Polar Bonds & Molecules

Objectives Describe how electronegativity values determine the distribution of charge in a polar molecule Describe what happens to polar molecules when they are placed between oppositely charged metal plates Evaluate the strength of intermolecular attractions compared with the strength of ionic and covalent bonds Identify the reason why network solids have high melting points

Important Vocabulary Nonpolar covalent bond Polar covalent bond Polar bond Polar molecule Dipole Van der Waals forces Dipole interactions Dispersion forces Hydrogen bonds Network solids

Bond Polarity Electronegativity is used to predict what kind of bond will form between two different atoms Atoms can share electrons equally or unequally due to different electronegativity values The bonding pairs of electrons in covalent bonds are pulled, as in a tug-of-war

Electronegativity Chart

Nonpolar Covalent Bonds When two atoms share electrons equally, a nonpolar covalent bond is formed Molecules of hydrogen (H2), oxygen (O2), and nitrogen (N2) have nonpolar covalent bonds Diatomic ________ molecules are also nonpolar

Polar Covalent Bonds When two atoms share electrons unequally, a polar covalent bond is formed It is also known as a polar bond The _____ electronegative atom attracts electrons more strongly and gains a slightly negative charge The ______ electronegative atoms has a slightly positive charge If the difference for electronegativity is greater than or equal to 2.0, an ionic bond will form

Predicting Bond Type

Polarity in Relation to Bond Strength How do you think polarity affects bond strength?

Practice Problems N and H F and F Ca and Cl Al and Cl H and Br Which type of bond will form between each of the following pairs of atoms? N and H F and F Ca and Cl Al and Cl H and Br Li and O C and O Cl and F

Partial Charges Having partial charges indicates that the shared pair of electrons will be closer to the atom with the greater electronegativity Thus making one end more negative and the other end more positive They are fractions of charge ranging between 0 and +1.6 x 10-19 or 0 and -1.6 x 10-19 The amount of the partial charge depends on the electronegativity differences between the elements involved

Polar Molecules Have positive and negative ends A molecule in which one end has a partial positive charge and the other end has a partial negative charge is called a dipole When polar molecules are placed between oppositely charged plates, they tend to become oriented with respect to the positive and negative plates For example: HF molecule We emphasize the nature of the dipole using the Greek letter delta to mean partial Hδ+Fδ-

Intramolecular vs. Intermolecular Intramolecular Forces Intermolecular Forces Are forces within ONE molecule Nonpolar covalent Electrons shared equally Polar covalent Electrons shared unequally Ionic Electrons are transferred Are forces between MULTIPLE molecules Van der Waals Forces London Dispersion Forces Hydrogen Bonding Dipole-Dipole

Attractions Between Molecules Molecules can attract each other by a variety of forces Intermolecular attractions are weaker than either ionic or covalent bonds However, these forces are still very important For example, intermolecular attractions help to determine if a molecular compound is a gas, liquid, or solid

Van der Waals Forces Are an umbrella term for intermolecular forces They are named after Dutch chemist Johannes van der Waals (1837-1923) Van der Waals forces consist of strong forces such as dipole-dipole interactions and hydrogen bonds, and weak London dispersion forces

Dipole-Dipole Interactions Occur when polar molecules are attracted to one another The electrical attraction involved occurs between the oppositely charged regions of polar molecules Dipole interactions are similar to ionic bonds but they are much weaker They also include hydrogen bonds

Hydrogen Bonds Are attractive forces in which a hydrogen covalently bonded to a very electronegative atom is also weakly bonded to an unshared electron pair of another electronegative atom Hydrogen bonding always involves hydrogen For example: water

London Dispersion Forces Are the weakest of all Van der Waals forces They occur between non-polar molecules They are believed to result from weak, momentary attractions called instantaneous dipoles An instantaneous dipole is formed when electrons moving in a molecule get “off balance” for an instant so that for an instant partial charges appear. This leads to short-live attractions For example: halogen diatomic molecules attract each other mainly by dispersion forces

Intermolecular Attractions & Molecular Properties The physical properties of a compound depend on the type of bonding it displays

Network Solids Are solids in which all of the atoms are covalently bonded to each other Melting a network solid would require breaking covalent bonds throughout the solid