CHAPTER OUTLINE Electronegativity Polarity & Electronegativity Lewis Structures Molecular Shapes

Least electronegative ELECTRONEGATIVITY Linus Pauling derived a relative Electronegativity Scale based on Bond Energies. Electronegativity (E.N.) is the ability of an atom involved in a covalent bond to attract the bonding electrons to itself. F 4.0 Cs 0.7 Most electronegative Least electronegative

Electronegativity increases

BOND POLARITY & ELECTRONEGATIVITY Polarity is a measure of the inequality in the sharing of bonding electrons The more different the electronegativity of the elements forming the bond The larger the electronegativity difference (EN) The more polar the bond formed

POLARITY & ELECTRONEGATIVITY As difference in electronegativity increases Bond polarity increases Most polar Least polar

POLARITY & ELECTRONEGATIVITY difference Bond Type EN = 0 Non-polar covalent 0 < EN <1.7 Polar covalent 1.7 < EN Ionic

The molecule is nonpolar covalent POLARITY & ELECTRONEGATIVITY The molecule is nonpolar covalent H Hydrogen Molecule Electronegativity 2.20 EN = 0

The molecule is polar covalent POLARITY & ELECTRONEGATIVITY The molecule is polar covalent H Cl Hydrogen Chloride Molecule + - Electronegativity 2.20 3.16 EN = 0.96

POLARITY & ELECTRONEGATIVITY No molecule exists The bond is ionic Sodium Chloride Na+ Cl- Electronegativity 0.93 3.16 EN = 2.23

(similar electronegativities) (small to moderate EN) SUMMARY OF BONDING Ionic Bond (large EN) EN > 1.7 Non-polar (similar electronegativities) EN = 0 Polar (moderate EN) Covalent Bond (small to moderate EN) 0 < EN < 1.7

COMPARING PROPERTIES OF IONIC & COVALENT COMPOUNDS Structural Unit Ions Molecules Melting Point High Low Boiling Point Solubility in H2O Low or None Electrical Cond. None Examples NaCl, AgBr H2, H2O

LEWIS STRUCTURES In Lewis symbols, valence electrons for each element are shown as a dot. Lewis structures use Lewis symbols to show valence electrons in molecules and ions of compounds. Lewis symbols for the first 3 periods of representative elements are shown below:

LEWIS STRUCTURES In a Lewis structure, a shared electron pair is indicated by two dots between the atoms, or by a dash connecting them. Unshared pairs of valence electrons (called lone pairs) are shown as belonging to individual atoms or ions.

LEWIS STRUCTURES Writing correct Lewis structures for covalent compounds requires an understanding of the number of bonds normally formed by common nonmetals.

LEWIS STRUCTURES Structures must satisfy octet rule (8 electrons around each atom). Hydrogen is one of the few exceptions and forms a doublet (2 electrons). Covalent molecules are best represented with electron-dot or Lewis structures.

LEWIS STRUCTURES Non-bonding electrons must be displayed as dots. Bonding electrons can be displayed by a dashed line.

LEWIS STRUCTURES More complex Lewis structures can be drawn by following a stepwise method: 1. Count the number of electrons in the structure. 2. Draw a skeleton structure. 3. Connect atoms by bonds (dashes or dots). 4. Distribute electrons to achieve Octet rule. 5. Form multiple bonds if necessary.

Example 1: H O H Write Lewis structure for H2O Step 1: H2O = 8 electrons 2 (1) + 6 = 8 Step 2:   H O H Step 3: Skeleton structure should be symmetrical Hydrogen has doublet 4 electrons used 4 electrons remaining Octet rule is satisfied Step 4:

Example 2: O C O Write Lewis structure for CO2 Step 1: CO2 = 16 electrons 4 + 2(6) = 16 Step 2:     O C O   Step 3:       Skeleton structure should be symmetrical Step 4: Octet rule is NOT satisfied 10 electrons used 6 electrons remaining 4 electrons used 12 electrons remaining Octet rule is satisfied Step 5:

Octet rule is satisfied Octet rule is NOT satisfied Example 3: Write Lewis structure for CO32- Step 1: CO32- = 24 electrons 4+3(6)+2 = 24 Step 2: 6 electrons remaining 12 electrons remaining 0 electrons remaining 18 electrons remaining     O C O O Step 3:     Step 4: Step 5: Octet rule is satisfied Octet rule is NOT satisfied

Octet rule is satisfied Example 4: Write Lewis structure for NH3 Step 1: NH3 = 8 electrons 5 + 3(1) = 8 Step 2:   H N H H Step 3: Step 4: Octet rule is satisfied

Octet rule is satisfied Example 5: Write Lewis structure for ClO3 Step 1: ClO3 = 26 electrons 7+3(6)+1 = 26 Step 2: 0 electrons remaining 20 electrons remaining 8 electrons remaining 2 electrons remaining 14 electrons remaining       O Cl O O Step 3:     Step 4: Octet rule is satisfied

EVALUATING LEWIS STRUCTURES When evaluating Lewis structures for correctness, two points must be considered: Are the correct number of electrons present in the structure? Is octet rule satisfied for all elements? (Hydrogen is an exception)

Structure is incorrect Example 1: Determine if each of the following Lewis structures are correct or incorrect. If incorrect, rewrite the correct structure. 2(1) + 4 + 6 = 12 Octet is incomplete Octet is complete Doublets are complete Structure is incorrect

Structure has 14 electrons Structure is incorrect Example 2: Determine if each of the following Lewis structures are correct or incorrect. If incorrect, rewrite the correct structure. Structure has 14 electrons Only 12 electrons shown 2(5) + 4(1) = 14 2 4 2 Structure is incorrect 2 2 Octets are complete

MOLECULAR SHAPES A very simple model , VSEPR (Valence Shell Electron Pair Repulsion) Theory, has been developed by chemists to predict the shape of large molecules based on their Lewis structures. The three-dimensional shape of the molecules is an important feature in understanding their properties and interactions. All binary molecules have a linear shape since they only contain two atoms. More complex molecules can have various shapes (linear, bent, etc.) and need to be predicted based on their Lewis structures.

MOLECULAR SHAPES Based on VSEPR, the electron pair groups in a molecule will repel one another and seek to minimize their repulsion by arranging themselves around the central atom as far apart as possible. Electron pair groups can be defined as any one of the following: bonding pairs non-bonding pairs multiple bonds

SUMMARY OF VSEPR SHAPES Number of electron pair groups around central atom Molecular Shape Bond Angle Examples Bonding Non-bonding 2 Linear 180 CO2 3 Trigonal planar 120 BF3 1 Bent SO2 4 Tetrahedral 109.5 CH4 Pyramidal NH3 H2O