Polar Bonds & Molecules

Objectives Describe how electronegativity values determine the distribution of charge in a polar molecule 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 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 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 Identify the reason why network solids have high melting points

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

Bond Polarity Electronegativity is used to predict what kind of bond will form between two different atoms 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 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 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 When two atoms share electrons equally, a nonpolar covalent bond is formed Molecules of hydrogen (H 2 ), oxygen (O 2 ), and nitrogen (N 2 ) have nonpolar covalent bonds Molecules of hydrogen (H 2 ), oxygen (O 2 ), and nitrogen (N 2 ) have nonpolar covalent bonds Diatomic halogen molecules are also nonpolar Diatomic halogen molecules are also nonpolar

Polar Covalent Bonds When two atoms share electrons unequally, a polar covalent bond is formed When two atoms share electrons unequally, a polar covalent bond is formed It is also known as a polar bond It is also known as a polar bond The more electronegative atom attracts electrons more strongly and gains a slightly negative charge The more electronegative atom attracts electrons more strongly and gains a slightly negative charge The less electronegative atoms has a slightly positive charge The less 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 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? How do you think polarity affects bond strength? The greater the difference between the electronegativity values of two elements joined by a bond, the greater the polarity of that bond The greater the difference between the electronegativity values of two elements joined by a bond, the greater the polarity of that bond The greater the polarity of a bond, the stronger the bond as well The greater the polarity of a bond, the stronger the bond as well

Practice Problems Which type of bond will form between each of the following pairs of atoms? A.N and H B.F and F C.Ca and Cl D.Al and Cl E.H and Br F.Li and O G.C and O H.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 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 Thus making one end more negative and the other end more positive They are fractions of charge ranging between 0 and +1.6 x or 0 and -1.6 x They are fractions of charge ranging between 0 and +1.6 x or 0 and -1.6 x The amount of the partial charge depends on the electronegativity differences between the elements involved The amount of the partial charge depends on the electronegativity differences between the elements involved

Polar Molecules Have positive and negative ends 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 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 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 For example: HF molecule We emphasize the nature of the dipole using the Greek letter delta to mean partial We emphasize the nature of the dipole using the Greek letter delta to mean partial H δ+ F δ- H δ+ F δ-

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

Attractions Between Molecules Molecules can attract each other by a variety of forces Molecules can attract each other by a variety of forces Intermolecular attractions are weaker than either ionic or covalent bonds Intermolecular attractions are weaker than either ionic or covalent bonds However, these forces are still very important However, these forces are still very important For example, intermolecular attractions help to determine if a molecular compound is a gas, liquid, or solid 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 Are an umbrella term for intermolecular forces They are named after Dutch chemist Johannes van der Waals ( ) They are named after Dutch chemist Johannes van der Waals ( ) Van der Waals forces consist of strong forces such as dipole-dipole interactions and hydrogen bonds, and weak London dispersion forces 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 Occur when polar molecules are attracted to one another The electrical attraction involved occurs between the oppositely charged regions of polar molecules 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 Dipole interactions are similar to ionic bonds but they are much weaker They also include hydrogen bonds 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 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 Hydrogen bonding always involves hydrogen For example: water For example: water

London Dispersion Forces Are the weakest of all Van der Waals forces Are the weakest of all Van der Waals forces They occur between non-polar molecules They occur between non-polar molecules They are believed to result from weak, momentary attractions called instantaneous dipoles 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. 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 This leads to short-live attractions For example: halogen diatomic molecules attract each other mainly by dispersion forces 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