Unit 5-K: Chemical Bonding

Essential Questions What types of elements combine to form ionic and covalent compounds? What factors determine the shape of covalent molecules? What is a polar molecule? Can you identify the Lewis Dot Structures for ionic and covalent compounds? How do you determine the type of bonding using electronegativity?

Vocabulary Review Ion-atom with a charge Cation-Positive ion, lost electrons Anion-Negative ion, gained electrons Oxidation number -the charge that represents the number of electrons lost or gained (New) Polyatomic ion -more than one element attached to the charge.

Chemical Bonding Bonds – the attractive force between atoms or ions in a compound. A bond depends upon: The electron configuration (involves valence electrons) Electronegativity Why do elements bond? To achieve a stable electron configuration (8 electrons; noble gas configuration) Octet rule – atoms lose, gain or share electrons to achieve a stable configuration of eight valence electrons To achieve the lowest possible energy state (lowest potential energy)

“NEW” LEWIS DOT STRUCTURES FAMILY SINGLE ELEMENT BONDED ELEMENT IA X X IIA X X IIIA X X IVA X X VA through VIIIA the bonded element structure is the same as the single element

Types of Bonds Ionic – complete transfer of electrons (loses or gains); example: NaCl Nonpolar covalent (also called pure covalent) – equal sharing of electrons between atoms; example: O2 Polar covalent – unequal sharing of electrons between atoms; example: H2O Electronegativity (EN) – indicates how strongly an atom wants to gain an electron (Decreases down, Increase Across)

Ionic Bonding Occurs when electrons are completely transferred from one atom to another. The strongest type of bond. Formed between a metal and a nonmetal.

Sodium Meets Chlorine http://www.beyondbooks.com/psc92/3b.asp

Ionic Bonding - Metals Lose e- to form same stable configuration as Noble Gas in preceding period; forms ions. Na Na+ + 1e- 1s22s22p63s1 1s22s22p6 Same Configuration as: Ne atom 1s22s22p6

Ionic Bonding - Nonmetals Gain e- to form same configuration as Noble Gas at end of the same period; forms ion. Cl + 1e- Cl1- 1s22s22p53s23p5 1s22s22p63s23p6 Same Configuration as: Ar 1s22s22p63s23p6

Ionic Bonding Make sure you understand valence electrons and electron configurations Draw Dot structures of valence electrons Know your oxidation numbers Make sure that positive and negative charges add up to zero! http://wine1.sb.fsu.edu/chm1045/notes/Bonding/Ionic/Bond02.htm

Ionic Bonds Equation showing an ionic bond Metals give up e- Non-metals gain e- Must get 8e- total Na• + F  Na+1 F -1 Na has no dots and F has all 8 dots You must show the charges (will equal 0) One Na+1 ion and one F-1 ion form a formula unit (NaF)

Ionic Bonding The simplest ratio of the ions represented in an ionic compound is called a formula unit We use this because no single particle of an ionic compound exists Total # of e- gained by nonmetals atoms = total # of e- lost by the metal,so overall charge = 0

Ionic Bonding Examples: KBr Potassium bromide (1:1) MgCl2 Magnesium chloride (1:2) Practice: Sodium phosphide, three sodium ions for every phosphide ion Na3P

Ionic Bonding Practice by writing the equations for the following ions: Magnesium & Sulfur Aluminum & Oxygen Magnesium & Iodine Copper & Bromine (note: Copper can be +1 or +2)

Covalent Bonds The result of sharing valence electrons. The shared electrons are part of the complete outer shell of both atoms. Occurs when elements are close together on the periodic table Between nonmetallic elements Molecule-formed when two or more atoms bond covalently.

Nonpolar covalent (also called pure covalent) – equal sharing of electrons between atoms; example: O2 Polar covalent – unequal sharing of electrons between atoms; example: H2O

Covalent Bonds Can exist as gases, liquids, or solids depending on molecular mass or polarity Usually have lower MP and BP than ionic compounds Do not usually dissolve in water Do not conduct electricity

TYPES OF COVALENT BONDS NONPOLAR COVALENT e- are equally shared No difference in electronegativity All diatomic molecules are nonpolar covalent H2,I2,O2,Cl2,N2,Br2,F2

TYPES OF COVALENT BONDS POLAR COVALENT unequal sharing E- spends more time with the more electronegative atom Difference in EN = 0.1-1.7 regions of partial charges also known as a dipole (two poles) + - d d Partial positive Partial negative H Cl pulled more by chlorine

Covalent Bonds Two Hydrogen Atoms (H2) http://web.jjay.cuny.edu/~acarpi/NSC/5-bonds.htm

