How do we formulate the ionic salt, potassium bromide?
Answers
Atoms on opposite sides of the periodic table tend to form ionic bonds. This is because the ones on the left want to lose electrons in order to obtain a stable electronic configuration, and the ones on the right want to take them in order to do so. (use the periodic table for this example such as the Dynamic Periodic Table)
KBr is an example of this. In order for K to lose all of its outer shell electrons, you must trace your finger from K, and go left. Nothing there! K, Potassium only has one electron in its outer shell to give away. After giving the electron away, K looks externally (in terms of electrons) like Argon (a noble gas).
Now, let's look at Br. We go right, because that is the closest way to get to the noble gases, and so have a complete outer shell with no extras (called valence electrons) to react with. Br needs exactly one electron.
Therefore, pairing K and Br results in a stable state where K donates and completely gives Br an electron, making K now positively charged (lost an electron) and Br negatively charged (gained one). This electron transfer forms an ionic bond because opposite charges attract.
Below is a 3d model for a lot of KBr. K would be the purple, and Br the green. Note that only + (purple) touches - (green) which makes the crystal (like a salt crystal) stable.