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Covalent Bonding Slide 2 Lesson 1:Covalent Bonding Covalent bonds: atoms held together by sharing electrons. Molecules: neutral group of atoms joined together by covalent bonds. Diatomic molecule: molecule consisting of 2 atoms. Remember them: F 2, Cl 2, I 2, Br 2, H 2, N 2, O 2 Molecules tend to have lower melting and boiling points than ionic compounds. YouTube - Making Molecules with Atoms Slide 3 Molecular Formula Shows how many atoms of each element a molecule contains. Naming binary molecular compounds Composed of two nonmetals; often combine in more than one way. Ex. CO and CO 2 Prefixes are used to name binary molecular compounds. PrefixMono-Di-Tri-Tetra-Penta-Hexa-Hepta-Octa-Nona-Deca- Number 12345678910 Slide 4 Binary Compounds Containing Two Nonmetals To name these compounds: 1)give the name of the less electronegative element first with the Greek prefix indicating the number of atoms of that element present 2) After give the name of the more electronegative non- metal with the Greek prefix indicating the number of atoms of that element present and with its ending replaced by the suffix ide. 3)Do not use the prefix mono- if required for the first element. Slide 5 Binary Molecular Compounds N 2 Odinitrogen monoxide N 2 O 3 dinitrogen trioxide N 2 O 5 dinitrogen pentoxide ICliodine monochloride ICl 3 iodine trichloride SO 2 sulfur dioxide SO 3 sulfur trioxide YouTube - Naming molecular compounds Slide 6 Binary Molecular Compounds Containing Two Nonmetals 1.________________ diarsenic trisulfide 2.________________sulfur dioxide 3. P 2 O 5 ____________________ 4.________________ carbon dioxide 5. N 2 O 5 ____________________ 6. H 2 O____________________ As 2 S 3 SO 2 diphosphorus pentoxide CO 2 dinitrogen pentoxide dihydrogen monoxide Slide 7 Naming Binary Compounds Binary Compound? Metal Present? Does the metal form more than one cation? Molecule Use Greek Prefixes Ionic compound (cation has one charge only) Use the element name for the cation. Ionic compound (cation has more than one charge) Determine the Charge of the cation; use a Roman numeral after the cation name. Yes No Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 98 Slide 8 Classwork #1: Do handout Naming Molecules Slide 9 Lesson 2: The Nature of Covalent Bonding Introduction with balloon activity octet rule: electron sharing occurs usually so that atoms attain the electron configurations of noble gases. Single covalent bond: two atoms held together by sharing a pair of electrons. Shown as two dots or as a long dash. A pair of valence electrons that is not shared between atoms is called an unshared pair. Slide 10 HH O H O H or O H H Slide 11 Double bonds: covalent bond formed by sharing two pairs of electrons Triple bonds: covalent bond formed by sharing three pairs of electrons. Slide 12 hydrogen chlorine iodine nitrogen Covalent bonding with equal sharing of electrons occurs in diatomic molecules formed from one element. A dash may replace a pair of dots. Slide 13 Classwork: introduction to lewis structures. Slide 14 Lesson 3:Molecular Structure Structural formula: uses symbols and bonds to show relative position of atoms. Steps to determine Lewis structures for molecules 1. Predict the location of certain atoms. Hydrogen is always an end atom The least electronegative atom is the central atom (usually the one closer to the left on periodic table) 2. Find the total number of electrons available for bonding. (# of valence electrons of atoms in molecule) 3. Determine the number of bonding pairs by dividing the total number of electron by 2 Slide 15 4. Place one bonding pair (single bond) between central atom and terminal atoms. 5. Subtract pairs used in step 4 from bonding pairs in step 3. Place lone pairs around each terminal atom bonded to the central atom to satisfy the octet rule. Any remaining pairs are assigned to the central atom. 6. If the central atom does not have an octet, convert one or two of the lone pairs on the terminal atoms to a double or a triple bond between central and terminal atom. Some elements like Be, B, Al do not form a complete octet, S and P can have more than 8 valence electrons. Slide 16 Ex. 1 Draw the lewis structure for ammonia, NH 3 1. Hydrogen is an end atom and nitrogen is the central atom. 2. Total number of valence electrons: (1 nitrogen x 5 valence electrons)+ (3 hydrogens x 1 valence electron)= 8 valence electrons. 3. Total number of bonding pairs= 8/2 = 4 4. Draw single bond from each H to N N H H H Slide 17 Ex. 1 Draw the lewis structure for ammonia, NH 3 5. Subtract the number of pairs of electrons used from the total pairs of electrons: 4-3 =1 pair available One lone pair remains, hydrogen can have only one bond, assign the lone pair to the central atom, N. N H H H Slide 18 Ex. 2 Draw the lewis structure for carbon dioxide, CO 2 1. Oxygen atoms are end atoms and carbon is the central atom. 2. Total number of valence electrons: (1 carbon x 4 valence electrons)+ (2 oxygen x 6 valence electron)= 16 valence electrons. 