Bonding
Ionic bonding is bonding which occurs between metals (when they have lost electrons) and non-metals (when they have gained electrons). Opposite charges attract and this attraction is referred to as an ionic bond.
Ions
Atoms are always trying to get a complete set of electrons in their outer energy levels. If they are metallic elements located in the first few groups of the Periodic Table and only have a couple of electrons, they will happily give away these electrons to form positive ions. If it is a non-metal, with six or seven electrons in their outermost energy level, they will grab electrons from another atom to get a complete octet. Gaining electrons turns the atom into a negative ion. An ionic bond is the electrostatic attraction between positive and negative ions.
The Noble gases, found in the last column of the Periodic Table already have a full outer shell containing eight electrons. This means that they don’t need to share or steal electrons from another atom, making them very unreactive. The Noble gases will always exist as atoms and never as ions.
Charges of ions
You need to know the charges of certain ions which are easy to predict depending on which group they are in. If they are in group 1, they will lose this single outer electron to form a 1+ ion. However, if they are in group 6, they need two extra electrons to achieve a full octet so will form a 2- ion. The block of elements in the middle (the transition metals) are a little trickier and you may need to memorise these.
| Group number | Charge of ion |
| 1 | +1 |
| 2 | +2 |
| 3 | +3 |
| 5 | -3 |
| 6 | -2 |
| 7 | -1 |
The ions that are formed from atoms in the transition metal block of the Periodic Table, or ions formed from molecules, are included in the table below. Make sure you can state the charge of each of these ions because it will be important for your exam.
| Positive ions | Negative ions |
| Copper, Cu2+ | Hydroxide, OH- |
| Iron (II), Fe2+ | Carbonate, CO32- |
| Iron (III), Fe3+ | Nitrate, NO3- |
| Lead, Pb2+ | Sulfate, SO42- |
| Zinc, Zn2+ | |
| Hydrogen, H+ | |
| Ammonium, NH4+ | |
Forming molecules from ions
You need to be able to write formulas for the molecules formed between two ions. Let’s say that a zinc ion (Zn2+) forms an ionic bond with a chloride ion (Cl-), forming zinc chloride. When the molecule forms, the charges need to cancel out. Since the zinc ion has a 2+ charge, whereas chloride only has -1 charge, that means we need two chloride ions to have an overall neutral molecule. The formula of zinc chloride is therefore ZnCl2.
Dot-and-cross diagrams of ionic compounds
We can show how the metallic atom loses electrons and the non-metallic atom gains electrons by drawing a dot and cross diagram as shown below. Typically we only show the electrons present in the outer energy level and we use different symbols for the electrons on each atom, so we can clearly see the electron transfer.
This dot and cross diagram shows how sodium, which has one electron in its outer shell, will lose this outer electron to become an ion with a +1 charge. An atom of chlorine takes the electron from sodium, so that it now has a full outer shell of eight electrons. Since the chloride atom has gained an additional electron, its ion has a -1 charge.
Giant ionic lattices
Ionic compounds can form giant lattices, with a large number of positive and negative ions forming numerous ionic bonds. These structures have high melting and boiling points because ionic bonds are strong and require a great deal of energy to break. They do not conduct electricity when solid, but can conduct electricity when molten or as part of an aqueous solution. Remember that in order to conduct electricity, there needs to be charged molecules which are free to move. The positive and negative ions can only move when the ionic compound is melted or dissolved in solution.
Did you know..
Living organisms depend on ions to function - even a simple movement such as moving your arm depends on your brain coordinating the movement by passing a wave of sodium ions and potassium ions to your arm muscles. The importance of sodium ions in muscle contraction can sometimes be seen when pouring soy sauce (rich in sodium ions) onto squid. The ions cause movements in the squids tentacles, turning your sashimi into a slimy dancing zombie. Watch a video here.
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