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Metallic Bonding(1 of 4)

Foglino, Paul. Cracking the AP Chemistry Exam. 2008. New York, Ny: The

Princeton Review, 2008.

Metallic substances are composed of atoms with smaller nuclei, and they tend to form stronger bonds with each other than atoms with larger nuclei. This is because the positively charged nuclei can be closer to the electrons. Although most metals are very hard, the delocalization of the electrons makes them very ductile and malleable. This delocalization also makes metals very good conductors of heat and electricity.

Hydrogen Bonding(2 of 4)



In a hydrogen bond, the positively charged end of a molecule with a hydrogen is attracted to the negatively charged end of a molecule with an extremely electronegative element (Oxygen, Fluorine or Nitrogen). These bonds are very strong because when a hydrogen atom gives up its lone electron, the positively charged nucleus is left unblocked. This causes substances with hydrogen bonds, like ammonia or water, to have relatively high boiling points.

Dipole-Dipole Forces(3 of 4)



Dipole-dipole forces are between two polar molecules that are neutral. The negative end of one polar molecule is attracted to the positive end of the other molecule. The larger the polarity, the greater the dipole-dipole attraction will be, and therefore, the higher the boiling and melting points will be. Since these attractions are fairly weak, substances with dipole-dipole forces are normally either gas or liquid at room temperature.

London Dispersion Forces(4 of 4)



London dispersion forces are also between neutral molecules. However, these molecules are non- polar, and the attractions are much weaker. These weak attractions occur because of the random motions of electrons. At any moment, a non-polar molecule will have more electrons on one side and it will be instantaneously polar, and the bond will act as if it were a weak dipole. The more electrons in a molecule, the more London dispersion forces. So the higher the number of electrons, the higher the melting and boiling points are. These are normally gases at room temperature.

DIPOLE MOMENTDipole moment is simply a measure of molecular polarity.Knowing or predicting the dipole moment of a molecule can help determine its properties.The following equation is used to calculate dipole moment:μ=QrWhere μ is the dipole momentQ is the value of the equal and opposite charges on a moleculeAnd r is the bond length.Dipole moments are reported in a unit called debyes. One debye equals one coulomb-meter (C-m.) (Brown, Lemay, Jr., Bursten, and Murphy),

The higher the dipole moment of a compound, the more polar it is.Compounds with higher dipole moments have a higher difference in their electronegatives, but lower bond length despite what the equation would have one believe.This is because the electronegativity difference – which affects the charge – has a greater effect on dipole moment than bond length does. (Brown, Lemay, Jr., Bursten, and Murphy .)

SIMPLE ISOMERISM IN ORGANIC COMPOUNDS

All organic compounds with more than four carbons exhibit some sort of isomerism.Organic compounds are called isomers if they share the same number of atoms of each element but have different names and structures. Although isomers share the same number of atoms of each element, they have differing chemical properties and, as such, cannot be considered the same substances. (Brown, Lemay, Jr., Bursten, and Murphy )

Structural IsomerismStructural isomerism occurs when two organic compounds have the same number of atoms of each element, but a different arrangement of connections between atoms.All organic compounds with four or more carbons can exist in either a branched or linear form. For example, butane (which has four carbons) has only a single branched isomer.(Gorga)

H3C-CH2-CH2-CH3

BUTANE (C4H10)Linear

H3C-CH-CH3 | CH32-METHYL PROPANE (C4H10)Branched(Brown, Lemay, Jr., Bursten, and Murphy )

Compounds with a higher number of carbons have a higher number of branched forms. (Gorga)

Organic compounds with double or triple bonds and four or more carbons also have multiple linear forms. This is because the double or triple bond can exist between different sets of carbons. For instance, in butene:

CH2=CH-CH2-CH3

1-BUTENEThe bond is on the first carbon.

