Ch. 8 Covalent Bonding Pre AP Chemistry

I. Molecular Compounds  A. Molecules & Molecular Formulas  1. Another way that atoms can combine is by sharing electrons.  2. Bonds formed by sharing electrons are called covalent bonds.  3. A neutral group of atoms joined together by covalent bonds are called molecules.  4. Atoms of the same element that are bonded covalently are called diatomic molecules.  5. Compounds composed of molecules are called molecular compounds and are represented by molecular formulas.

I. Molecular Compounds B. Comparing Molecular & Ionic Compounds  1. Ionic compounds have high electronegativity differences while molecular compounds have small differences in electronegativity.  2. Ionic compounds tend to be crystalline solids with high melting points and are nonconductive, however, in the molten (liquid) state, ionic compounds will conduct electricity.  3.Molecular compounds are mostly liquids and gases at room temperature and do not conduct electricity.  4. Ionic compounds are soluble in water while molecular compounds are not soluble in water.

II. The Nature of Covalent Bonding A. The Octet Rule in Covalent Bonding  In covalent bonds, electron sharing usually occurs so that the atoms attain a noble gas configuration.  For all atoms that form covalent bonds, except for hydrogen, eight electrons represents a full outer level.  In a Lewis dot structure, the outer shell electrons are arranged as dots around the element symbol.  There are two types of electron pairs: a. Shared pairs - electrons involved in bonding. b. Unshared pairs - electrons not involved in bonding.  A single shared pair form a single bond.  Pairs of unshared electrons are also called lone pairs.  In order to describe the shape of a molecule, you need to draw a Lewis dot structure.

II. The Nature of Covalent Bonding A. The Octet Rule in Covalent Bonding 8.Atoms sometimes share more than one pair of electrons to attain noble gas configuration. 9.There are two types of bonds: a. Double bonds. b. Triple bonds. 10.Double bonds contain two shared pairs of electrons. 11.Triple bonds contain three shared pairs of electrons. 12.Double bonds consist of four electrons in between the bonded atoms. 13.Triple bonds consist of six electrons in between the bonded atoms. 14.Double and triple bond compounds are more reactive than compounds with only single bonds.

II. The Nature of Covalent Bonding B. Coordinate Covalent Bonds 1.It is possible for atoms to achieve the noble gas configuration by type of bonding called coordinate covalent bonding. 2.A covalent bond in which one atoms contributes both bonding electrons is a coordinate covalent bond. 3.In a coordinate covalent bond, the shared electron pair comes from one of the bonding atoms. 4.This type of bonding is evident in polyatomic ions which are a tightly bound group of atoms that has a positive or negative charge and behave as a unit.

II. The Nature of Covalent Bonding C. Exception to the Octet Rule 1.For some molecules or ions, it is impossible to draw structures that satisfy the octet rule. 2.The octet rule cannot be satisfied in molecule whose total number of valence electrons is an odd number. 3.There are also molecules in which an atom has less, or more, than a complete octet of valence electrons. 4.An unpaired electron that is left over or an atom that does not aquire less than an octet rule example of not meeting the octet rule.

II. The Nature of Covalent Bonding D. Bond Dissociation Energies 1. The total energy required to break the bond between two covalently bonded atoms is known as bond dissociation energies. 2. Compounds with only single C-H and C-C bonds are quite unreactive because the dissiociation energies are high.

II. The Nature of Covalent Bonding E. Resonance 1. Resonance structures are structures that occur when it is possible to write two or more valid electron dot formula that have the same number of electrons pairs for a molecule or ion. 2. Electron pairs do not resonate back and forth between the different structures but are actually a hybrid.

III. Bonding Theories A. Molecular Orbitals 1.When two atoms combine, this model assumes that the atomic orbitals overlap to produce molecular orbitals, which are orbitals that apply to entire molecule. 2.There are two types of bonds: a. sigma bonds. b. pi bonds. 3.When two orbitals (s or p) form a bond that lies directly along the axis, it is called a sigma bond ( s ). 4.When p orbitals overlaps, they tend to be sideways (parallel), they form a pi bond ( π ) Pi bonds hybridize to sp 2 orbitals.