3. Total number of bonding pairs= 16/2 = 8 4. Draw single bond from each C to O C OO Slide 19 Ex. 1 Draw the lewis structure for carbon dioxide, CO 2 5. Subtract the number of pairs of electrons used from the total pairs of electrons: 8-2 =6 pair available Add three pairs of electrons to each oxygen. C OO Slide 20 Ex. 1 Draw the lewis structure for carbon dioxide, CO 2 6. No lone pairs remain for carbon. Carbon does not have an octet, use a lone pair from each oxygen to form a double bond with the carbon atom. C OO C OO Slide 21 CW: lewis structures handout part 1 Slide 22 Lesson :4 Exception to octet rule Some molecules have an odd number of valence electrons and cannot form an octet around each atom. Some molecules form with fewer than eight electrons present around an atom. Ex. Boron Some compounds have central atoms with more than 8 electrons. This is called an expanded octet. Ex. S, Xe and P Slide 23 Ex. 3 Draw the lewis structure for XeF 4 (exception octet rule) 1. F is an end atom and nitrogen is the central atom. 2. Total number of valence electrons: (1 xenon x 8 valence electrons)+ (4 fluorines x 7 valence electron)= 36 valence electrons. 3. Total number of bonding pairs= 36/2 = 18 4. Draw single bond from each F to Xe Xe F F F F Slide 24 Ex. 1 Draw the lewis structure for XeF 4 (exception octet rule) 5. Subtract the number of pairs of electrons used from the total pairs of electrons: 18-4 =14 pairs available 14 lone pairs remain, place them around each fluorine so that each fluorine has 8 valence electrons Xe F F F F Slide 25 Ex. 1 Draw the lewis structure for XeF 4 (exception octet rule) 6. There are 2 pairs of electrons still available, place around Xe which is capable of having more than 8 valence electron. Xe F F F F Slide 26 Molecular Shape VSEPR (Valence shell electron pair repulsion) Model The repulsion between electron pairs in a molecule result in atoms existing at fixed angles from each other. (Remember balloon activity) Shared electron pairs repel each other A greater repulsion occurs between unshared electron pairs and shared electron pairs. Slide 27 Bonding and Shape of Molecules: Count number of bonds and unshared pairs of electrons AROUND CENTRAL ATOM and then use table below to determine shape of molecule. Number of Bonds Number of Unshared Pairs ShapeExamples 2343223432 0001200012 Linear Trigonal planar Tetrahedral Pyramidal Bent BeCl 2 BF 3 CH 4, SiCl 4 NH 3, PCl 3 H 2 O, H 2 S, SCl 2 -Be- B C N : O : : Covalent Structure Slide 28 Use table on last slide to determine shape of molecule. HH O Shape: bent 2 bonds and 2 unshared pairs SO 2 Slide 29 Carbon tetrachloride CCl CCl 4 C 109.5 o Cl Carbon tetrachloride carbon tet had been used as dry cleaning solvent because of its extreme non-polarity. Shape: Tetrahedral 4 bonds and 0 unshared pairs. Slide 30 Classwork: do in your notebook Determine the shape for the following molecules (first draw the lewis structure for the molecule and then use the table on slide #7 to determine the shape taking in consideration the number of bonds and unshared pairs of electrons around the CENTRAL ATOM.) 1. BF 3 2. OCl 2 3. CF 4 4. NH 3 5. BeI 2 Slide 31 Lesson 5: Strength of covalent bonds: Depends on distance between bonded nuclei. The shorter the bond length the stronger the bond. Triple bond strength> double bond strength> single bond strength Slide 32 Electronegativity and polarity The type of bond can be predicted by using the electronegativity difference of the elements that are bonded. Slide 33 For identical atoms, the bond they form is a nonpolar bond because the pair of electrons is shared equally. Bonds between different atoms can be ionic or covalent. If electronegativity difference is greater than 1.70 it is considered an ionic bond. If electronegativity diffence is less than 1.70 it is considered a covalent bond. If electronegativity difference is 0, the bond is nonpolar covalent. Slide 34 Polar covalent bonds Form when pair of electrons is not shared equally by bonding atoms (like a tug-of-war) Partial charges occur at the ends of the bond. Using the symbols -, partially negative, and +, partially positive, next to the model of a molecule indicates the polarity of the polar covalent bond. Slide 35 Molecular Polarity Molecules are either polar or nonpolar The nature of the covalent bond and the shape of the molecule result in a polar or nonpolar molecule. Symmetric molecules tend to be nonpolar H-H (H 2 ) has a nonpolar bond thus is a nonpolar molecule. H 2 O has polar bonds and is a polar molecule. Slide 36 CO 2 has polar bonds but do to the molecules shape is a nonpolar molecule. Polar bonds in this molecule are opposite to each other and cancel each other, so molecule is nonpolar. Slide 37 CH 4 has polar bonds but do to the molecules shape is a nonpolar molecule. Polar bonds in this molecule are opposite to each other and cancel each other, so molecule is nonpolar. Slide 38 CH 3 Cl has polar bonds and do to the molecules shape is a polar molecule. Polar bonds in this molecule do not cancel each other, so molecule is polar.