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Polarity

Polarity. Explain Polarity Theoretical Prediction of Molecular Polarity. Let’s Clarify Some Terms. Covalent Bond?? Polar Covalent Bond?? Non Polar Bond?? Ionic Bond?? Let’s go with our text found on page 252. Please read from page 251-255. Review The Terminologies.

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Polarity

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  1. Polarity Explain Polarity Theoretical Prediction of Molecular Polarity

  2. Let’s Clarify Some Terms • Covalent Bond?? • Polar Covalent Bond?? • Non Polar Bond?? • Ionic Bond?? Let’s go with our text found on page 252. Please read from page 251-255

  3. Review The Terminologies • Covalent Bond – Chemical bond formed between two atoms by “equal sharing” ofelectrons: non-polar • Polar Covalent Bond – electrons spend more closer to one atom than the other: “unequal sharing” of electrons. • Ionic Bond – when electronegativity difference is greater than or equal to 1.7 • Electronegativity – a measure of the ability of an atom in a molecule todraw bonding electrons to itself.

  4. Think Continuum percent ionic character IonicCovalent 100% 0% 3.3 1.7 0 electronegativity difference Ionic BondPolar Covalent Bond

  5. Polarity of Individual Bonds • The polarity of an individual bond is determined by calculating the absolute difference in electronegativity of two atoms. • For example: HCl , • From the Periodic Table the electronegativity of H = 2.1, Cl = 3.0 • We use the symbol δ+ or δ- to indicate the electronegativity of an individual atom. • The + or – is assigned relative to the bonded atom. • Difference in electronegativity of HCl = │ 2.1 – 3.0 │ is 0.9, we said that HCl is polar.

  6. Evaluating Polarity of a Molecule • Theoretical Prediction of Molecular Polarity: • Step 1: Draw Lewis Structure for the molecule • Step 2: Determine VSEPR shape • Step 3: Use electronegativities to determine the polarity of each bond. (calculate △δ) • Step 4: Add the vectors to determine if the final result is zero or non-zero. If △δ=0 then it’s non-polar, if △δ>0 then we say it is polar.

  7. Example • NH3 • Step1: Draw Lewis structure • Step 2: VSEPR – said it is Trigonal Pyramidal (3bonds 1 lone pair) • Step 3: Find △δ = ?? • △δ = 0.9 for each H-N bonds • Step 4: Add vectors

  8. Quicker Way Estimates • Draw arrows in the direction that points toward the most electronegative atom • Ask yourself: are all the arrows skew towards one side? Or are they evenly distributed in opposite directions. Keep in mind the size of the arrows by using the periodic table trends Hint: Flourine. • Decision Time: Skewed = Polar Evenly Distributed in opposite directions = nonpolar.

  9. Few Trends

  10. Chapter 4 Homework Questions • Lewis Structure Questions • P.229 # 10, 12 • P.230 # 4e (structural diagram = VSEPR shape) Valence Bond Theory / Hybridization p.235 #8, 9, 10, 11 p. 238 #18 p.232 # 1-3 p.234 # 5a,b

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