Transcript Chapter 5 Chemical Bonds: The Ties That Bind

Chapter 5
Chemical Bonds: The Ties That
Bind
Chemical Bonds
• Forces responsible for holding together atoms in
molecules and ions in crystals
• Determine shape of molecules
• Predict chemical and physical properties of materials
• Related to arrangement of electrons in compounds
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Stable Electron Configurations
• Valence electrons – outermost level with
electrons
• Core electrons – all other electrons in an
atom
• Isoelectronic – same number of valence
electrons
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Electron-Dot Structures
• Valence electrons represented by dots
• Electron-dot symbols
– Examples: Na•, •Mg•, …
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Sodium Reacts with Chlorine
• Sodium dropped in flask filled with chlorine
gas
– Produces sodium chloride (i.e., table salt)
• Chlorine extracts electron from sodium
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Ionic Bonds
• Na+ and Cl–
• Opposite charges attract
• Ions organize themselves in orderly manner
– Crystal of NaCl
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More Ionic Compounds
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Octet Rule
• Atoms tend to gain or lose electrons to
have eight valence electrons
– Same as noble gases
– He and H are exceptions, get only two
valence electrons
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Naming Ions
• For cations, simple positive ions
– Add the word ion
– Examples: Na+ – sodium ion
Al3+ – aluminum ion
• For anions, simple negative ions
– Change the usual ending to -ide
– Examples: Cl– – chloride
S2– – sulfide
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Naming Binary Ionic Compounds
• Two components in compound
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Covalent Bonds
• Bond formed by a shared pair of electrons
• Gives atom an octet of electrons
– Shared pair of electrons – bonding pair
– Other electrons not involved in bonding –
nonbonding pairs
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Multiple Bonds
• Sometimes atoms need to share more
than one pair of electrons
• Share two pairs – double bond
• Share three pairs – triple bond
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Naming Covalent Compounds
Use prefixes to
indicate the number
of each kind of atom
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Electronegativity
• Measure of the attraction of an atom in a
molecule for a pair of shared electrons
– Fluorine most electronegative element
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Polar Covalent Bonds
• If elements do not have the same
electronegativity, they get unequal
sharing of electrons
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Polyatomic Ions
• Charged groups of atoms that remain together
through most chemical reactions
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Draw Electron-Dot Formula
1. Determine total number of valence
electrons
2. Write skeletal structure and connect
atoms by dashed line
3. Give outer atoms an octet of electrons
4. Subtract number of electrons assigned
5. Add remaining electrons to central
atom
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6. If central atom does not have an octet, form
multiple bonds using nonbonding electrons from
outer atoms
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Number of Covalent Bonds/Element
• Follow the electron-dot rules for the
following elements
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Free Radicals
• Have odd number of electrons
– One unpaired electron
– Tend to be very reactive
• Nitrogen oxides (such as NO, NO2)
common radical
– Key component in smog
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Molecular Shape
• Determine reactive, chemical and
physical properties
– Very important in biological systems
• Must draw electron-dot diagram to
predict shape of molecule
• Look at central atom
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Valence Shell Electron Pair Repulsion
(VSEPR) Theory
• Electron pairs arrange
themselves to minimize
repulsion
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Possible Shapes
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Shapes and Properties
Polar molecule
has separate
centers of
positive and
negative charge
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States of Matter
• 3 phases of matter: solid, liquid, and gas
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Intermolecular Forces
• Glue that holds matter together
• Melting and boiling points measure the
relative strength
• Ionic forces – strongest
– Found in salts
– NaCl melts at 800°C
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Dipole Forces
• Not as strong as ionic forces
• Must have polar molecule
– HCl melts at –112°C
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Hydrogen Bonding
• Hydrogen must be attached to
electronegative atom
– N, O, F
• Plays important role in biological
systems
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Dispersion Forces
• Present in all molecules
• Weak momentary attractive forces
– Arise for electrons moving about in
molecules and atoms
– Strong in larger molecules
– Important in nonpolar compounds
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Solutions
• Solutions are homogeneous
mixtures of two or more
substances
– Homogeneous: thoroughly
mixed, even composition
throughout
• Solute: substance being
dissolved
• Solvent: substance doing the
dissolving
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Forces in Solution
• Like dissolves like
• Nonpolar compounds dissolve best in
nonpolar solvents
– Oil and gasoline mix
• Polar compounds dissolve best in polar
solvents
– Water and HCl mix
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Ionic Compounds in Solution
• Ions dissolve in water
– Charges on ions are attracted to partial charges
on water
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End Of Chapter 5
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