Electronegativity and Polarity

Electronegativity and Polarity 1 What is electronegativity? • Electronegativity: the ability of an atom to attract bonding electrons to itself. 2 Electronegativity Trend • Electronegativity increases as you go from left to right • Electronegativity increases as you go from bottom to top 3 Electronegativity Trend 4 Electronegativity Chart You will be given these values on the test/ exam 5 Electronegativity Difference • Electronegativity difference (ΔEN) is the difference in electronegativities of two bonded atoms or ions. Example: The electronegativity difference for N-H: ΔEN of N-H = 3.0 – 2.1 = 0.9 6 Practice Problems What are the ΔEN of the following bonds: 1. C-H = 2. O-H = 3. H-H = 4. Ca-F = 7 Practice Problems What are the ΔEN of the following bonds: 1. C-H = 2.5-2.1 = 0.4 2. O-H = 3.5 – 2.1 = 1.4 3. H-H = 2.1 – 2.1 = 0 4. Ca-F = 1.0 – 4.0 = 3.0 8 Electronegativity in Bonds • Electronegativity can create localized charges in a molecule. These localized regions are called “poles” (negative poles and positive poles) • Negative poles occur around the atoms with the higher electronegativity, because it tends to pull the electrons towards itself. • Positive poles occur around the atoms with lower electronegativity, because they are do not attract the bonding electrons to themselves as much 9 Electronegativity in Bonds Bonds Covalent Bonds Ionic Bonds Polar covalent bond 0.5-1.6 Non-polar covalent bond 0-0.4 10 Electronegativity in Bonds 0.5 1.6 11 Molecular Polarity • Polar molecule: a molecule in which there is an uneven distribution of electrons. This results in a positive charge at one end and a negative charge at the other end. • Non-polar molecule: a molecule in which the electrons are equally distributed among the atoms, and therefore no localized charges. • Dipole moment: contains a magnitude and direction in which charges are distributed. This is responsible for negative poles and positive poles 12 Determining Polarity To figure out if a molecule is polar or non-polar, you must: 1. Draw it’s structural formula 2. Look at the electronegativity differences of the individual bonds in the molecule You try! • Is HCl polar or non-polar? • Is CO2 polar or non-polar? 13 Is HCl polar or non-polar? Is CO2 polar or non-polar? 14 Is HCl polar or non-polar? HCl is polar because there is unequal sharing of electrons with an electronegativity difference of 0.9 Is CO2 polar or non-polar? CO2 is non-polar because there is equal sharing of electrons because both oxygen atoms are pulling at carbon’s electrons equally 15 Polarity & Symmetry •Even if the individual bonds in a molecule are polar bonds, the overall molecule can be non-polar if it is symmetrical. •If the molecule has two lines of symmetry, then the compound is non-polar The C-O bond has an electronegativity difference of 0.8 but the molecule is non-polar because there are two oxygen on either end making the compound symmetrical 16 Polarity & Symmetry CO2 is non-polar, but what about H2O? They both have a central element bonded to two other atoms of the same element. They are both symmetrical. But what is different? 17 Polarity & Symmetry CO2 is non-polar, but what about H2O? CO2 has two lines of symmetry, but H2O only has one, so H2O is polar 18 Polarity & Lone Pairs • Lone pairs can affect the magnitude of the dipole moment since they are more localized. • As a result, the central atom has a high electron density, affecting the polarity of the molecule. • Therefore, water is a polar molecule 19