III. Bonding Theories B. VSEPR Theory 1.The behavior of molecules depends on their structural characteristics and electron configurations. 2.There are two ways in which structure can account for the shape of a molecule:  repulsive forces  atomic orbital overlap.  A molecular shape considers the different ways s and p orbitals can overlap when electrons are shared.  The resulting electron cloud will occupy more space than a single bond.  Thus, the bond angle between the other atoms will decrease to allow room for the multiple bond.

III. Bonding Theories B. VSEPR Theory 6.Electrons carry negative charges. 7.Electron forces spread as far apart as possible to minimize repulsive forces. 8.The repulsion between two unshared pairs is greater than the repulsion of an unshared pair and a shared pair. 9.The repulsion between an unshared pair and a shared pair is greater than the repulsion of two shared pairs. 10.There are six common shapes a. Linear triatomic b. Bent triatomic c. Trigonal planar d. Pyramidal e. Tetrahedral f. Trigonal bipyramidal

III. Bonding Theories C. Hybrid Orbitals 1.VSPER theory works well when accounting for molecular shapes, but it does not help much in describing the types of bonds formed. 2.Orbital hybridization provides information about both molecule bonding and molecular shape. 3.Several atomic orbitals mix to form the same total number of equivalent hybrid orbitals. 4.When an s orbital and p orbital hybridize, they form four identical sp 3 orbitals which form single bonds. 5.When a 2s orbital and two 2p orbitals combine, they form two sp 2 orbitals that form double (π) pi bonds. 6.When a 2s orbital and a 2p orbital combine, they form one sp orbital which form triple π pi bonds.

IV. Polar Bonds and Molecules A. Bond Polarity 1.An atom’s electronegativity is its ability to attract the electrons involved in bonding. 2.Because of the differences in electronegativity of elements, in a covalent bond between different elements, one atom attracts the shared pair of electrons much more than the other atom. 3.This type of molecule is polar covalent. 4.When the two atoms share the electrons equally, the bond is a nonpolar covalent bond. 5.The presence of a polar bond in a molecule often makes the entire molecule. 6. Because polar molecule have a positive and negative pole, a polar molecule is called a dipole ( or has a dipole moment). 7.The dipole moment is the measure of the strength of the dipole and is the result of the asymmetrical charge distribution in a polar molecule.

IV. Polar Bonds and Molecules B. Attractions Between Molecules 1.Weak forces are involved in the attraction of electrons and protons of atoms. 2.Intermolecular forces account for a wide range of properties among covalent molecules. 3.These forces are called Van der Waals forces. 4.These forces are weaker than chemical bonds. 5.There are several types of Van der Waals forces: a. Dipole - Dipole forces b. Induced dipole forces c. Dispersion forces.

IV. Polar Bonds and Molecules B. Attractions Between Molecules 6.Dipole - dipole forces are when two molecules of the same of different substances that are both permanent dipoles and are attracted to each other. 7.When a dipole approaches a molecule that is not a dipole, it induces a dipole in the molecule and the molecule is now attracted to the dipole. 8.Dipoles occur in polar covalent molecules with higher boiling and melting points than molecules of the same size. 9.Dispersion forces occur when a nonpolar molecule forms a temporary dipole and induces a dipole in another molecule a. They are also called London Forces. b. Dispersion forces occur in nonpolar substances with low melting and boiling points.

IV. Polar Bonds and Molecules B. Attractions Between Molecules 10. Liquids that have hydrogen in the molecular structure experience hydrogen bonding. 11. This occurs when the hydrogen of one of the molecules is attracted to another atom on another molecule with unshared pair of electrons. 12. The attractive force of the hydrogen bond is not as strong as a chemical bond.

IV. Polar Bonds and Molecules C. Intermolecular Attractions and Molecular Properties 1.The physical properties of a compound depend on the type of bonding it displays — whether it is ionic or covalent. 2. The melting and boiling points of most molecular compounds are low. 3.However, a few molecular solids do not melt until the temperature reaches 1000°C or higher or they decompose without melting. 4.These are called network solids or network crystals and are very stable.