CH3-CH=CH-CH3

2-BUTENEThe double bond is on the second carbon.(Brown, Lemay, Jr., Bursten, and Murphy )

Geometric IsomerismGeometric Isomerism, also sometimes called stereoisomerism, only occurs in substances with one or more double bonds, and have to do with the differences between spatial relationships between carbon groups. (Gorga)Since double bonds do not bend as easily as single bonds, the carbon chains attached to the carbons that share the bond have a fixed position. They can either be located on the same side of the double bond or on opposite sides. (Brown, Lemay, Jr., Bursten, and Murphy.) (Gorga.)For example, in butene:

H3C CH3

\ / C = C / \ H HCis-2-buteneMethyl groups are located on the same side of the double bond.

H3C H \ / C = C / \ H CH3

Trans-2-butene Methyl groups are located on opposite sides of the double bond.(Brown, Lemay, Jr., Bursten, and Murphy.)

When naming geometric isomers, if carbon groups are on opposite sides the compound gains the prefix trans- and if they are on the same side it gains the prefix cis-. (Gorga)

SIMPLE ISOMERISM IN COORDINATION COMPLEXESAs in organic compounds, coordination complexes can also have isomerism. This occurs when two or more coordination complexes share the same composition (number of each atom) but different arrangement of atoms. Coordination complexes can experience many different kinds of isomerism, falling under two groups: Structural Isomers (which have different bonds) and Stereoisomers (which have the same bonds but different arrangements). (Brown, Lemay, Jr., Bursten, and Murphy.) (Lancashire)

Structural IsomersCoordination complexes can undergo two different types of geometric isomerism: coordination-sphere isomerism and linkage isomerism. (Brown, Lemay, Jr., Bursten, and Murphy.)

Linkage Isomerism is relatively rare, but occurs when a ligand is capable of coordinating in more than one way. For instance, NO2

- can attach either at the N or at one of the O atoms. Though very few ligands are capable of doing this, another example is CN-. (Brown, Lemay, Jr., Bursten, and Murphy.)Coordination-Sphere Isomerism is much more common and can exist in every coordination compound. Coordination-sphere isomers differ in the kind of ligands attached to the metallic atom. (Lancashire)For instance, there are three substances with the formula CrCl3(H2O)6[Cr(H2O)6]Cl3 [Cr(H2O)5Cl]●H2O [Cr(H2O)4Cl2]Cl● 2 H2O (Brown, Lemay, Jr., Bursten, and Murphy.)Be this as it may, stereoisomerism is far more important in coordinate compounds than geometric isomerism. (Brown, Lemay, Jr., Bursten, and Murphy.)

StereoisomersThe two types of stereoisomerism that coordination compounds undergo are geometric isomerism and optical isomerism. (Brown, Lemay, Jr., Bursten, and Murphy.)Geometric isomerism occurs when the position of different ligands in space differs. For example, with diamminechloroplatinum(II) compounds, these types of arrangements can exist:

("Geometric Isomerism of Cisplatin")

Like with organic compounds, if similar atoms are on the same side, the prefix is cis- and if they are on opposite sides it is trans- (Lancashire)Optical Isomers, also called enantiomers, are three dimensional mirror images that cannot be superimposed upon one another. Molecules that cannot be superimposed upon one another are called chiral. (Brown, Lemay, Jr., Bursten, and Murphy.) (Lancashire)To predict whether a coordination compound has optical isomers, the structural diagram is flipped. If it cannot be superimposed on the original, then optical isomerism is occurring. (Lancashire)

Molecular Models: VSEPR and Valence Bonds

VSEPR (Valence Shell Electron-Pair Repulsion) Theory

states that electron domains will arrange themselves in an optimum geometry to minimize repulsion between themselves and the other negatively charged electrons domains.

o electron domains are any place that electrons reside, they can be non-bonded electrons, a single bond, a double bond, and so on.

Can be used to predict the shape of individual molecules (their electron domain geometry and molecular geometry)

Steps to predict shapes of molecules or ions:

1. Draw the Lewis structure of the molecule or ion. Count the number of electron domains around the central atom.

2. Determine the electron-domain geometry by arranging the electron domains around the central atom so that repulsions are minimized.

