Electronegativity and Polarity

Electronegativity: Attracting Electrons When two atoms form a bond, each atom attracts the other atom’s electrons in addition to its own. Electronegativity of an atom is a measure of an atom’s ability to attract electrons in a chemical bond. As you move from left to right on the periodic table, the EN increases. As you move from top to bottom on the periodic table, the EN decreases.

Nonpolar Covalent Bond When electrons are shared between 2 atoms, a covalent bond is formed. If the atoms are identical, e.g. Cl2, the electrons are shared equally (nonpolar)

Polar Covalent Bond If the electrons are shared between 2 different atoms, e.g. HBr, the sharing is unequal The bonding electrons spend more time near the more electronegative atom H Br

Electronegativity Values

Electronegativity Differences The absolute value of the difference in electronegativities of two bonded atoms provides a measure of polarity of a bond. The greater the difference, the more polar the bond.

The ionic range is from 1.7 to 3.3. The covalent range is from 0 to 0.5. The polar covalent range is from 0.5 to 1.7.

Predicting Bond Type Using EN You can use the differenced between EN to decide whether the bond between two atoms is ionic or covalent. ΔEN = larger EN – smaller EN For example, K-F EN for K is 0.8 EN for F is 4.0 4.0 – 0.8 = 3.2 According to the diagram, KF is ionic.

What type of bond is N-O? EN for N is 3.0 EN for O is 3.5 3.5 – 3.0 = 0.5 According to the diagram, the bond is mostly covalent.

Polar Covalent Bonds (The in-between bonds) When two bonding atoms have an EN difference that is greater than 0.5, but less than 1.7, they are considered to be polar covalent bonds. The difference is not great enough for the more EN atom to take the electrons from the less EN atom. The difference is great enough though, for the bonding electrons to spend more time near the more EN atom.

Look at the H-O bond in water. EN for O is 3.5 EN for H is 2.1 Difference is 1.4 and this falls in the polar covalent region.

Which bond is more polar? B – Cl or C – Cl? The difference in electronegativities between chlorine and boron is 1.0 (3.0 – 2.0 = 1.0). The difference in electronegativities between chlorine and carbon is 0.5 (3.0 – 2.5 = 0.5). The B – Cl bond is more polar and the chlorine atom will hold the partial negative charge because of its higher electronegativity.

Which bond is more polar? P – F or P – Cl? The difference in electronegativities between fluorine and phosphorus is 1.9 (4.0 – 2.1 = 1.9). The difference in electronegativities between chlorine and phosphorus is 0.9 (3.0 – 2.1 = 0.9). The P – F bond is more polar and the fluorine atom will hold the partial negative charge because of its higher electronegativity.

Polar Molecules Note: Not all molecules with polar bonds are polar molecules

Molecular Shapes and Molecular Polarity Recall: Bond polarity is a measure of how equally the electrons in a bond are shared between two atoms. As the difference in electronegativity between two atoms increases, so does bond polarity.

In molecules containing two atoms, a dipole exists if there is a difference in electronegativity. For a molecule with more than two atoms, the dipole moment depends on both the polarities of the individual bonds and the geometry of the molecule.

Consider linear CO2 Each C=O bond is polar, but since the bonds are identical, the bond dipoles are equal in magnitude. The overall dipole moment = 0.

Bond dipoles and dipole moments are vector quantities, which means they have magnitude and direction. The overall dipole moment of a molecule is the sum of its bond dipoles. Consider the bent molecule, water, with two polar bonds.

Both bonds are identical so the bond dipoles are equal, but since the molecule is bent, the bonds do not directly oppose each other. Therefore, the bond dipoles do not cancel each other out. The water molecule has a non zero dipole moment so it is polar. The oxygen carries a partial negative charge and the hydrogen atoms each carry partial positive charges.

Comparing Models

Drawing Lewis Structures Arrange the element symbols. Central atoms are generally those with the highest bonding capacity. Carbon atoms are always central atoms Hydrogen atoms are always peripheral atoms Add up the number of valence electrons from all atoms. Add one electron for each negative charge and subtract one for each positive charge. Draw a skeleton structure with atoms attached by single bonds. Complete the octets of peripheral atoms. Hydrogen will not have any lone pairs! Place extra electrons on the central atom. If the central atom doesn’t have an octet, try forming multiple bonds by moving lone pairs. 23

Structural Formula 7. From the Lewis structure, remove dots representing lone pairs 8. Replace bond dots with a dash H can only accommodate two electrons H and O are common exceptions to rule 2 Organic compounds are not compact nor symmetrical. 24

Water Hydrogen (1) + Hydrogen (1) + Oxygen (6) = 8

SAMPLE PROBLEM A) Writing Lewis Structures for Molecules with One Central Atom PROBLEM: Write a Lewis structure for CCl2F2, one of the compounds responsible for the depletion of stratospheric ozone. SOLUTION: Cl Step 1: Carbon has the highest bonding capacity and is the central atom. The other atoms are placed around it. Cl C F F Steps 2-4: C has 4 valence e-, Cl and F each have 7. The sum is 4 + 4(7) = 32 valence e-. : C Cl F : : Make bonds and fill in remaining valence electrons placing 8e- around each atom. : 27

Ammonia (NH3) Nitrogen (5) and Hydrogen 3(1) = 8

Drawing Lewis Structures · · Cl C Cl O · · · · COCl2 24 ve’s · · · · · · · · · · · · · · · · HOCl 14 ve’s H O Cl · · · · · ·  · · O Cl O O · · · · · · · · ClO3 26 ve’s · · · · · · · · · · H C O H H · · CH3OH 14 ve’s · · 31

H : H C O H : H SAMPLE PROBLEM B) Writing Lewis Structure for Molecules with More than One Central Atom PROBLEM: Write the Lewis structure for methanol (molecular formula CH4O), an important industrial alcohol that is being used as a gasoline alternative in car engines. SOLUTION: Hydrogen can have only one bond so C and O must be next to each other with H filling in the bonds. There are 4(1) + 4 + 6 = 14 valence e-. C has 4 bonds and O has 2. O has 2 pair of nonbonding e-. H : H C O H : H 32

Multiple Bonds So far we have only looked at single bonds. The sharing of a pair of electrons constitutes a single bond. In many molecules atoms attain complete octets by sharing more than one pair of electrons. When two electron pairs (4 electrons) are shared, two lines are drawn to represent the double bond. A triple bond corresponds to sharing of three pairs of electrons (6 electrons).

: C C H H : : : SAMPLE PROBLEM C) Writing Lewis Structures for Molecules with Multiple Bonds. PROBLEM: Write Lewis structures for the following: (a) Ethylene (C2H4), the most important reactant in the manufacture of polymers (b) Nitrogen (N2), the most abundant atmospheric gas PLAN: For molecules with multiple bonds, there is a Step 5 which follows the other steps in Lewis structure construction. If a central atom does not have 8e-, an octet, then e- can be moved in to form a multiple bond. SOLUTION: (a) There are 2(4) + 4(1) = 12 valence e-. H can have only one bond per atom. C H : C H (b) N2 has 2(5) = 10 valence e-. Therefore a triple bond is required to make the octet around each N. N : . N : . N : 34

Polyatomic Ions Many compounds contain a combination of covalent and ionic bonds. E.g. NaOH OH- is a polyatomic ion with a covalent bond between O and H Ionic bond between Na+ and OH-

Coordinate Covalent Bonds A covalent bond in which both of the shared electrons come from the same atom. E.g. NH3 (ammonia) and H+ (hydrogen ion) to form NH4 (ammonium)