chapter 10 molecular structure: solids and liquids electron configurations and dot formulas review
TRANSCRIPT
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Chapter 10 Molecular Structure: Solids and Liquids
Electron Configurations and Dot Formulas
Review
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Electron-Dot Formulas
Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings
Why look at Electron-Dot Formulas?
Ionic Compounds: Helps to determine formulas
Covalent Compounds: Help us to understand molecular
structures or molecular geometries,
and molecular properties.
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Preview of Molecular Geometries or Shapes
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Electron-Dot Symbols
Electron-dot symbols show
• the valence electrons of an atom
• one to four valence electrons as single dots on the sides of an atomic symbol
• Five to eight valence electrons with one or more pairs of dots on the sides of an atomic symbol
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Valence Electrons in Some Electron-Dot Symbols
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Guide to Writing Electron-Dot Formulas
STEP 1 Determine the arrangement of atoms.
STEP 2 Add the valence electrons from all the atoms.
STEP 3 Attach the central atom to each bonded atom
using one pair of electrons.
STEP 4 Add remaining electrons as lone pairs to
complete octets (2 for H atoms).
STEP 5 If octets are not complete, form one or more
multiple bonds.
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Multiple Covalent Bonds
In a single bond • One pair of electrons is shared.
In a double bond, • Two pairs of electrons are shared.
In a triple bond.• Three pairs of electrons are shared.
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Electron-Dot Formulas
Electron-dot formulas show
• The order of bonded atoms in a covalent compound.
• The bonding pairs of electrons between atoms.
• The unshared (lone, non-bonding) valence electrons.
• A central atom with an octet.
Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Number of Covalent BondsThe number of covalent bonds can be determinedfrom the number of electrons needed to completean octet.
Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Electron-Dot Formula of SF2
Write the electron-dot formula for SF2.
STEP 1 Determine the atom arrangement.
S is the central atom.F S F
STEP 2 Total the valence electrons for 1S and 2F.
1S(6e-) + 2F(7e-) = 20e-
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Electron-Dot Formula SF2
STEP 3 Attach F atoms to S with one electron pair. F : S : F
Calculate the remaining electrons.20e- - 4 e- = 16e- left
STEP 4 Complete the octets of all atoms by placing remaining e- as 8 lone pairs to complete
octets.
: F : S : F : or : F─S─F :
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Electron-Dot Formula ClO3-
Write the electron-dot formula for ClO3− .
STEP 1 Determine atom arrangement. Cl is the central atom. O −
O Cl O
STEP 2 Add all the valence electrons for 1Cl and 3O plus 1e- for negative charge on the ion.
1Cl(7e-) + 3 O(6e-) + 1e− = 26e-
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Electron-Dot Formula ClO3-
STEP 3 Attach each O atom to Cl with one electron pair.
O −
O : Cl : O
Calculate the remaining electrons.
26e- - 6 e- = 20e- left
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Electron-Dot Formula ClO3-
STEP 4 Complete the octets of all atoms by placing the remaining 20 e- as 10 lone pairs to complete
octets. − −
:O: :O: │
: O : Cl : O : or : O─Cl─O :
Note: Bonding electrons can be shown by a
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Remember: Multiple Bonds
In a single bond • One pair of electrons is shared.
In a double bond, • Two pairs of electrons are shared.
In a triple bond.• Three pairs of electrons are shared.
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Electron-Dot Formula of CS2
Write the electron-dot formula for CS2.
STEP 1 Determine the atom arrangement. The C
atom is the central atom. S C S
STEP 2 Total the valence electrons for 1C and 2S.
1C(4e-) + 2S(6e-) = 16e-
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Electron-Dot Formula CS2
STEP 3 Attach each S atom to C with electron pairs.
S : C : S
Calculate the remaining electrons. 16e- - 4 e- = 12e- left
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Electron-Dot Formula CS2
STEP 4 Attach 12 remaining electrons as 6 lone pairs to complete octets. .. .. : S : C : S : .. ..
STEP 5 To complete octets, move two lone pairs between C and S atoms to give two double
bonds.
.. .. .. .. : S : : C : : S : or : S = C = S :
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Multiple Bonds in N2
In nitrogen N2,
• Octets are achieved by sharing three pairs of electrons, which is a triple bond.