3. Use the arrangement of bonded atoms to determine the molecular geometry.

(See tables 9.2 and 9.3 below)

Brown, Theodore L., H. Eugene Lemay, Jr., Bruce E. Bursten, and Catherine J. Murphy. Chemistry: TheCentral Science.

http://apchemcyhs.wikispaces.com

***Know the bond angles associated with each electron domain geometry:

http://apchemcyhs.wikispaces.com/

Valence Bond Theory: hybridization of orbitals

a covalent bond forms when an atomic orbital from one atom overlaps with an atomic orbital from another atom

the orbitals mix (or undergo hybridization) to form new orbitals called hybrid orbitals—the number of orbitals formed equals the number of orbitals mixed

o An example of hybridization occurs with BeCl2. The electron configuration and the orbital diagrams of Be (1s22s2) suggest that Be should not form bonds since all of its electrons are paired. However, if one of the two paired electrons in the Be s orbital is promoted to a p orbital and the s and the p orbitals are hybridized, each lone electron in one of the two sp hybrid orbitals can overlap with the lone electron in a p orbital on Cl to form BeCl2.

the superscript refers to the number of orbitals of a specific type that were mixed (in sp2 hybridization, one s orbital and two p orbitals were mixed)

Foglino, Paul. Cracking the AP Chemistry Exam. 2008. New York, Ny: The Princeton Review, 2008.



Sigma and Pi bonds

the first covalent bond formed between two atoms is a sigma bond—all single bonds are sigma bonds

if additional bonds between the two atoms form, they are called pi bonds—the 2nd bond in a double bond is a pi bond and the 2nd and 3rd bonds in a triple bond are also pi bonds

Bond type: Single Double TripleBond designation: One sigma One sigma

and one piOne sigma and two pi

Bond order: One Two ThreeBond length: Longest Intermediate ShortestBond energy: Least Intermediate greatest


Geometry of molecules and ions:

The overall shape of a molecule is determined by its bond angles, the angles made by the lines joining the nuclei of the atoms in the molecule.

Along with the bond lengths, the bond angles accurately define the shape and size of the molecule.

These geometries are important because they include all the commonly

occurring shapes found for molecules and ions that obey the octet rule.

Electron-Domain

Geometry

Bonding

Domains

Nonbonding

Domains

Molecular Geometry Lewis

Structure

2 0

3 0

2 1

4 0

3 1

2 2

From: http://crescentok.com/staff/jaskew/isr/tigerchem/oxidation/shape.htm

http://crescentok.com/staff/jaskew/isr/tigerchem/oxidation/shape.htm

For molecules and ions with a central atom surrounded by other atoms, there are five fundamental shapes:


Relation of properties to structure:

Molecules have three-dimensional shapes determined by the relative orientations of their covalent bonds, which is maintained whether it is a solid, liquid, or gas

Properties of the molecule depend on the arrangement of the atoms.

For example, enzymes can lose their function if there are slight alterations in its structure, because they require precise fits between molecules.

Factor such as lone pairs and molecular weight of the elements in a molecule effect the way that a molecule is bonded and the angles that are formed from a molecule.

Trigonal Planar- 0 lone pairs Bent- 1 lone pair

CH3OH


Citations

Brown, Theodore L., H. Eugene Lemay, Jr., Bruce E. Bursten, and Catherine J. Murphy. Chemistry: TheCentral Science. AP Ed. Upper Saddle River, NJ: Pearson Education, Etc., 2009. Print.

"Chemical Bonds and the Shape of Molecules." Crescent Public Schools – Crescent, Oklahoma - Home of the Tigers. Web. 14 Apr. 2011. <http://crescentok.com/staff/jaskew/isr/tigerchem/oxidation/shape.htm>.


"Geometric Isomerism of Cisplatin." Cisplatin. Web. 14 Apr 2011. <http://www.chemcases.com/cisplat/cisplat12.htm>.

Gorga, Frank R. "Isomers of Organic Compounds, An Introduction." Bridgewater State College Website.Bridgewater State College, 12 Mar 2007. Web. 14 Apr 2011.

<http://webhost.bridgew.edu/fgorga/Stereochem/default.htm>.

Lancashire, Robert J. "Isomerism in Coordination Compounds." Chemistry, UWI-Mona. University of theWest Indies, 08 Jan 2011. Web. 14 Apr 2011.<http://wwwchem.uwimona.edu.jm:1104/courses/IC10Kiso.html>.



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