Bonding

CHEMICAL BONDING : 

CHEMICAL BONDING Cocaine Chemistry 11DL – Unit 5

Chemical Bonding : 

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 : 

Review of Chemical Bonds There are 3 forms of bonding: _IONIC___—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 __COVALENT___—some valence electrons shared between atoms Most bonds are somewhere in between ionic and covalent.

The type of bond can usually be calculated by finding the difference in electronegativity of the two atoms that are going together. : 

The type of bond can usually be calculated by finding the difference in electronegativity of the two atoms that are going together.

Electronegativity Difference : 

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!

Ionic Bonds : 

Ionic Bonds All those ionic compounds were made from ionic bonds. 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).

Electron Distribution in Molecules : 

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)

Bond and Lone Pairs : 

Bond and Lone Pairs Valence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS. This is called a LEWIS structure.

Bond Formation : 

Bond Formation A bond can result from an overlap of atomic orbitals on neighboring atoms. Overlap of H (1s) and Cl (2p) Note that each atom has a single, unpaired electron.

Review of Valence Electrons : 

Review of Valence Electrons Remember 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 1s22s22p63s23p64s2 3d104p5How many valence electrons are present?

Review of Valence Electrons : 

Review of Valence Electrons Number of valence electrons of a main (A) group atom = Group number

Steps for Building a Dot Structure : 

Steps for Building a Dot Structure Ammonia, NH3 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!) 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

Slide 13: 

3. Form a single bond between the central atom and each surrounding atom (each bond takes 2 electrons!) Building a Dot Structure 4. Remaining electrons form LONE PAIRS to complete the octet as needed (or duet in the case of H). 3 BOND PAIRS and 1 LONE PAIR. Note that N has a share in 4 pairs (8 electrons), while H shares 1 pair.

Slide 14: 

Check to make sure there are 8 electrons around each atom except H. H should only have 2 electrons. This includes SHARED pairs. Building a Dot Structure 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!

Carbon Dioxide, CO2 : 

Carbon Dioxide, CO2 1. Central atom = 2. Valence electrons = 3. Form bonds. 4. Place lone pairs on outer atoms. This leaves 12 electrons (6 pair). 5. Check to see that all atoms have 8 electrons around it except for H, which can have 2. C 4 e-O 6 e- X 2 O’s = 12 e-Total: 16 valence electrons

Carbon Dioxide, CO2 : 

Carbon Dioxide, CO2 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. C 4 e-O 6 e- X 2 O’s = 12 e-Total: 16 valence electrons How many are in the drawing?

Double and even triple bonds are commonly observed for C, N, P, O, and S : 

Double and even triple bonds are commonly observed for C, N, P, O, and S H2CO SO3 C2F4

Now You Try One!Draw Sulfur Dioxide, SO2 : 

Now You Try One!Draw Sulfur Dioxide, SO2

MOLECULAR GEOMETRY : 

MOLECULAR GEOMETRY

Slide 20: 

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. MOLECULAR GEOMETRY

Some Common Geometries : 

Some Common Geometries Linear Trigonal Planar Tetrahedral

VSEPR charts : 

VSEPR charts 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!

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Other VSEPR charts

Structure Determination by VSEPR : 

Structure Determination by VSEPR Water, H2O The electron pair geometry is TETRAHEDRAL The molecular geometry is BENT. (ANGULAR) 2 bond pairs 2 lone pairs

Structure Determination by VSEPR : 

Structure Determination by VSEPR Ammonia, NH3 The electron pair geometry is tetrahedral. The MOLECULAR GEOMETRY — the positions of the atoms — is TRIGONAL PYRAMID.

Bond Polarity : 

Bond Polarity HCl is POLAR because it has a positive end and a negative end. (difference in electronegativity) Cl has a greater share in bonding electrons than does H. Cl has slight negative charge (-d) and H has slight positive charge (+ d)

Slide 28: 

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

Bond Polarity : 

Bond Polarity “Like Dissolves Like” Polar dissolves Polar Nonpolar dissolves Nonpolar