Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 107

Fundamentals of Chemistry

Fall 2009

Lecture Notes: 20 October

© R. Paselk 2005
 
     
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Chemical Bonds

Chemical bonds are the strongest forces that exist between atoms. They are the forces that hold atoms together in molecules and atoms or ions together in solids. We will look at other weak bonds and forces later.

The two most important and common strong bond types in chemistry are ionic bonds and covalent bonds.

Electronegativity

So how do we determine whether two atoms will form an ionic or a covalent bond? Use a new property - electronegativity (EN). Electronegativity is a periodic measure of how electrons are shared by atoms with the highest value for F and the lowest for Cs. There are a couple of ways of determining EN's:

Bond Type: So how do we use this to predict whether a bond is covalent or ionic?

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 & Ions

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 

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:

Structures of Ionic Crystals

ball & stick crystal structure for NaCl

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.

Multiple Bonds & Resonance

Recall we must show an octet (or duet for Period I) in the outer-most shell (valence electrons). When this does not occur with single electron pairs (bonds) between atoms can sometimes make it happen with multiple bonds. You might find "Clark's Method" useful for determining the bonding patterns of various molecules.

Clark's Method for determining bonding in covalent Lewis Structures

Additional exercises on Lewis Structures are available in the Lewis Structure Module.

A Quantum Picture of Bonding

For a more in-depth understanding of bonding as illustrated with QuickTime movies based on quantum calculations check out the Bonding supplement.

 

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Last modified 19 October 2009