Lecture 44 Electronegativity and Polarity Ozgur Unal NIS – CHEMISTRY Lecture 44 Electronegativity and Polarity Ozgur Unal

Electron Affinity and Electronegativity Electron affinity is a measure of the tendency of an atom to accept an electron. Electronegativity indicates the relative ability of an atom to attract electrons in a chemical bond. Check out Figure 8.20 for electronegativities. A chemical bond between atoms of different elements is never completely ionic or covalent. The difference between electronegativities of atoms forming a bond indicates the character of the bond. Check out Table 8.7 for the bond character for a given electronegativity difference.

Electronegativity and Bond Character Molecules of the same atoms share the electrons in the covalent bond equally  Non polar covalent bond Example: N2, H2, O2, CO2 Some molecules made up of different elements do not share the electrons in the covalent bond equally  Polar covalent bond Example: H2O, NH3 Bonding is not often clearly ionic or covalent. An electronegativity (EN) difference of 1.70 is considered 50% ionic and 50% covalent. As the difference in the EN increases, the bond becomes more ionic in character.

Polar Covalent Bonds Polar covalent bonds form because not all atoms that share electrons attract them equally. When a polar bond forms, the shared electron pair or pairs are pulled toward one of the atoms. Thus the electrons spend more time atound that atom. The Greek letter delta (δ) is used to represent a partial charge. δ-  partial negative charge δ+  partial positive charge The more electronegative atom is at the partially negative end, while the more electronegative atom is at teh partially positive end. The resulting polar bond is called a dipole.

Polar Covalent Bonds Nonpolar molecules such as H2 or CO2 are not attracted by an electric field. Polar molecules align with an Polarity and Molecular Shape: The shape of a molecule is important in determining the polarity of the molecule. Example: H2O and NH3 are polar CO2 and CCl4 are nonpolar Check out Figure 8.23

Polar Covalent Bonds Solubility of Polar Molecules: The physical property known as solubility is the ability of a substance to dissolve in another substance. Example: Sugar and salt dissolve in water well. Polar molecules and ionic compounds are ususally soluble in polar substances. But nonpolar molecules dissolve only in nonpolar substances.

Properties of Covalent Compounds Intermolecular Forces: In a covalent compound, the covalent bonds between atoms in molecules are strong, but the attraction forces between molecules are relatively weak. The weak attraction forces are known as intermolecular forces, or van der Waals forces. There are different types of intermolecular forces: Dispersion force  between nonpolar molecules Dipole-dipole force  between polar molecules Hydrogen bond  between the H end of the dipole and a F, O or N atom on another dipole

Properties of Covalent Compounds Forces and Properties: The properties of molecular compounds are related to the intermolecular forces holding the molecules together. Low melting and boiling points compared to ionic compounds These weak intermolecular forces explain why many molecular substances exist as gases (O2, CO2, H2S etc) Many covalent molecules are relatively soft solids.