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Some Electron-Dot Formulas
Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings
2(1) + 6 = 8
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Electronegativity and Polarity
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Electronegativity• is the relative ability of atoms to attract shared
electrons • is higher for nonmetals, with fluorine as the
highest with a value of 4.0 • is lower for metals, with cesium and francium as
the lowest with a value of 0.7 • increases from left to right going across a period
on the periodic table• decreases going down a group on the periodic
table
Electronegativity
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Some Electronegativity Values for Group A Elements
Low values
High values
Electronegativity increases
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A nonpolar covalent bond • Occurs between nonmetals.• Is an equal or almost equal sharing of electrons.• Has almost no electronegativity difference (0.0 to
0.4).
Examples: Atoms Electronegativity Type of Bond
DifferenceN-N 3.0 - 3.0 = 0.0 Nonpolar covalent
Cl-Br 3.0 - 2.8 = 0.2 Nonpolar covalentH-Si 2.1 - 1.8 = 0.3 Nonpolar covalent
Nonpolar Covalent Bonds
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A polar covalent bond • Occurs between nonmetal atoms.• Is an unequal sharing of electrons.• Has a moderate electronegativity difference
(0.5 to 1.7).
Examples: Atoms Electronegativity Type of
Bond DifferenceO-Cl 3.5 - 3.0 = 0.5 Polar covalentCl-C 3.0 - 2.5 = 0.5 Polar covalentO-S 3.5 - 2.5 = 1.0 Polar covalent
Polar Covalent Bonds
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Comparing Nonpolar and Polar Covalent Bonds
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Ionic BondsAn ionic bond • Occurs between metal and nonmetal ions.• Is a result of complete electron transfer.• Has a large electronegativity difference (1.8 or more).
Examples: Atoms Electronegativity Type of Bond
Difference Cl-K 3.0 – 0.8 = 2.2 Ionic
N-Na 3.0 – 0.9 = 2.1 Ionic
S-Cs 2.5 – 0.7 = 1.8 Ionic
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Electronegativity and Bond Types
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Predicting Bond Types
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Use the electronegativity difference to identify the type of bond between the following as:
nonpolar covalent (NP), polar covalent (P), or
ionic (I).
A. K-N 2.2 ionic (I) B. N-O 0.5 polar covalent (P)
C. Cl-Cl 0.0 nonpolar covalent (NP)
D. H-Cl 0.9 polar covalent (P)
Learning Check
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Determining Molecular Polarity
The polarity of a molecule is determined from its• electron-dot formula• shape • polarity of the bonds• dipole cancellation
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Polar Bonds and Polar Molecules
In water, the molecule is not linear and the bond dipoles do not cancel each other.Therefore, water is a polar molecule.
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Polar Molecules
A polar molecule • Contains polar bonds.
• Has a separation of positive and negative charge called a dipole indicated with + and - or arrow.
• Has dipoles that do not cancel.
+ - • •
H–Cl H—N—H dipole │
H dipoles do not cancel
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Nonpolar Molecules
A nonpolar molecule • Contains nonpolar bonds.
Cl–Cl H–H
• Or has a symmetrical arrangement of polar bonds.
O=C=O Cl │
Cl –C–Cl │
Cl
dipoles cancel
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Polar Bonds and Nonpolar Molecules
For example, the bond dipoles in CO2 cancel each other because CO2 is linear.
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Attractive Forces Between
Particles
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Ionic Bonds
In ionic compounds, ionic bonds • Are strong attractive forces.• Hold positive and negative ions together.
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Dipole-Dipole Attractions
In covalent compounds, polar molecules
• exert attractive forces called dipole-dipole attractions
• form strong dipole attractions called hydrogen bonds between hydrogen atoms bonded to F, O, or N, and other atoms that are strongly electronegative
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Dipole-Dipole Attractions
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Dispersion Forces
Dispersion forces are• weak attractions between nonpolar molecules• caused by temporary dipoles that develop
when electrons are not distributed equally
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Comparison of Bonding and Attractive Forces
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Melting Points and Attractive Forces
• Ionic bonds require large amounts of energy to break apart. Ionic compounds have very high melting points.
• Hydrogen bonds are the strongest type of dipole-dipole attractions. They require more energy to break than other dipole attractions. Compounds with hydrogen bonds have moderate melting points.
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Melting Points and Attractive Forces (continued)
• Dipole-dipole attractions are weaker than hydrogen bonds, but stronger than dispersion forces. They have low to moderate melting points.
• Dispersion forces are weak and little energy is needed to break them. Compounds with dispersion forces have the lowest melting points.