Why do atoms form bonds? To attain a noble gas configuration How do atoms form bonds? By gaining, losing, or sharing electrons Gain or loss of electrons  ionic bonding Sharing of electrons  covalent bonding

Covalent Bonding Results from: electrostatic attraction between nucleus (protons) one atom & electrons of neighbor’s atom

electrons are shared, not transferred bond formed between 2 nonmetal atoms - sometimes two atoms of same element If attractions are > than repulsions: bond is formed

Different ways of representing a covalent bond Compounds with covalent bonds are molecular!

Recognizing covalent formulas Covalent bonding occurs between nonmetal atoms so… –formulas contain only nonmetals

Identify Bond Type from Formula ZnO N2O5N2O5N2O5N2O5Al CH 3 OH Al 2 O 3 CaBr 2 AuAg CO 2 Li 3 N Na 2 S Mg CsF H2OH2OH2OH2ONaCl SO 2 Cu CH 4 CovalentCovalent CovalentCovalent Covalent CovalentIonic Ionic Ionic IonicIonicIonic Ionic Metallic Metallic Metallic MetallicMetallic

Making a bond Liberates energy! Separate Atoms Molecule

A + B  AB A and B are both atoms AB is a molecule in equation: –no bonds broken; one bond formed –energy is released reaction is exothermic

A + B  AB bond is formed energy is released AB has less PE than A + B AB is more stable than A + B

Breaking a Bond Absorbs Energy!

CD  C + D CD is molecule; C and D are atoms in equation: –one bond broken –no bonds formed reaction is endothermic C & D have higher PE than CD

Structure of Covalent Compounds form molecules more reactants have, the more molecules can make

Representations of Molecules Space-filling model Ball-and-stick model Ball-and-stick and electron cloud

Representations of Molecules Lewis Structures of compound H 2 = Molecular Formula H-H = Structural Formula [- represents 1 pair of shared electrons ]

Halogens Share 1 pair (2 electrons): form single covalent bond

Rules for drawing Lewis Diagrams 1.Arrange symbols on paper the way think atoms are arranged - Diatomics easy – next to each other - Hydrogens always terminal (on outside) - Atom with least attraction (lowest electronegativity) for shared electrons placed in center 2.Add up # valence electrons all atoms start with single bonds (electron pairs) between all atoms 3. Distribute electrons, start with single bonds (electron pairs) between all atoms 4. Test validity (2 tests)

Tests for Lewis Structures Must Pass Both! 1.# dots = # valence electrons found in rule 2 2.Every atom has octet of electrons around it (except H: needs 2) Bonding electrons get counted 2 times – once for each atom sharing them

Assessing Lewis Diagrams If dot structure passes both tests, you’re finished If fails one or both tests, try again If single bonds don’t work - try multiple bonds –Single bond = 2 electrons shared –Double bond = 4 electrons shared –Triple bond = 6 electrons shared

Try some examples H 2, F 2, HF, O 2, N 2 Step 1: Draw symbols the way think atoms are arranged –Diatomics easy – atoms next to each other!

Drawing Lewis Diagrams H H Step 2: Count up the valence electrons valence electrons Each H has 1 valence electron so the total = 2 Step 3: Distribute the valence electrons, starting with single bonds between all atoms :

Testing Lewis Diagrams H H : Test 1: 2 dots in diagram = 2 valence electrons Test 2: Each H has 2 valence electrons (Remember H only wants 2)

Testing Lewis Diagrams F F Step 1 Step 2 2 X 7 = 14 valence electrons Step 3 F F : ::

Try HF H F Step 1 Step = 8 valence electrons Step 3 H F : :....

Types of Covalent Bonds Single: 2 atoms share 1 pair of electrons Double: 2 atoms share 2 pairs of electrons Triple: 2 atoms share 3 pairs of electrons

O O Step 1: Step 2: 2 X 6 = 12 valence electrons Step 3: O O : :: Step 4: Test failed No Good! Try O 2

Try O 2 Again! O O Step 1: Step 2: 2 X 6 =12 Step 3: Distribute electrons (single bond between atoms didn't work so try a double bond) :: Step 4: Test!

Try N 2 N N Step 1: Step 2: 2 X 5 = 10 electrons Step 3: Distribute electrons starting with a single bond between the nitrogen atoms :.. :.. : Step 4: Test No Good!

Try N 2 with a double bond N N :: : :.. No Good!

Try N 2 with a triple bond N N ::::: Step 4: Test