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Components of Cells
The Macromolecules
Electronegativity

Electrons in covalent bonds

A covalent bond is the force that holds two atoms together in close proximity while they share two electrons.

These shared electrons are, at the same time, found in the form of waves and tiny particles of negative charge. All negatively charged electrons are attracted towards any positive charge, and a major source of positive charges are the protons at the center of the quantum atom.

Shared electrons in a covalent bond, therefore, are pulled towards the positively charged protons at the centers of the two atoms.

How strongly the electrons are pulled depends on the number of protons in the center of each atom, the distance the electrons are from the nucleus (the energy level and orbital), and the amount or number of electrons between them and the atomic center (the amount of "screening" by inner electrons).

Electronegativity is a way of quantifying the amount of "pull" on the shared electrons in a covalent bond towards one atomic center, or the other.

Range of Values

Fluorine is the most electronegative element (type of atom). It is arbitrarily given a value of 4.0, the highest value, and all other values for all other elements range down to caesium and francium at values of 0.7. These are the least electronegative elements (the noble gasses are not usually rated). This is called the Pauling scale and is the one most commonly used for defining relationships between atoms and elements.

Pulling

In covalent bonds between two identical elements (H-H, O-O, C-C, etc.) the electronegativity of both the sharing atoms is the same, so the electrons will be shared equally between them. But when electrons are shared between elements with different degrees of electronegativity then the forces pulling on the two electrons will not be the same.

The element having the highest electronegativity will have the strongest pulling force, and the shared electrons will be attracted more to this atom than to the other in the covalent bond partnership. This distorts the distribution, or the amount of time, a shared electron will spend near one atom rather than the other.

Carbon-to-Carbon

The shared electrons in a carbon-to-carbon (C-C) covalent bond are pulled equally by the six protons at the center of each carbon atom (6+) and shielded by the inner, 1s, electrons (2-). The forces are balanced and the electrons spend the same amount of time distributed around each carbon atomic center.

Carbon-to-Oxygen However, the shared electrons in a carbon-to-oxygen (C-O) covalent bond are pulled in one direction by a carbon atom with six protons (6+) and in the other direction by an oxygen atom with eight protons at its center (8+). Both centers are shielded by the same inner, 1s, electrons (2-). Now the forces are not balanced, there is a greater pull on the shared electrons in the direction of the oxygen atom, and thus they spend more time distributed around the oxygen atom than they do around the carbon atom.

As a result, there is slightly more negative charge around the oxygen atom that around the carbon atom. The covalent bond between a carbon and an oxygen atom is therefore polarized.

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© 2005, Professor John Blamire