Chemical Bonding Chapter 6

Chemical Bonding Chemical bond —mutual electrical attraction between the nuclei and valence electrons of different atoms that binds the atoms together. Why do bonds form? stability (lower energy) is achieved when compared to the unbonded states.

Types of Intramolecular Bonds ionic bonds (electrical attraction between ions) {Electron Transfer} metallic bonds (“delocalized” electrons) {Electron Sea} covalent bonds (sharing of electrons) All involve electron interactions. The difference arises in how they interact.

Distinguishing between covalent and ionic bonds: Look at ELECTRONEGATIVITY of the atoms involved. PAGE 151 The DIFFERENCE in the electronegativities of the bonding atoms will tell you what type of bond is formed.

Distinguishing between covalent and ionic bonds cont… Electronegativity Difference Type of Bond 0.00 – 0.39 Nonpolar-covalent 0.40 – 1.7 Polar-covalent 1.71 – 4.00 Ionic

Hydrochloric Acid

Distinguishing between covalent and ionic bonds cont… Polar bonds —arise when electron density (orbitals) are NOT evenly distributed (electrons are held closer to atom with the LARGER electronegativity) partially positive partially negative

Distinguishing between covalent and ionic bonds cont… Nonpolar bonds —arise when atoms have similar electronegativities (electrons evenly distributed between atoms) Nonpolar Bond Cl Cl Electronegativity Difference 3.0 - 3.0 = 0 Polar Bond Electronegativity Difference l 2.1 - 3.0 l = 0.9 H Cl

Octet Rule Compounds tend to form so that each atom has eight valence electrons (noble gas configuration) gain, lose, or share electrons

Octet Rule Exceptions: H, Be, B, and some nonmetallic elements in 3rd period in these elements’ families Hydrogen needs 2 valence electrons total Beryllium needs 4 valence electrons total Boron needs 6 valence electrons total

Octet Rule Cont… Some need more than 8 valence electrons and are known as expanded octets because they are not full until they have 10 e- or even 12 e- Elements that are expanded octet can be satisfied with only 8 e- in certain molecules. most common: Sulfur (12) may do this Phosphorus (10) may do this Halogens (10 & 12) may do this Noble gases (10 & 12) may do this

Electron-dot (Lewis) Structures Cont… We Use Electron Dot diagrams for elements by telling the number of valence electrons Draw the Dot Diagram for Elements Nitrogen Hydrogen N H Bonding Site Bonding Site Bonding Site Nitrogen needs 3 electrons to be full Hydrogen only needs 2 electrons to be full, so it needs 1 more

Electron-dot (Lewis) Structures We only use Electron Dot Diagrams for covalent compounds , ions or single elements. Rules for Drawing Uncombined Elements: show VALENCE electrons (use periodic table) and do NOT need to follow the octet rule Monatomic Ions: Use Straight line brackets with the charge outside. Cations = lose electrons Anions = gain electrons Covalent Compounds & Polyatomic Ions: Use the following slides of information and follow all steps exactly. Follow Octet Rule

Lewis Dots for Elements = one valence electron v Symbol of Element 1 5 8 X 2 You must place one Electron (dot) into each position, before you pair. (Top,Right,Bottom,Left) 4 6 7 3

Cations -vs- Anions CATIONS Example: K+1 ANIONS Example: O -2

Electron-dot (Lewis) Structures Cont… When bonding: H is never central atom (O is rarely) The number of electrons EACH atom needs to complete its octet is the number it needs to SHARE with other atoms 2 shared electrons (1 pair) = 1 bond Bonds are indicated with a Unshared electrons do NOT go between atoms NOT counting the shared electrons, each atom should still have its correct number of valence electrons*

Electron-dot (Lewis) Structures Cont… Multiple Bonds Double bond 4 e- shared O2 Triple bond  6 e- shared N2 Quadruple bond  8 e- shared C2

Step 1: Count the number of valence electrons. Steps to Follow… Step 1: Count the number of valence electrons. Step 2: Draw a structure with single bonds joining the atoms to the central atom. Central atoms are the atom that can make the most bonds usually (or they are the single atom in the compound).

Steps to Follow… Step 3: Place lone pairs (unshared pairs) around each element to make them follow the octet rule. (Don’t forget the exceptions.) Step 4: Count the electrons in the newly made structure. If they match it is probably the correct structure…so use it.

Steps to Follow… If there are TOO MANY electrons around the new structure, take off 2 sets of lone pairs (one off each atom) and make a double bond or more until the totals match. If there are NOT ENOUGH electrons then add lone pairs to the central atom until the totals match.

Polyatomic Ions: put entire structure in a [ ] charge is superscript outside brackets

Resonance Structure Some molecules and ions need to be represented by more than one Lewis structure. Ex. O3 Actual structure: average of all structures used to represent resonance structures NOTE: substances with odd number of valence electrons are stable because multiple resonance structures can be drawn to show different bonding configurations.

Resonance Structure Cont… When can you usually draw resonance structures? at least one double bond present, along with single bonds between similar atoms

Electron-dot (Lewis) Structures Cont… Occasionally, one atom must contribute BOTH electrons to make a bond coordinate covalent bonds IDENTICAL to regular covalent bonds Indicated with an