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, a third bond type, found in metallic solids, will be discussed later.
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:
So how do we use this to predict whether a bond is covalent or ionic?
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)
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:
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||1s2 2s2 2p6||3s2 3p3|
1s2 2s2 2p6 3s2 3p6 3d10
(≠ [Ar] = 1s2 2s2 2p6 3s2 3p6 )
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 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:
In this case can consider that we get two equations each involving a homo dissociation to give radicals, that is atoms with unpaired electrons:
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© R A Paselk
Last modified 8 April 2013