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Unformatted text preview: CHEMICAL BONDING
Cocaine 1 2 Chemical Bonding
Problems and questions -- How is a molecule or polyatomic ion held together? Why are atoms distributed at strange angles? Why are molecules not flat? Can we predict the structure? How is structure related to chemical and physical properties? Review of Chemical Bonds 3 Most bonds are somewhere in between ionic and covalent. There are 3 forms of bonding: _________--complete transfer of 1 or more electrons from one atom to another (one loses, the other gains) forming oppositely charged ions that attract one another _________--some valence electrons shared between atoms _________ holds atoms of a metal together 4 The type of bond can usually be calculated by finding the difference in electronegativity of the two atoms that are going together. Electronegativity Difference If the difference in electronegativities is between: 1.7 to 4.0: Ionic 0.3 to 1.7: Polar Covalent 0.0 to 0.3: Non-Polar Covalent
Example: NaCl Na = 0.8, Cl = 3.0 Difference is 2.2, so this is an ionic bond! 5 6 Ionic Bonds
All those ionic compounds were made from ionic bonds. We've been through this in great detail already. Positive cations and the negative anions are attracted to one another (remember the Paula Abdul Principle of Chemistry: Opposites Attract!) Therefore, ionic compounds are usually between metals and nonmetals (opposite ends of the periodic table). 7 Electron Distribution in Molecules Electron distribution is depicted with Lewis (electron dot) structures This is how you decide how many atoms will bond covalently! (In ionic bonds, it was decided with charges) G. N. Lewis 1875 - 1946 8 Bond and Lone Pairs Valence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS. H Cl shared or bond pair lone pair (LP) This is called a LEWIS structure. Bond Formation
A bond can result from an overlap of atomic orbitals on neighboring atoms. 9 H + Cl H Cl Overlap of H (1s) and Cl (2p) Note that each atom has a single, unpaired electron. 10 Review of Valence Electrons Remember from the electron chapter that valence electrons are the electrons in the OUTERMOST energy level... that's why we did all those electron configurations! B is 1s2 2s2 2p1; so the outer energy level is 2, and there are 2+1 = 3 electrons in level 2. These are the valence electrons! Br is [Ar] 4s2 3d10 4p5 How many valence electrons are present? Review of Valence Electrons
Number of valence electrons of a main (A) group atom = Group number 11 Steps for Building a Dot Structure
1. Decide on the central atom; never H. Why? If there is a choice, the central atom is atom of lowest affinity for electrons. (Most of the time, this is the
least electronegative atom...in advanced chemistry we use a thing called formal charge to determine the central atom. But that's another story!) 12 Therefore, N is central on this one 2. Add up the number of valence electrons that can be used. H = 1 and N = 5 Total = (3 x 1) + 5 = 8 electrons / 4 pairs Building a Dot Structure
3. Form a single bond between the central atom and each surrounding atom (each bond takes 2 electrons!) 13 H N H H 4. Remaining electrons form LONE PAIRS to complete the octet as needed (or duet in the case of H). H N H H 3 BOND PAIRS and 1 LONE PAIR. Note that N has a share in 4 pairs (8 electrons), while H shares 1 pair. Building a Dot Structure
1. Check to make sure there are 8 electrons around each atom except H. H should only have 2 electrons. This includes SHARED pairs. 14 H N H H 6. Also, check the number of electrons in your drawing with the number of electrons from step 2. If you have more electrons in the drawing than in step 2, you must make double or triple bonds. If you have less electrons in the drawing than in step 2, you made a mistake! 15 Carbon Dioxide, CO2
1. Central atom = 2. Valence electrons = 3. Form bonds.
C 4 eO 6 e- X 2 O's = 12 eTotal: 16 valence electrons O C O This leaves 12 electrons (6 pair). 4. Place lone pairs on outer atoms. 5. Check to see that all atoms have 8 electrons around it except for H, which can have 2. 16 Carbon Dioxide, CO2
C 4 eO 6 e- X 2 O's = 12 eTotal: 16 valence electrons How many are in the drawing? 6. There are too many electrons in our drawing. We must form DOUBLE BONDS between C and O. Instead of sharing only 1 pair, a double bond shares 2 pairs. So one pair is taken away from each atom and replaced with another bond. O C O O C O Double and even triple bonds are commonly observed for C, N, P, O, and S 17 H2CO SO3 C2F4 O C O 18 Now You Try One! Draw Sulfur Dioxide, SO2 Violations of the Octet Rule
(Honors only) 19 Usually occurs with B and elements of higher periods. Common exceptions are: Be, B, P, S, and Xe. Be: 4 B: 6 P: 8 OR 10 S: 8, 10, OR 12 SF4 BF3 Xe: 8, 10, OR 12 MOLECULAR GEOMETRY 20 21 MOLECULAR GEOMETRY VSEPR Valence Shell Electron Pair Repulsion theory. Most important factor in determining geometry is relative repulsion between electron pairs. Molecule adopts the shape that minimizes the electron pair repulsions. 22 Some Common Geometries
Linear Trigonal Planar Tetrahedral VSEPR charts 23 Use the Lewis structure to determine the geometry of the molecule Electron arrangement establishes the bond angles Molecule takes the shape of that portion of the electron arrangement Charts look at the CENTRAL atom for all data! Think REGIONS OF ELECTRON DENSITY rather than bonds (for instance, a double bond would only be 1 region) 24 25 Other VSEPR charts 26 Structure Determination by VSEPR
The electron pair geometry is TETRAHEDRAL H O H 2 bond pairs 2 lone pairs The molecular geometry is BENT. 27 Structure Determination by VSEPR
lone pair of electrons in tetrahedral position H H N H The MOLECULAR GEOMETRY -- the positions of the atoms -- is TRIGONAL PYRAMID. Bond Polarity 28 HCl is POLAR because it has a positive end and a negative end. (difference in electronegativity)
+ - H Cl Cl has a greater share in bonding electrons than does H. Cl has slight negative charge (- ) and H has slight positive charge (+ ) Bond Polarity 29 This is why oil and water will not mix! Oil is nonpolar, and water is polar. The two will repel each other, and so you can not dissolve one in the other Bond Polarity 30 "Like Dissolves Like" Polar dissolves Polar Nonpolar dissolves Nonpolar 31 Diatomic Elements These elements do not exist as a single atom; they always appear as pairs When atoms turn into ions, this NO LONGER HAPPENS! Hydrogen Nitrogen Oxygen Fluorine Chlorine Bromine Iodine Remember: BrINClHOF ...
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- Spring '10