Sunday, April 8, 2012

Covalent Bonding Continued

So if covalently bonded structures do not form lattices of repetitive atoms then what kind of molecular arrangements do they form?

In a covalently bonded molecule, the outer electrons of one atom overlap with the outer electrons of another atom. While this does involve a bit of +/- attraction (like ionic bonding) in many cases, it does not create a strictly attraction/repellent nature of charged ions in the ionic bond.

In the figure above, a fluorine atom is covalently bonded to a hydrogen atom. You could also say that they are sharing their outer electrons. Because fluorine and hydrogen have different properties of a term called electronegativity they are not equally sharing the electrons in each of their outer shells. However, the sharing is equal enough to consider the bond a covalent one.

Often, groups of covalently bonded atoms are held together in liquid or solid form by forces of attraction between molecules. While these forces are not strong enough to be considered covalent bonds, they are often strong enough to render a substance a liquid or solid at room temperature instead of a gas (molecules flying all over the place in a gas and not held together by forces).

The diagram in blue shows a three-dimensional picture of a single molecule of water. Many people even outside of the chemical world are familiar with the word H2O as being synonymous with water. The purpose of this simple drawing is to show the outer electrons in water as both bonded to atoms (in this case hydrogen) and existing as free-floating electrons called lone pairs. The two lone pairs each have their own balloon . Since there are four sets of outer electrons here, they distribute themselves at equal angles from the center to minimize repelling against each other. This creates the most basic covalently bonded structure that we commonly refer to as a tetrahedral.

Interestingly, water molecules use these lone pairs to stack themselves into repetitive patterns that are not lattices (no ionic bonding involved) but allow them to stick together strong enough to form a liquid at room temperature. (Note that most other molecules the same size as water as gases at room temperature) See my post on this for more information on my other blog.

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