Atoms and Molecules
Bonds and Octet Rules
VISUAL AIDSFunctional Groups
QUIZZESPractice Quiz I
Practice Quiz II
Practice Quiz III
Bonds and the Octet Rule
Chemical bonds are the forces—analogous to glue—that hold atoms together in molecules and ions in ionic crystals.
Valence Shells and Bonding
Electrons orbit the nucleus in specific energy levels/shells at super-high speed. The valence shell, or valence energy level, of an atom is the outermost energy level of the atom that is occupied by electrons, and it is the valence electrons that atoms use in forming bonds with other atoms.
Consider, for example, a sodium (Na) atom (atomic number 11), which is a metal atom (elements to the left of the heavy, stepped diagonal line in the periodic table are metals). The valence shell of the sodium atom is its third energy level, and it contains one electron. The first and second energy levels of the sodium atom are completely filled and contain, respectively, 2 electrons (since n = 1 for the first energy level, 2n2 = 2) and 8 electrons (since n = 2 for the second energy level, 2n2 = 8). Thus, the sodium atom contains one valence electron. The single electron in the valence shell of the sodium atom is the electron involved in bond formation.
Another example is the chlorine (Cl) atom (atomic number 17), which is a nonmetal atom (elements to the right of the heavy, stepped diagonal line in the periodic table are nonmetals). The valence shell of the chlorine atom also happens to be its third energy level, and it contains 7 electrons. The first and second energy levels of the chlorine atom are also completely filled and contain, respectively, 2 electrons (once again, since n = 1 for the first energy level, 2n2 = 2) and 8 electrons (once again, since n = 2 for the second energy level, 2n2 = 8). Thus, the chlorine atom contains 7 valence electrons. The 7 electrons in the valence shell of the chlorine atom are the electrons involved in bond formation.
The electrons in valence shells of atoms are key to the forming of chemical bonds between atoms. As you can see in the illustrations of the sodium and chlorine atoms in the previous section, most atoms do not have completely filled valence shells. Atoms such as these are not stable, meaning they tend to react with other atoms to form chemical bonds. They share, donate, or receive electrons from other atoms in order to attain the electron configuration of their nearest noble gas neighbors (which are stable).
Noble gases (members of group 8A of the periodic table ) are stable due to the fact that they each have a complete set of valence electrons (2 for helium [He] and 8 for the others). Thus, if an atom achieves the electron configuration of the nearest noble gas neighbor that has 8 valence electrons, the octet rule, or “rule of eight” applies. For some atoms, such as hydrogen (H) and lithium (Li), the rule of two applies, because their closest noble gas neighbor is helium (He), which has 2 valence electrons in energy level 1.
Two types of chemical bonds are considered in Core Studies 7.1. One type is the ionic bond —which involves the transfer of electrons from metal atoms to nonmetal atoms—and the other type is the covalent bond —which can be either polar or nonpolar and which involves the sharing of electrons between nonmetal atoms.
The Ionic Bond
The formation of an ionic bond, which involves the transfer of electrons from metal atoms to nonmetal atoms, can be illustrated by considering the formation of the ionic compound, sodium chloride (NaCl), from the reaction of a sodium atom with a chlorine atom. The sodium atom (atomic number 11), with 1 valence electron in its third energy level, would like to achieve the stability of its nearest noble gas neighbor, the neon (Ne) atom (atomic number 10), which contains 8 valence electrons in its second energy level. So the sodium atom has a strong tendency to give up its single valence electron. By doing so, the sodium atom becomes a positively charged sodium ion (Na+1)—specifically, a sodium cation —which now contains, like neon, 8 electrons in its outermost, occupied energy level (energy level 2).
The chlorine atom (atomic number 17), on the other hand, has 7 valence electrons. According to the octet rule the chlorine atom needs one more electron to become stable (like its nearest noble gas neighbor, the argon (Ar) atom, which contains 8 valence electrons in its third energy level). So the chlorine atom gains the electron lost by the sodium atom. When this occurs, the chlorine atom becomes a negatively charged chloride ion (Cl-1)—specifically, a chloride anion —and the electrostatic attraction between the oppositely charged sodium cation and chloride anion constitutes the ionic bond.
