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Dot and Cross Diagrams of Ionic Bonding

We can represent ionic compounds by drawing dot and cross diagrams. In order to do this, we first need to know how to draw dot and cross diagrams of ions, so if you have forgotten how to do that, go back and refresh your memory now.

To draw a dot and cross diagram of an ionic compound, we simply draw dot and cross diagrams of the ions involved, in the correct ratio.

For example, to draw a dot and cross diagram of sodium chloride (NaCl), we draw a sodium ion (Na+) next to a chloride ion (Cl-). Because the ratio of sodium to chloride is 1:1, we draw one of each ion:

A dot a cross diagram of sodium chloride. On the left is a sodium ion. It is surrounded by square brackets, with the a plus sign (+) written to the top right to represent the charge. To the right of the sodium ion is a chloride ion. It is surrounded by square brackets, with a minus sign (-) written to the top right to represent the charge.

A dot and cross diagram of sodium chloride.

Notice that we use different symbols for the electrons on each ion. In this case, we used dots for sodium's electrons and crosses for chloride's electrons (although we could have done it the other way round).

The only exception is that one of chloride's electrons is shown as a dot. This is to represent the fact that a chlorine atom would have to gain one electron to become a chloride ion. We could think of the overall diagram as showing the way that sodium chloride could form from atoms if each sodium atom transferred one electron to a chlorine atom.

As another example, a dot and cross diagram for magnesium bromide (MgBr2) is shown below. This time, there are two bromide ions and one magnesium ion shown. This represents the 1:2 ratio of magnesium to bromide ions in the compound. In this example, magnesium's electrons are represented by dots and bromide's electrons are represented by crosses.

A dot and cross diagram of magnesium bromide. A magnesium ion is shown in the centre, with a charge of 2+. There are two bromide ions, on either side of the magnesium ion. Each has a charge of -.

Dot and cross diagram of magnesium bromide.

Note that the bromide ions are shown on either side of the magnesium ion rather than next to each other. This is because in an ionic compound, each cation is always surrounded by anions and each anion is always surrounded by cations.

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How do you draw a dot and cross diagram of an ionic compound?

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