Download Chapter 8. Chemical Bonds

Chapter 8. Chemical Bonds
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Why Chemical Bonds (bonding rule)
Electronegativity
Ionic Bonds, ionic compounds, properties
Covalent Bonds, covalent compounds
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Nonpolar & polar bonds
Lewis dot structure
VSEPR and molecular shapes (geometry)
Hydrogen bond
Disclaimer: This material is only a study-aid; it is not comprehensive
and may have many gaps that can only be filled by attending the class.
Lewis Dot Symbols (Ch. 5, pp. 137–9)
• The valence electrons (outer electrons) in an
atom can be represented with dots in the Lewis
dot symbol. Each outer electron is
represented by a dot around the atomic symbol
Octet Rule
When there are “0” or “8” electrons in the outer shell
of an atomic center, it has the noble gas configuration
and the maximum stability
(exception: octet rule for H: 2 electrons, i.e. He-like)
Bonding Rule
The objective of all atoms is to achieve an Octet
by
• EITHER: Losing and gaining electrons to form Ionic Compound
• OR: Sharing Electrons to form Covalent Compound
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Ionic Bond
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Negative charges are attracted to positive charges
Negative anions are attracted to positive cations
The result is an ionic bond
Typical ionic compounds contain IA or IIA cations
Charge neutrality (electroneutrality) Æ correct formula
Typical ionic compounds: solids, brittle, high melting
point
Covalent Bond
• Through sharing electron pair(s) between two
atoms
• Lewis Dot Structures: distribution of valence
electrons in a covalent molecule
– Observing Octet rule
• Bonding pair; Lone pair
• Bond order: number of electron pairs shared
– Single bond: one pair
– Double bond: two pairs
– Triple bond: three pairs
• Resonance structures
Electronegativity (χ)
• The ability of an atom to attract electrons
when forming chemical bond.
• Periodic trend (excluding VIIIA):
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Period: left Æ right, _________
Group: top Æ bottom, ________
most electronegative, where?
most electropositive, where?
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Electronegativities of Elements
Insert Figure 8.6
Figure 8.6, p.220
Polar and Nonpolar Bonds
• Nonpolar: equal sharing of electron pair(s)
– A-A
• Polar: unequal sharing of electron pair(s)
– A-B, χA ≠ χB
– Larger the difference between χA and χB, more polar
• Notation of polar bonds
Metallic Bonding (p.223, self study)
Hydrogen Bonding (sec. 14, pp242-3)
• Intermolecular bonds of A-H …A-H type
– Directional; strong
• A is either F, O, or N.
• H-bonding responsible for
– high boiling point of water
– DNA double helix
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Electrolytes & NonNon-electrolyte
• When ionic compounds are dissolved in
water, they dissociate to form aqueous ions:
• NaCl(s) H2O
Na+(aq) + Cl-(aq)
• The resulting solution conducts electricity
and is called an electrolyte.
• Non-electrolyte: strongly covalent
compounds that do not dissociate in water,
and the resultant solution is not conducting
Secs.
Secs. 8 -13: Molecular Shapes
• Valence Shell Electron Pair Repulsion Theory
(VSEPR)
– Valence electron pairs around the central atom stay as far
apart as possible
– Consider both lone pairs and bonding pairs
– Double and triple bonds are treated as a single pair
• Idealized arrangement (electron pairs)
– Two pairs: ____________
– Three pairs: ____________
– Four pairs: _____________
• Molecular shape: Atomic arrangement
• Shape and polarity
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