Chem. Exam 1 Outline

Chem. Exam 1 Outline - 8.4 1) Bond Polarity and Electro...

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8.4 1) Bond Polarity and Electro negativity a) When two identical atoms bond, as in Cl 2 or N 2 , the electron pairs must be shared equally. b) The concept of bond polarity helps describe the sharing of electrons between atoms. 2) Electronegativity a) We use a quantity called electronegativity to estimate whether a given bond will be nonpolar covalent, polar covalent, or ionic. b) Within each period there is generally a steady increase in electronegativity from left to right; that is, from the most metallic to the most nonmetallic elements. 3) Electronegativity and Bond Polarity a) We can use different in electronegativty between two atoms to gauge the polarity of the bonding between them. b) In F 2 , the electrons are shared equally between the fluorine atoms, and thus the covalent bond is nonpolar. c) In HF the fluorine atom has a greater electronegativity than the hydrogen atom, with the result that the sharing of electrons is unequal- the bond is polar. d) In LiF the electronegativity difference is very large, meaning the electron density is shifted far toward F. 4) Dipole Moments a) The difference in electronegativity between H and F leads to a polar covalent bond in the HF molecule. b) We can indicate the polarity of the HF molecule in two ways. i) H δ + --- F δ - or H --- F c) Polarity helps determine many of the properties of substances that we observe at the macroscopic level, in the laboratory and in everyday life. d) How can we qualify the polarity of a molecule? Whenever two electrical charges of equal magnitude but opposite sign are separated by a distance, a dipole is established.
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e) The dipole moment increases as the magnitude of charge that is separated increases and as the distance between the charges increases. f) Dipole moments are usually reported in Debyes (D), a unit that equals 3.34 x 10^-30 coulomb-meters (C-m). 5) Bond Types and Nomenclature a) This is a good point for a brief interlude about nomenclature. There are two general approaches to naming binary compounds: one used for ionic compounds and the other for molecular ones. b) One reason for the overlap in the two approaches to nomenclature is that many compounds of metals with higher oxidation numbers have properties more similar to molecular compounds (which contain covalent bonds) than to ionic compounds (which contain ionic bonds). 8.5 1) Drawing Lewis Structures a) Lewis Structures can help us understand the bonding in many compounds a dare frequently used when discussing the properties of molecules. 2) Formal Charge a) When we draw a Lewis Structure, we are describing how the electrons are distributed in a molecule (or polyatomic ion). b) To calculate the formal charge on any atom in a Lewis structure, we assign the electrons to the atoms as follows: i) All unshared (nonbonding) electrons are assigned to the atom on which they are found. ii) For any bond –single, double, or triple- half of the bonding
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This note was uploaded on 09/14/2011 for the course CHEM 1001 taught by Professor Staff during the Fall '09 term at Cornell.

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Chem. Exam 1 Outline - 8.4 1) Bond Polarity and Electro...

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