Covalent Bonding …electrons are shared

Terms Valence electrons - in the outer shell/orbital Nonmetals – on the right side of the p.t. Noble gases – group 18, have 8 valence e - Valence electrons

Covalent bonds Nonmetals hold onto their valence electrons. They can’t give away electrons to bond. Still want noble gas configuration. Get it by sharing valence electrons with each other. By sharing both atoms get to count the electrons toward noble gas configuration.

Covalent Bonding In a covalent bond the electrons are the “glue” that holds the atoms together. Only nonmetals and Hydrogen. Different from an ionic bond because they actually form molecules. G.N. Lewis

Covalent Bonding: Hydrogen H + H  H : H The two electrons are shared evenly between the two hydrogen atoms. It is as if each atom has two electrons – the noble gas configuration of [He]. H : H

Lewis Structures A Lewis structure is a way of drawing a molecule that shows all valence electrons as dots or lines that represent valence electrons. eg. The Lewis structure for H 2 can be drawn in two ways: H : HorH–H A single line represents two covalently shared electrons – also known as a single bond.

Lewis Structures The Lewis Structure for F 2 : Two electrons are shared between the two F atoms (one single covalent bond). Each F atom also has three unshared electron pairs. These non-bonding electron pairs are called lone pairs.

Single Covalent Bond A sharing of two valence electrons. Two specific atoms are joined.

The Octet Rule Note that by sharing electrons, it is as if each F atom has eight electrons - the noble gas configuration. The Octet Rule: Main group elements with more than two valence electrons gain, lose, or share electrons to achieve a noble gas configuration characterized by eight valence electrons.

Covalent bonding Fluorine has seven valence electrons F A second atom also has seven F By sharing electrons …both end with full orbitals 8 Valence electrons 8 Valence electrons

How to show how they formed It’s like a jigsaw puzzle. You have to know what the final formula is. You put the pieces together to end up with the right formula. For example- show how water is formed with covalent bonds.

Water H 2 O H O Each hydrogen has 1 valence electron Each hydrogen wants 1 more The oxygen has 6 valence electrons The oxygen wants 2 more They share to make each other happy

Put the pieces together The first hydrogen is happy The oxygen still wants one more HO

Water The second hydrogen attaches Every atom has full energy levels A pair of electrons is a single bond HO H H HO

Multiple Bonds Sometimes atoms share more than one pair of valence electrons. A double bond is when atoms share two pair (4) of electrons. A triple bond is when atoms share three pair (6) of electrons.

Carbon dioxide CO 2 Carbon is central atom –Carbon has 4 valence electrons –Wants 4 more Oxygen has 6 valence electrons –Wants 2 more OC

Carbon dioxide Attaching 1 oxygen leaves the oxygen 1 short and the carbon 3 short O C

Carbon dioxide l Attaching the second oxygen leaves both oxygen 1 short and the carbon 2 short O C O

l The only solution is to share more l Requires two double bonds l Each atom gets to count all the atoms in the bond 8 valence electrons Carbon dioxide O CO

Lewis Structures

How to draw them Add up all the valence electrons. Count up the total number of electrons needed to make all atoms have 8. Subtract. Divide by 2 This tells you how many bonds to draw. Fill in the rest of the valence electrons to fill atoms up.

Tips for Lewis Structures Group 14 almost always goes in the center Hydrogen and halogens always go on the outside. Hydrogen and halogens only form 1 bond Group 15 always has 1 lone pair Group 16 always has 2 lone pairs Group 17 (halogens) always has 3 lone pairs

Examples NH 3 –N - has 5 valence electrons wants 8 H - has 1 valence electrons wants 2 NH 3 has 5+(3*1) = 8 NH 3 wants 8+(3*2) = 14 –(14-8)/2= 3 bonds 4 atoms with 3 bonds N H

NHH H Examples Draw in the bonds All 8 electrons are accounted for Everything is full

Examples HCN C is central atom –C - has 4 valence electrons wants 8 –N - has 5 valence electrons wants 8 –H - has 1 valence electrons wants 2 HCN has = 10 HCN wants = 18 –(18-10)/2= 4 bonds 3 atoms with 4 bonds -will require multiple bonds (not to H)

HCN Put in single bonds Need 2 more bonds Must go between C and N NHC

HCN l Put in single bonds l Need 2 more bonds l Must go between C and N l Uses 8 electrons - 2 more to add NHC

HCN l Put in single bonds l Need 2 more bonds l Must go between C and N l Uses 8 electrons - 2 more to add l Must go on N to fill octet NHC

Another way of indicating bonds Often use a line to indicate a bond Called a structural formula Each line is 2 valence electrons HHO  HHO

Structural Examples H CN C O H H C has 8 electrons because each line is 2 electrons Ditto for N Ditto for C here Ditto for O

Draw a Lewis structure for methane (CH 4 ) that obeys the octet rule.

Draw a valid Lewis structure for formaldehyde (CH 2 O).

Draw a Lewis structure for Sulfur Dichloride (SCl 2 ).

Draw a Lewis structure for Phosphorus Trifluoride (PF 3 ).