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In this lesson, we will review the three major types of bonding found in compounds.
Consider a fluorine atom. Fluorine has an electron configuration of 1s2 2s2 2p5. Its highest occupied energy level is n = 2. The 2s sublevel contains two electrons and the 2p contains five, so a fluorine atom has seven valence electrons. One of these electrons occupies a 2p orbital by itself and thus is unpaired. This is obviously one electron short of a complete octet, so a fluorine atom may achieve greater stability by the addition of an electron.
This tug-of-war, often referred to as sharing electrons, is the basis of a covalent bond. The fluorine atoms have no attraction for each other; however, each atom is attracted to the pair of electrons that they are sharing. This causes the atoms to stick together quite strongly.
Consider now a sodium atom, with an electron configuration of 1s2 2s2 2p6 3s1. Sodium has only one valence electron. But if it were to lose its 3s electron, its highest occupied energy level would then be the n = 2 level, which would contain eight electrons—an octet!
But where might this electron go? We have already seen that fluorine is one electron short of an octet. If a fluorine atom were nearby, it could easily take this electron and add it to its 2p sublevel, giving it eight valence electrons as well. So by the simple transfer of one electron from sodium to fluorine, each atom is able to achieve eight valence electrons and satisfy the Octet Rule.
But remember that electrons are negatively charged particles; therefore transferring them from one atom to another has implications for the charge of each atom involved. Losing the electron causes the sodium to become positively charged, and gaining an electron causes the fluorine to become negatively charged; thus, they are no longer atoms but ions. Because the ions have opposite charges, they experience a strong electrostatic attraction for each other that we call an ionic bond.
Is there a “middle ground” between covalent bonding, in which two atoms share a pair of electrons equally between them, and ionic bonding, in which an electron is completely transferred from one atom to another? Can a pair of electrons be shared unequally? The answers to these questions can best be understood in the context of electronegativity.
In simplest terms, electronegativity is how hard an atom pulls on electrons in a bond. In a covalent bond, the electrons are shared equally because the two atoms involved have equal or nearly equal electronegativities; that is, each atom pulls the electrons to itself with equal strength. An ionic bond occurs when one atom’s electronegativity is so much larger than the other’s that the first atom can literally pull the electron away from the second.
Between these two extremes is the polar covalent bond. In this type of bond, each atom is trying to pull the electron pair to itself, but because one atom has a higher electronegativity, it is able to pull harder and the electrons are drawn closer to that atom. This means that the electrons spend more time near the atom with the higher electronegativity, making it slightly negative (δ−), and less time near the atom with lower electronegativity, making it slightly positive (δ+). The bond is considered covalent because the electrons are shared, but polar due to the slight opposite charges produced at each end.
We classify a bond as ionic, covalent or polar covalent based upon the difference in electronegativities between the two atoms in the bond. Since electronegativities vary in a regular fashion on the Periodic Table, we can use the atoms’ positions relative to each other to quickly categorize our bonding type.
Ionic bonding requires a large difference in electronegativity. Since electronegativities are lowest on the lower left of the Periodic Table (where metals are) and highest on the upper right (where nonmetals are), ionic bonds can be generalized to occur between metals and nonmetals. Examples would include NaCl or CaI2.
Covalent bonding occurs when electronegativities are virtually equal. This most often occurs when two atoms of the same element bond to each other. The most common examples would include diatomic elements. The diatomic gases are H2, N2, O2, F2, Cl2, Br2, and I2.
Polar covalent bonding happens when two atoms have a moderate difference in their electronegativities. This is most likely to happen when two different nonmetal atoms bond with each other, as in H2O or SO2.
Which of the following correctly describes the formation of an ionic bond?
The correct answer is D. Ionic bonds occur when there is a large difference in the atoms’ attraction for electrons, resulting in electron transfer and ion formation.
What type of bonding is present in MgBr2?
Choice A is correct. This bond is between a metal and a nonmetal, which means a large difference in electronegativities. The metal (Mg) will transfer one electron to each of the nonmetal atoms (Br), creating one Mg2+ ion and two Br1− ions that attract each other electrostatically. This is an ionic bond.