Kaplan MCAT General Chemistry Review

We classify chemical bonds into two distinct types: ionic and covalent. In ionic bonding, one or more electrons from an atom with lower ionization energy, typically a metal, are transferred to an atom with greater electron affinity, typically a nonmetal, and the electrostatic force of attraction between opposite charges holds the resulting ions together. This is the nature of the bond in sodium chloride, where the positively charged sodium cation is electrostatically attracted to the negatively charged chloride anion. In covalent bonding, an electron pair is shared between the two atoms, typically two nonmetals, which have relatively similar values of electronegativity. The degree to which the pair of electrons is shared equally or unequally between the two atoms determines the degree of polarity in the covalent bond. If the electron pair is shared equally, the covalent bond is nonpolar; if the pair is shared unequally, the bond is polar. If both electrons being shared were contributed by only one of the two atoms, the bond is called coordinate covalent.

Ionic Bonds

Ionic bonds form between atoms that have significantly different electronegativities. The atom that loses the electron(s) becomes a cation, and the atom that gains the electron(s) becomes an anion. The resulting ionic bond is the electrostatic force of attraction between the opposite charges of the ions. There is no sharing of electrons in an ionic bond. For this electron transfer to occur, the difference in electronegativity must be greater than 1.7 on the 4.0-Pauling scale. In general, you will recognize ionic bonds forming between the alkali metals and the alkaline earth metals of Groups IA and IIA (Groups 1 and 2) and the halogens of Group VIIA (Group 17). The atoms of the active metals have one or two electrons, which they hold onto only loosely. The atoms of the halogens are strongly “interested in” gaining one more electron to complete their valence shells. These tendencies explain the formation of the ionic bond in, say, sodium chloride, which we’ve already discussed.

Mnemonic

The t in cation looks like a plus sign: ca + ion.

Ionic compounds have characteristic physical properties, which you should recognize for Test Day. Because of the strength of the electrostatic force between the ionic constituents of the compound, ionic compounds have very high melting and boiling points. The melting point of sodium chloride is greater than 800°C. Many ionic compounds dissolve readily in aqueous and other polar solvent solutions and, in the molten or aqueous state, are good conductors of electricity. In the solid state, the ionic constituents of the compound form a crystalline lattice consisting of repeating positive and negative ions in which the attractive forces between oppositely charged ions maximize, while the repulsive forces between ions of like charge minimize.

For example, if you were to analyze the atomic structure of the salt you just spread over the meat that you’re going to grill up for dinner, you would see that each sodium ion is surrounded by six chloride ions and each chloride is surrounded by six sodium ions. This is a lattice formation known as 6:6 coordinated. (The optimal time to salt meat is about 30 minutes prior to cooking so that the salt has sufficient time to draw fluid to the surface by osmosis, creating a salt solution, which then gets drawn back into the muscle fiber once again. This results in well-seasoned meat through the entire thickness, not just at the surface.)

Covalent Bonds

When two or more atoms with similar electronegativities interact, the energy required to form ions through the complete transfer of one or more electrons is greater than the energy that would be released upon the formation of an ionic bond. That is to say, when two atoms of similar tendency form a compound to attract electrons in a bond, it is energetically unfavorable to form ions. So, rather than struggling to form ions, the atoms simply opt to share the electrons as a compromise, which allows them both to fill their valence shells. The binding force between the atoms is not ionic; rather, it is the attraction that each electron in the shared pair has for the two positive nuclei of the bonded atoms.

MCAT Expertise

Think of bonds as a tug-of-war between two atoms. When the difference in electronegativity is great (more than 1.7) then the “stronger” molecule wins all of the electrons and becomes the anion. When the electronegative values are relatively similar, then we have a stalemate, or a covalent bond with mostly equal sharing of electrons.