Polar Covalent Bonds Unequal sharing of electrons Have poles (dipoles) – regions that are positive & regions that are negative Electrons are pulling toward more electronegative element Symbols: δ+ δ- : show regions of partial charge : arrow points to more electronegative element

Nonpolar Covalent Electrons are equally shared No difference in electronegativity (ex. diatomic molecules) Also, can be due to shape of molecule Electrons pulled equally in all directions, polar effect cancels (ex. I Be I )

Predicting Bond Type Look at table of EN values and subtract the values for the 2 atoms involved in the bond take the absolute value If EN difference is… 0 – 0.4  nonpolar covalent bond 0.5 – 1.6  polar covalent bond >1.7  ionic bond

DIFFERENCE IN ELECTRONEGATIVITY 0 = NONPOLAR COVALENT >0 TO 1.6 = POLAR COVALENT 1.7 TO 3.4 = IONIC POLAR COVALENT IONIC 0 1.7 3.4 NONPOLAR

To Determine Molecular Shape Use VSEPR (valence shell electron pair repulsion) rules: 1) Draw the Lewis dot structure for the molecule 2) Identify the central atom 3) Count total # of electron pairs around the central atom 4) Count # of bonding pairs of electrons around the central atom 5) Count # of lone pairs of electrons around the central atom 6) Look at summary chart, identify shape

# of e- pairs around central atom Molecular Shape # of e- pairs around central atom # of bonding pairs of e- # of lone pairs of e- Name Shape 2 linear 3 trigonal planar 4 tetrahedral 1 trigonal pyramidal angular (bent)

Practice Determine the shape: BCl3 2. CH4

Polarity TO DETERMINE THE POLARITY OF A MOLECULE, not a bond, you must know the type of bond and the shape. POLAR MOLECULES must meet 2 criteria: Must have a polar covalent bond (EN difference between 0.5 and 1.6) AND Must have an asymmetrical shape: trigonal pyramidal, angular, or 2 element linear If both criteria are not met, it is not a polar molecule, it is either a nonpolar molecule or an ionic compound.

Practice Label the following MOLECULES as polar or nonpolar. 1. NH3 2. CH4 3. HCl

Multiple Covalent Bonds Atoms of some elements attain a noble-gas configuration by sharing more than one pair of electrons between two atoms When writing structural formulas a line can represent a pair of shared electrons

Multiple Covalent Bonds Double Bond: share 2 pairs of e- O=O Triple Bond: share 3 pairs of e- N N

Covalent Bonds Exceptions to Octet Rule Beryllium (2 valence e-, full with 4 valence e-) BeI2 Aluminum (3 valence e-, full with 6 valence e-) AlCl3 Boron (3 valence e-, full with 6 valence e-) BH3

More Exceptions to the Octet Rule Odd number of valence e-, cannot form an octet around each atom NO2 Compounds can form with fewer than 8 e- (rare) BH3 Expanded octet – can hold more than 8 valence e- due to empty d orbital S and P most common elements SF6 or PCl5

Expanded Octets Some atoms bond so they have more than 8e- in the outer level Occur only around a central nonmetallic atom from period 3 or higher PCl5 SF6 http://www.wou.edu/las/physci/poston/ch222/VSEPR-Geometries.htm

EXAMPLES FOR FAMILY VA PCl5 PCl3 Cl Cl Cl P P Cl Cl Cl Cl Cl

EXAMPLES FOR FAMILY VIA SCl6 SCl2 Cl Cl Cl S S Cl Cl Cl Cl Cl

Property Ionic Bonds Covalent Bonds Electrons are: Transferred Shared Difference in Electronegativity: 0-nonpolar >0 to 1.7-polar ≥1.8 A metal and nonmetal Bond between: 2 nonmetals State at room temperature Solid Usually gas (can be solid or liquid) Particle Name: Formula Unit Molecule High Low Melting Point: Conducts Electricity? Yes No Dissolves in water? Yes No (usually) Yes No Flammable Forms a more stable configuration? Yes Yes Examples: NaCl, MgI2 NH3, CHCl3

Metallic Bonds Metals often form lattices in the solid state similar to ionic crystals Even though metal atoms have at least one valence e- they do not share or lose electrons Electron sea model – all the metal atoms in a metallic solid contribute their valence e- to form a “sea” of e-

Metallic Bonds Positive ions in a “sea of electrons,” belongs to the crystal as a whole Force applied (left to right) highlighted cation unchanged. Explains ease of deformation of metals http://cwx.prenhall.com/petrucci/medialib/media_portfolio/12.html