Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2015

Lecture Notes 27: 3 April

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Chemical Bonds, cont.

Last time were tlking abput how to determine if a bond is likely to be ionic or covalent, particularly in aqueous systems.

Ionic Bonds

An ionic bond is the result of the electrostatic force of attraction between ions that carry opposite electrical charges, as described by Coulomb's Law:

E = 2.31 x 10-19J*nm (Q1Q2/r)

where r is the distance between ion centers in nm.

Formation of ionic bonds

We can visualize the formation of ionic bonds as the transfer of an electron from a metal atom to a non-metal atom to form an ion pair. in vacuo:

M(g) + energy right arrow M(g)+ + e-

X(g) + e- right arrow X(g)- + energy

M(g)+ X(g) right arrow MX(g)

Lewis Structures for Atoms

Lewis Dot Structures are a very simple way of modeling atoms, ions, and molecules involving the representative elements (IUPAC groups 1, 2 & 13 - 18). In a Lewis Structure the nucleus and "core" electrons (all but the outermost shell) are represented by the symbol of the element, now referred to as a "kernel." (Note that kernals are not the same as Noble gas cores for atoms in Period 4 and up because of the d electrons which are included in the kernal.) Examples:

 Name  Lewis Structure Kernel electrons  Valence electrons
Sodium  Na.   1s2 2s2 2p6  3s1
 Phosphorus  phosphorus Lewis dot structure  1s2 2s2 2p6  3s2 3p3
 Bromine  bromine Lewis dot structure

 1s2 2s2 2p6 3s2 3p6 3d10

(≠ [Ar] = 1s2 2s2 2p6 3s2 3p6 )

4s24p5 

Lewis Structures for Ions and Ionic Compounds

For ions the charge is always shown. Thus for metal ions such as calcium the Lewis Structure simply becomes the symbol for the ion. For negative ions such as we see for oxygen (2-) we enclose the ion and its electrons in brackets to indicate that the electrons are all "owned" by the oxygen - it does not share. Notice that the Lewis Structures of monoatomic ions are isoelectronic with the nearest Noble gas. Thus sodium loses an electron to leave a kernel isoelectronic with neon, whereas bromine gains an electron to become isoelectronic with krypton. Examples:

Brackets are particularly important when we make ionic compounds:

Covalent Bonds

Covalent Bonds occur with the sharing of electrons by two atoms with similar tendencies to gain and loose electrons.

Let's look at the formation of HCl as an example of the creation of a covalent bond:

H2 + Cl2 right arrow 2 HCl

In this case can consider that we get two equations each involving a homo dissociation to give radicals, that is atoms with unpaired electrons:

Note Lewis Structure homework set in Discussion Manual.

Lewis Structures for Covalent Molecules

Covalent Bonds occur with the sharing of electrons by two atoms with similar tendencies to gain and loose electrons.

Formal Charge

This is another mode of "electron bookkeeping." Like oxidation numbers it uses a very simple set of rules to enable us to make realistic guesses about how atoms behave in molecules without having to have a pocket supercomputer to do that "quick" quantum mechanics calculation.

Formal charge helps us to determine how charges are distributed on atoms in a molecule or molecular ion. It is not always terribly accurate, but is very useful for approximating how molecules will behave in some situations. It is particularly useful in choosing among resonance structures in organic chemistry to determine which are likely to make the greatest contribution to the "real" structure.

Formal Charge (FC) = the charge an atom would have if all bonding pairs were shared equally (polar bonds don't exist in this model).

To assign Formal Charges:

  1. Draw a correct Lewis Structure.
  2. Assign both electrons of a lone pair to its associated atom.
  3. Divide all bonding pairs, giving one electron of each pair to each atom in the bond.
  4. Calculate FC = # electrons on the unbonded (elemental) atom - # electrons assigned to the bonded atom.

Examples:

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© R A Paselk

Last modified3 April 2015