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Trigonal bipyramidal
Square pyramidal
Octahedral
http//chemlab.truman.edu/CHEM121Labs/MolecularMod
eling1.htm
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Polarity

• Electrons are not always shared equally in
  molecules
• Creates a partial charge within the molecule
• Atoms with uneven electronegativities share
  electrons unequally
• The greater the difference, the greater the
  polarity
• Polar having opposite ends one atom attracts
  electrons more strongly than the others
• Nonpolar doesnt have opposite ends electrons
  shared equally among bonding atoms

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Examples

• CO2 Is a symmetrical molecule therefore it is
  nonpolar
• H2O H 2.20, O 3.44
• 3.44 2.20 1.24
• Water molecules are asymmetrical, so the molecule
  is polar covalent, with the electrons
  concentrating around the O atom (higher
  electronegativity)

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Differences in Electronegativity
Difference in Electronegativities Characteristic of Bond
gt 2.1 Mostly ionic (electrons transferred)
2.1 0.4 Polar Covalent (electrons unevenly shared)
lt0.4 Nonpolar Covalent (electrons equally shared)
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Diatomic Molecules

• A diatomic molecule is a molecule formed from two
  identical atoms
• The atoms join together because they are more
  stable that way than if they exist as single
  atoms
• Remember HOFBrINCl
• H2, O2, F2, Br2, I2, N2, and Cl2