Nature of Covalent Bonding

Objectives Describe how electrons are shared to form covalent bonds and identify exceptions to the octet rule Demonstrate how electron dot structures represent shared electrons Describe how atoms form double or triple covalent bonds Distinguish between a covalent bond and a coordinate covalent bond and describe how the strength of a covalent bond is related to its bond dissociation energy Describe how oxygen atoms are bonded in ozone

Important Vocabulary Coordinate covalent bond Polyatomic ion Single covalent bond Structural formula Unshared pair Double covalent bond Triple covalent bond Coordinate covalent bond Polyatomic ion Bond dissociation energy Resonance structure

Octet Rule in Covalent Bonding In forming covalent bonds, electron sharing usually occurs so that atoms attain the electron configurations of noble gases Combinations of atoms of nonmetallic elements in groups 4A, 5A, 6A, & 7A are likely to form covalent bonds For example, hydrogen has 1 electron Thus it will form a covalent bond with itself to share 2 electrons and attain the noble gas configuration of helium

Single Covalent Bonds Form when two atoms are held together by sharing a pair of electrons Of the diatomic molecules: hydrogen, chlorine, fluorine, bromine & iodine form single covalent bonds An electron dot structure such as H:H represents the shared pair of electrons of the covalent bond by two dots The pair of shared electrons forming the covalent bond is also represented as a dash H‒H

Single Covalent Bonds

Structural Formulas Represent the covalent bonds by dashes Shows the arrangement of covalently bonded atoms

Unshared Pair An unshared pair is a pair of valence electrons that are not shared between atoms It is also called a lone pair of a nonbonding pair For example: in the water molecule oxygen has 2 unshared pairs of electrons and forms 2 single covalent bonds

Unshared Pair Examples Identify the unshared pairs in the pictures below

Drawing Lewis Structure for Simple Molecules Step 1: draw the symbols for the molecule side by side, leaving space for the dots Step 2: place two dots, one on top of the other, between the two symbols to represent the single covalent bond Step 3: Add remaining number electrons for both symbols!

Examples H Cl H-F

Practice Problems #1 F I H-Cl

Multiple Bonds Atoms can share more than one pair of electrons in a covalent bond Atoms form double or triple covalent bonds if they can attain a noble gas structure by sharing two or three pairs of electrons Sharing two pairs of electrons results in a ___________________ Sharing three pairs of electrons results in a ___________________

Double Covalent Bonds

Triple Covalent Bonds

Drawing Lewis Structures with Multiple Elements Draw a Lewis structure for each atom in the compound Determine the number of valence electrons in the compound Arrange the Lewis structure to show how the atoms bond in the molecule Halogen and hydrogen atoms often bind to only one other atom and are usually at the end of the molecule Carbon is often placed in the center of the molecule The atom with the lowest electronegativity is usually placed in the center

Drawing Lewis Structures with Multiple Elements Distribute the dots so that each atom except for hydrogen, beryllium, and boron, satisfies the octet rule Change each pair of dots that represents a shared pair of electrons to a long dash Count the number of electrons surrounding each atom making sure you satisfy the octet rule for each one (including hydrogen, beryllium, & boron) and that you have the same number of valence electrons from step 1

Examples CH3I H2S CH2Cl2

Practice Problems #2 SCl2 SiH4 H2O CHF3

Coordinate Covalent Bonds Is a covalent bond in which one atom contributes both bonding electrons In a coordinate covalent bond, the shared electron pair comes from one of the bonding atoms For example: carbon monoxide (CO)

Polyatomic Ions A polyatomic ion is a group of 2 or more atoms joined by covalent bonds that has a positive or negative charge and behaves as a unit Most polyatomic cations and anions contain both covalent and coordinate covalent bonds Compounds containing polyatomic ions also include both _____________________

Ammonium Ion Formation Forms when a positively charged hydrogen ion (H+) attaches to the unshared electron pair of an ammonia molecule (NH3)

Drawing Lewis Structures for Polyatomic Ions Is different in that we must account for the charges on them For example: SO32- Step 2 Step 1 Step 4 Step 3

Examples ClO3‒ H3O+

Practice Problems #3 NO3- PO43- NH4+ CO32-

Bond Dissociation Energy Is the energy required to break a bond between two covalently bonded atoms The ___________ the bond the ______ energy is required to break it Bond energy is measured in ____________ Examples: H2 requires 435 kJ/mol to break C‒C requires 347 kJ/mol

Resonance Some molecules can not be represented by a single Lewis structure Resonance structures are two or more possible configurations for the same compound that differ in the arrangement of electrons For example: ozone, O3 The actual bonding of oxygen atoms in ozone is a hybrid, or mixture, of the extremes represented by the resonance forms

Exceptions to the Octet Rule The octet rule cannot be satisfied in molecules whose total number of valence electrons is an _________ There are also molecules in which an atom has fewer, or more, than a complete octet of valence electrons For example: nitrogen dioxide has a total of 17 valence electrons

Exceptions to the Octet Rule Several molecules with an even number of electrons fail to follow the octet rule For example: boron trifluoride (BF3) The boron atom within this compound is deficient by two electrons

Exceptions to the Octet Rule A few atoms, especially phosphorus and sulfur, can expand the octet to include 10 or 12 electrons