Sec. 8.5: Electronegativity and Bond Polarity Covalent Bonding Sec. 8.5: Electronegativity and Bond Polarity

Objectives Describe how electronegativity is used to determine bond type. Compare and contrast polar and nonpolar covalent bonds and polar and nonpolar molecules. Describe the characteristics of compounds that are covalently bonded.

The character of the bond formed is determined by electron affinity or electronegativity. Electron affinity measures the tendency of an atom to accept an electron. Electronegativity (ability to attract electrons in a bond) and electronegativity values allow scientists to evaluate affinity.

Pg. 194 in your text

INCREASES DECREASES

Chemical bonds between atoms of different elements are never complete ionic or covalent and the character of the bond depends on how strongly the bonded atoms attract electrons.

The electronegativity difference between H and Cl is 0. 7 The electronegativity difference between H and Cl is 0.7. Because the difference is NOT zero, electrons are NOT shared equally. Generally ionic bonds form when the electronegativity difference greater than 1.70

The shared electron pair is pulled towards Large differences in electronegativity indicate that an electron was transferred from one atom to another, resulting in bonding that is primarily ionic. The shared electron pair is pulled towards the Cl atom. The electrons spend more time with the Cl atom than the H atom.

Pg. 266

Bonds between different atoms are never completely covalent or ionic The bond character is determined by electronegativity difference. As electronegativity difference increases, the bond becomes more ionic in character. At an electronegativity of 1.7, the bond is considered to be 50% covalent and 50% ionic. (see graph pg. 266)

Practice Problems Determine the electronegativity difference between the atoms listed. Indicate what type of bond would form between them. 1. C-Cl 2. N-H 3. O-S 4. Na-Br

Molecular Polarity Polarity of a molecule depends on the polarity of the bonds in the molecule AND the shape of the molecule. Molecules that are polar are dipoles - they have a partial negative charge on one side of the molecule and a partial positive charge on the other side.

CCl4: Polar or not? Each C-Cl bond is polar covalent with an electro- negativity difference of 0.61 The molecular shape is tetrahedral. The bonds are symmetrical. The electric charge measured at any distance from the center is identical on all sides and partial charges are balanced. The molecule is nonpolar.

H2O: Polar or not? Each O-H bond is polar covalent with an electronegativity difference of 1.24. The molecular shape is bent. The polar O-H bonds are NOT symmetrical; there are definite negative and positive ends. The molecule is polar.

Polar or not? If bonds are polar, asymmetrical molecules are polar symmetrical molecules are nonpolar Practice: NH3, CHCl3, SF2, SF4, SF6

Properties of Covalent Compounds Solubility is the ability of a substance to dissolve another substance. Following the phrase “like dissolves like” Polar (or ionic) molecules dissolve in other polar (or ionic) substances. Nonpolar molecules dissolve ONLY in nonpolar substances.

Intermolecular Forces Covalently bonded compounds have weak intermolecular or van der Waals forces. Non-polar molecules exhibit a weak dispersion force, or induced dipole. These forces are the weakest. Between polar molecules, these forces are called dipole-dipole forces. One end of the dipole attracts the oppositely charged end of another dipole. The strength of attraction increases with the polarity of the molecule.

Intermolecular Forces The third type of van der Waals force is called a hydrogen bond. This is an especially strong dipole-dipole force. It is formed between the H end of a dipole and a F, O, or N atom on another dipole.

Properties of Covalent Compounds Many physical properties depend on the strength of intermolecular forces. Molecular compounds have LOW melting & boiling points (compared to ionic compounds) Molecular substances exist mostly as gases OR vaporize easily at room temperature. Molecular substances are SOFT if solid. Solids form crystal lattice structures but have much less attraction between particles than in an ionic crystal.

Covalent Network Solids Solids composed only of atoms interconnected by a network of covalent bonds. Network solids are brittle and extremely hard. They do not conduct electricity or heat. They have very high melting points. Examples are graphite and diamonds – both are networks of C atoms. Graphite Diamond