Covalent Bonding

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Covalent Bonding

• Chemistry 5th Edition
• McMurry/Fay

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Electronegativity
? (Ei Eea)/2 Usually converted to a unitless
number between 0 and 4. Low ? ? form cation High
? ? form anion
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Electronegativity

• Bond polarity is due to electronegativity
  differences between atoms.
• Pauling Electronegativity is expressed on a
  scale where F 4.0

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Electronegativity
Pauling Electronegativities
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Electronegativity
Predicting ? 1. Increases across periods 2.
Decreases down groups 3. Very low for noble gases.
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Electronegativity

• Ionic Character As a general rule for two
  atoms in a bond, we can calculate an
  electronegativity difference (?EN ) ?EN EN(Y)
  EN(X) for XY bond.
• If ?EN lt 0.5 the bond is covalent.
• If ?EN lt 2.0 the bond is polar covalent.
• If ?EN gt 2.0 the bond is ionic.

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The Covalent Bond

• Covalent bonds are formed by sharing at least one
  pair of electrons.

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The Covalent Bond

• Every covalent bond has a characteristic length
  that leads to maximum stability.
• This is the bond length.

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The Covalent Bond

• Every covalent bond has a characteristic length
  that leads to maximum stability.
• This is the bond length.

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Polar Covalent Bonds

• Using electronegativity values, predict whether
  the following compounds are nonpolar covalent,
  polar covalent, or ionic
• SiCl4 CsBr FeBr3 CH4
• HCl CCl4 NH3 H2O

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Electron-Dot Structures

• Using electron-dot (Lewis) structures, the
  valence electrons in an element are represented
  by dots.
• Valence electrons are those electrons with the
  highest principal quantum number (n). Valence
  electrons normally occupy s and p subshells

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Electron-Dot Structures

• Use the chemical symbol of the element, and add
  dots for each valence electron.
• N ? He 2s22p3
• ?? ? ? ?
• 2s 2p
• N

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Electron-Dot Structures

• Octet The s2p6 configuration. The complete
  octet is the most stable configuration.
• Ar ? Ne 3s23p6
• ?? ?? ?? ??
• 3s 3p
• Ar

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Electron-Dot Structures

• Ion Formation
• Elements may form ions in order to complete
  their octets.
• s-block and the lower left p-block elements will
  lose electrons elements on the right of the
  p-block will gain electrons.

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Electron-Dot Structures

• Ionic Bonds Two atoms form octets for each by
  one donating electron(s) to the other.
• Covalent Bonds A pair of electrons is shared
  between two atoms, giving both an octet.
• (predicted by electronegativity)

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Electron-Dot Structures

• The electron-dot structures provide a simple, but
  useful, way of representing chemical reactions.
• Ionic
• Covalent

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Electron-Dot Structures

• Double Bond Two atoms share two electron pairs,
  again forming octets for each.
• Triple Bond Three electron pairs are shared.

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Electron-Dot Structures

• Single Bonds
• Double Bonds
• Triple Bonds

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Drawing Lewis Structures

• Lewis Structure A diagram showing how electrons
  are shared between atoms in a molecule.
• Octet Rule Atoms proceed as far as possible
  toward completing their octets by sharing
  electron pairs. H needs only 2 electrons (and
  can accept no more).

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Drawing Lewis Structures

• Exceptions to the Octet Rule
• 1. Boron may form an incomplete octet with only
  6 valence electrons.
• 2. Phosphorus may form an extended octet with 10
  valence electrons.
• 3. Sulfur may form an extended octet with 12
  valence electrons.

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Drawing Lewis Structures

• Lone Pair A pair of electrons residing on a
  single element and thus not involved in bonding.
• Bonding Pair A pair of electrons shared in a
  covalent bond. Often represented as a line
  between atoms.
• F F

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Drawing Lewis Structures

• Step 1 Determine the total number of valence
  electrons in the molecule.
• For anions add 1 electron for each negative
  charge. For cations subtract 1 electron for each
  positive charge.
• Step 2 Find the total number of electron pairs
  by dividing by 2.

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Drawing Lewis Structures

• Step 3 Identify the central atom(s) and arrange
  the other atoms around it (them).
• Central atoms are usually C, N, B, P, S
• Step 4 Connect the atoms with single electron
  pairs first, then attempt to complete each octet
  by adding lone pairs or multiple bonds until all
  electrons are used.

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Drawing Lewis Structures