Lesson Plan - Get It!
You and a friend want to buy a pizza for lunch. The pizza costs $10.00. You have two dollars, and your friend has eight dollars. You can both chip in to buy the pizza, but do you think it's easier to give up $2.00 or $8.00?
It is probably easier to give up two dollars than to give up eight dollars.
An ionic bond in chemistry follows this same trend. In an ionic bond, electrons are transferred from one element to another:
By Mhowison [Public domain], from Wikimedia Commons
Notice how the electron is leaving Na, sodium, for Cl, chlorine, in the image above, provided by WikiMedia Commons. This is very different from a covalent bond where the electrons are shared between the two bonded elements. An ionic bond forms between a metal and a non-metal.
Metals are represented by the darker blue color above. Non-metals are represented as the light blue and pink elements. Ionic bonds are trying to achieve the same goal as a covalent bond: eight valence electrons in the outer layer of the atom. Metals have so few electrons in the outer layer, it is easier to give up electrons to achieve the octet than it is to take them from other atoms.
Remember, elements in group 1A have one valence electron, elements in group 2A have two, and the trend continues through groups 3A–8A.
Cs, or cesium, has one valence electron, while I, iodine, has seven valence electrons. Iodine needs one electron to reach the octet, and cesium can give one away to achieve a full outer layer. These two atoms form an ionic bond. Cesium gives one electron, which gives it a positive charge with a value of 1. Iodine accepts one electron, giving it a negative-one (-1) charge. The charges of each element balance out so the compound has a net charge of 0.
To determine the chemical formula, you crisscross the charges. So, if cesium has a plus-one and iodine has a negative-one, each element in the formula would have a subscript of 1. Scientists don’t keep the charges or values of 1 in chemical formulas, in order to keep them efficient. The chemical equation for the compound would be CsI, and the name would be cesium iodide. Always write the metallic element first, in both the formula and the name! Notice, there are no prefixes in an ionic compound name. Metals follow the same pattern, so you can rely on the charges to determine how many of each atom are in a compound.
Try another one: Bond Ca, calcium, with F, fluorine.
Calcium has two valence electrons, and fluorine has seven. Calcium will give up one valence electron to a fluorine atom, fulfilling the octet.
It hasn't reached octet — it still has one valence electron, so we add another fluorine atom that is able to take the extra electron. Calcium gave up two electrons, and is now carrying a positive-two charge. Each fluorine now has a negative-one charge because it took one electron. Crisscross the charges into subscripts. You should end up with CaF2. The positive-two charge became the subscript on the fluorine, indicating that you need two atoms of fluorine to fulfill the octet rule for both elements involved in the bond! The compound would be named "calcium fluoride." Again, always write the metal first and never include prefixes!
Atoms that become positively charged as they give up electrons are called "cations" (pronounced cat-ions — think feline). "Anions" are negatively-charged ions. Every ionic compound is made up of cations and anions bonded together. The charge is always determined by the number of electrons given or received. There is a pattern on the periodic table that you can use to quick-check your work:
Metals in group 1A always ionize to a positive-one charge, because they have one valence electron to give away. Group 2A ionizes to a positive-two charge, and group 3A to a positive-three charge. Group 4A can be a little challenging, either choosing a positive or negative charge with a value of four. This has to do with the other elements involved in the bond. Group 5A becomes negatively charged with a value of three. These elements have five valence electrons, so they take three to fill the octet. Group 6A ionizes to negative two, and group 7A to negative one. Remember that group 8A doesn't bond, so it doesn't form ions either, because of the already-full outer layer of electrons.
Ionic compounds have high melting and boiling points because of the metallic atoms held in the bonds. These bonds hold a lot of chemical energy. Some common ionic bonds you might have heard of include table salt (NaCl) and calcium carbonate, or chalk (CaCO3). Ionic bonds are hard to represent with dot structures because the electrons are actually moving from one element to another. Summarize what you have learned about ionic bonds in a table like the one below:
Ionic bonds form between metal and a non-metal. Each element is trying to achieve a full outer layer of electrons, either by taking or giving away electrons. As the atom gains or loses electrons, it becomes charged, forming an ion. These ions follow a pattern on the periodic table that makes it easy to use the chemical names to determine how many of each atom exist in a compound.
In the Get It? section, you will practice more naming and identification of ionic compounds.