As a general rule, most metals tend to lose electrons to become positive ions (cations) and most non-metals tend to gain electrons to become negative ions (anions). The ionic bond occurs between these oppositely charged ions.
In the illustration below, the sodium atom loses an electron to the chlorine atom. The sodium atom becomes a positive ion and the chlorine atom becomes a negative ion. The two oppositely charged ions attract, forming the ionic bond.
Ionic bonding: process of making sodium chloride
This is an imaginary construct being used for illustrative purposes.
The Covalent Bond
A covalent bond is a bond between nonmetals in which the atoms involved share valence electrons. As noted previously, two types of covalent bonds are considered in Core Studies 7.1, namely nonpolar and polar covalent bonds. In a nonpolar covalent bond, electrons are shared equally between atoms. In a polar covalent bond, electrons are most often closer to one atom, creating negative and positive “poles.”
To help us distinguish between nonpolar and polar covalent bonds, we must first consider electronegativity. This term reflects the ability of an atom in a molecule to attract a pair of shared electrons to itself. Every atom has its own electronegativity value. When the electronegativity difference between two atoms is zero or negligible (less than 0.5), the bond is nonpolar covalent, which means that electrons are shared equally between the nonmetal atoms involved. In a polar covalent bond, the electronegativity difference between the atoms is greater than or equal to 0.5 and less than 2.0, which means that electrons are shared unequally between participating nonmetal atoms. When the electronegativity difference is significant (greater than or equal to 2.0), the bond is ionic, as is the case with sodium chloride that is considered above (sodium has an electronegativity of 0.9, and chlorine has an electronegativity of 3.0, so that the difference is 2.1).
To illustrate the formation of a nonpolar covalent bond, let us consider the formation of the chlorine molecule (Cl2) from two chlorine (Cl) atoms. Keep in mind that each chlorine atom would like to achieve the electron structure of the argon atom, its nearest noble gas neighbor. The only way that this can occur is for each atom to share one of its valence electrons with the other. This shared pair of electrons constitutes the covalent bond. Note that when the sharing of electrons occurs, each chlorine atom will have a total of 8 electrons in its third energy level (its valence shell), like the argon atom. Furthermore, because the chlorine atoms are identical and are, therefore, characterized by an electronegativity difference of zero, the sharing of the electron pair is “equal,” and the bond (the electron pair) is a nonpolar covalent bond. The unshared electron pairs in the chlorine molecule are called nonbonding electrons.
The illustration of polar covalent bonding can be considered within the context of the water molecule (H2O). The oxygen (O) atom (atomic number 8) has six electrons in its valence level or shell (energy level 2). To achieve the electron configuration of its nearest noble gas neighbor, neon (Ne), which contains 8 electrons in its valence energy level or shell (energy level 2), oxygen needs two more valence electrons. The hydrogen (H) atom has one valence electron (energy level 1), and since its nearest noble gas neighbor is helium (with two valence electrons in energy level 1), the hydrogen atom needs one more electron to become stable.
When two hydrogen atoms (each a nonmetal atom like oxygen) share their valence electrons with the oxygen atom, the oxygen atom gains two electrons, one from each hydrogen atom. Thus the oxygen atom is able to satisfy the octet rule. Each hydrogen atom shares one electron from oxygen atom's valence shell, thereby satisfying the rule of two. The overall process of sharing of electrons leads to the formation of two covalent bonds between the oxygen atom and each hydrogen atom. The remaining two unshared pairs of electrons on the oxygen atom of the water molecule are called nonbonding electron pairs.
Since the electronegativity of hydrogen is 2.1, and the electronegativity of oxygen is 3.5, the bonds are polar covalent (electronegativity greater than 0.5 but less than 2.0), because the electronegativity difference between the oxygen and hydrogen atoms is 1.4 (3.5 – 2.1). This means that the hydrogen and oxygen atoms involved in each bond share the electron pair unequally. Given that oxygen is more electronegative than hydrogen, the oxygen atom attracts the two electron pairs more strongly than each of the two hydrogen atoms attract them. Consequently, the water molecule that is formed from an oxygen atom and two hydrogen atoms is a polar molecule because its oxygen “end” is slightly more negative than each of the two positive hydrogen “ends” or “poles.”
Covalent bonding: process of forming water
This is an imaginary construct being used for illustrative purposes.