Ionic Bonds: Why and How Ions Are Formed
Ionic bonding is the type of bonding that holds salts together. To better understand why and how ions — atoms that have a charge due to the loss or gain of electrons — are formed, you can study what happens during the chemical reaction to create salt.
The components: sodium and chlorine
The process of creating table salt is pretty remarkable. You take two substances that are both very hazardous, and from them you make a substance that’s necessary for life:
Sodium is an alkali metal, a member of the IA family on the periodic table. The Roman numerals at the top of the A families show the number of valence electrons (s and p electrons in the outermost energy level) in the particular element. So sodium has 1 valence electron and 11 total electrons because its atomic number is 11.
Chlorine is a member of the halogen family — the VIIA family on the periodic table. It has 7 valence electrons and a total of 17 electrons.
You can use an energy level diagram to represent the distribution of electrons in an atom. The energy level diagram for sodium and chlorine is shown in the following figure.
Instead of using the energy level diagram to represent the distribution of electrons in an atom, you can use the electron configuration. Write, in order, the energy levels being used, the orbital types (s, p, d, and so on), and — in superscript — the number of electrons in each orbital.
Here are the electronic configurations for sodium and chlorine:
The chemical reaction
The noble gases are the VIIIA elements on the periodic table. They’re unreactive because their valence energy level (outermost energy level) is filled. Achieving a filled (complete) valence energy level is a driving force in nature in terms of chemical reactions, because that’s when elements become stable. They don’t lose, gain, or share electrons.
The other elements in the A families on the periodic table do gain, lose, or share valence electrons in order to fill their valence energy level and become stable.
Because this process, in most cases, involves filling the outermost s and p orbitals, it’s sometimes called the octet rule — elements gain, lose, or share electrons to reach a full octet (8 valence electrons: 2 in the s orbital and 6 in the p orbital).
Sodium has one valence electron; by the octet rule, it becomes stable when it has eight valence electrons. Two possibilities exist for sodium to become stable:
It can gain seven more electrons to fill energy level 3.
It can lose the one 3s electron so that energy level 2 (which is filled at eight electrons) becomes the valence energy level.
In general, the loss or gain of one, two, or sometimes even three electrons can occur, but an element doesn’t lose or gain more than three electrons. So to gain stability, sodium loses its 3s electron.
At this point, it has 11 protons (11 positive charges) and 10 electrons (10 negative charges). The once neutral sodium atom now has a single positive charge [11(+) plus 10(-) equals 1+]. It’s now an ion. And ions that have a positive charge (such as sodium) due to the loss of electrons are called cations.
You can write an electron configuration for the sodium cation:
Chlorine has seven valence electrons. To obtain its full octet, it must lose the seven electrons in energy level 3 or gain one at that level. Because elements don’t gain or lose more than three electrons, chlorine must gain a single electron to fill energy level 3.
At this point, chlorine has 17 protons (17 positive charges) and 18 electrons (18 negative charges). So chlorine becomes an ion with a single negative charge. The neutral chlorine atom becomes the chloride ion. Ions with a negative charge due to the gain of electrons are called anions.
The electronic configuration for the chloride anion is:
Ending up with a bond
Sodium can achieve its full octet and stability by losing an electron. Chlorine can fill its octet by gaining an electron. If the two are in the same container, then the electron sodium loses can be the same electron chlorine gains.
The transfer of an electron creates ions — cations (positive charge) and anions (negative charge) — and opposite charges attract each other. The sodium cation attracts the chlorine anion and forms the compound NaCl, or table salt.
This is an example of an ionic bond, which is a chemical bond (a strong attractive force that keeps two chemical elements together) that comes from the electrostatic attraction (attraction of opposite charges) between cations and anions.
The compounds that have ionic bonds are commonly called salts. In sodium chloride, a crystal is formed in which each sodium cation is surrounded by six different chloride anions, and each chloride anion is surrounded by six different sodium cations. The crystal structure is shown in the following figure. Notice the regular, repeating structure.
Different types of salts have different crystal structures. Cations and anions can have more than one unit of positive or negative charge if they lose or gain more than one electron. In this fashion, many different kinds of salts are possible.