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Molecular Structure & Properties

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Surya Lombela

on 22 October 2013

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Transcript of Molecular Structure & Properties

Molecular Structure & Physical Properties
Dipole Moments
A dipole moment is the the mathematical product of the separation of the ends of a dipole and the magnitude of the charges. --Google
The charges can be represented by Q and -Q and the distance between the two can be represented by R so that a dipole moment u = QxR
The magnitude of a dipole moment is determined by the shape of the molecule and the electronegativity of the atoms involved
Ionic Compounds
High melting and boiling points because of ionic bonds
Interlocking of positive and negative ions
Tends to stay in solid state
Made up of non-metal anions and metallic cations, so there is a big difference between their electronegativities.
Covalent Compounds
Covalent compounds are relationships between nonmetals. They have stable chemical bonds called covalent bonds which involve shared electron pairs between atoms.
1. Electronegativity
2. Dipole Moments
3. Ionic Compounds
4. Covalent Compounds

Video on the differences between ionic and covalent compounds (10 min.).
Low melting and boiling points: covalent compounds create distinct molecules with weak forces in between, so low energy is needed to break them.
Poor electrical conductors: since charged ions are what allow electricity flow and covalent compounds don't have ions, they are not very good conductors.
Easily shift between solid, liquid, and gas states
Many are soluble in nonpolar liquids, but not in water

References Describing Ionic Compounds
Important Things to Know about Ionic Compounds,

Covalent Vs Ionic Bonds,
http://www.diffen.com/difference/Covalent_Bonds_vs_Ionic_Bonds ionic vs covalent

Papers on Electronegativity for Further Reading
Mapping the Periodic Landscape of Elements,
by Amato, I.; Science News, 12/16/1989, Vol 136, Issue 25, pg 390

Redefining Hydrogen Bonds, the Givers of Life, by Melissae Fellet; New Scientist, 7/30/2011, Vol 211, Issue 2823, pg 9
Websites for Further Reading
Molecular Dipole Moments,

Molecular Structure and Polarity, http://www.chem.ufl.edu/~itl/2045/lectures/lec_16.html
A covalently bonded compound called a buckminster fullerene
Dipole Moment
Electrostatic Potential Map of propane with corresponding ball-and-stick structure
Electrostatic Potential Maps of polyatomic ions
Periodic Table Showing Electronegativity
Electrostatic Potential Map of water shown with a scale of negativity and positivity
Electronegativity is an atom's tendency to attract electrons while in a chemical bond
Since it is not a physical property, it is measured in comparison to fluorine, which has a the highest electronegativity. This scale is called the Pauling Scale
Electronegativity effects how bonds form and determines the dipole moments of asymmetrical molecules

What Influences Electronegativity? And What Does Electronegativity Influence In Bonds?
Atoms that have a high electronegativity are usually small non-metals.
Electronegativity increases across periods because the greater nuclear charge exerted by the protons in the nucleus pulls electrons in more tightly
Electronegativity decreases down groups because larger atoms have more shielding and so attract atoms less
When a bond occurs between high electronegativity atoms, it will be covalent. If there is one high electronegativity atom and one low one, the electrons from the low electronegativity atom will be taken completely by the other atom and the bond will be ionic
Linus Pauling
An American scientist, peace activist, author, and teacher
A founder of quantum chemistry and molecular biology
Two-time Nobel Prize winner
Explored covalent and ionic bonds and the role that electronegativity played
Worked on developing concept of hybridization
Peter Debye
a Dutch-American physicist and physical chemist
applied the concept of a dipole moment to asymmetrical molecules
related dipole moments to temperature and dielectric constant
for these contributions, the units for dipole moments were called debyes
Video on Ionic Compounds
Ionic lattices: space-filling and ball-and-stick
Intermolecular forces
Intermolecular forces are magnetic attractions that hold compounds together based on polarity, that are present in covalent compounds.
Since the forces are not necessarily that strong, covalent compounds have several characteristics specific to them.
Gilbert N. Lewis
American physical chemist
Developed idea of covalent bonds and the Lewis Dot structure system
Received PhD at Harvard and eventually became Dean of the College of Chemistry at the University of California, Berkeley
Experimental evidence of the differences between ionic and covalent bonds: the values for the ionic compounds are significantly higher than those for the covalent compounds
Values for Melting and Boiling Points of Ionic and Covalent Compounds
Calculating Dipole Moments
Think of the electronegativities of the atoms in a molecule as vectors pointing towards the more electronegative atom.
To find the dipole moment, add these vectors. The result is the dipole moment.
Sometimes the vectors cancel. This occurs when the molecule in question is symmetrical, and these molecules do not have dipole moments
Physical Properties
Overall, the differences between the bonds in ionic and covalent compounds are influenced by electronegativity, dipole moments, and intermolecular forces. These differences are shown most clearly by the melting and boiling points.
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