Covalent bondingï¿½-ï¿½The WikiPremed MCAT Course

At this stage in the MCAT course, we are carefully studying the nature of chemical bonds in themselves. In a few weeks, we will be studying chemical reactions in which old bonds are broken and new bonds formed. Let us preview an important conceptual cluster, the internal energy changes involved in chemical bond formation that will be so important to our understanding of the thermochemistry of reactions later.

The internal energy change of two atoms undergoing bond formation involves a decrease in electrostatic potential energy. The total energy of the bound molecule, with the atoms separated by the bond distance, is less than the energy of the system with the atoms fully separated. Why is this so?

Imagine bond formation.

As electrons within nonbonded atoms form molecular orbitals, the nuclei are drawn inward along the bond axis and internal energy decreases. Let us say that again. Try to picture it in your mind. As the bonding electrons form molecular orbitals, their presence in the internuclear space draws the nuclei inward, an electrostatic potential energy decrease. Unlike charges are being drawn together, the negatively charged electrons in the bonding orbital with the positively charged nuclei on either side.

This is a blended classical/quantum description designed to help you conceptualize bond formation. Most chemical reactions involve the breaking of old bonds (internal energy increase) coinciding with new bonds forming (internal energy decrease). Whether the system has lost or gained internal energy depends on whether the stronger bonds are the bonds broken or the bonds formed.

Don't blow past this. Think about electrostatic potential energy. Think about bond formation as a trip two atoms take down into a potential energy well. How deep is the well? That's how strong the bond is, i.e. how much energy the system loses when the bond is formed.

The WikiPremed MCAT Course is a free comprehensive course in the undergraduate level general sciences. Undergraduate level physics, chemistry, organic chemistry and biology are presented by this course as a unified whole within a spiraling curriculum.

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