Updated: Sep 4, 2021
The reactivity of an element defines how easily the element takes part in chemical reactions. For example, potassium is highly reactive and will burn violently when it comes into contact with water. On the other hand, gold is unreactive, so you can get your jewelery wet without worrying about being blown up! We can visualise the relative reactivities of a group of elements by creating a reactivity series - where the elements are listed from most to least reactive. In this article, we will learn why some elements are more reactive than others and how to work out the reactivity series for a group of unknown elements from their reactions. We will also learn a fun technique for remembering a reactivity series, which is sure to come in handy for a variety of subjects, not just chemistry.
Why are some elements more reactive than others?
Visualise the structure of an atom - there is a central nucleus made up of protons and neutrons, around which are 'shells' of electrons (see a good outline of this structure here). Each shell has a maximum amount of electrons it can contain before moving on to the next shell, which is further away from the nucleus. Elements react with each other by trading the electrons in the outer shell of their atoms. Atoms want to have full outer shells, so if the outer shell is nearly full, the atom will receive electrons to fill that shell up. If the outer shell is almost empty, the atom will want to lose those electrons so that the full shell beneath it becomes the outer shell. The amount of these outer electrons determines how reactive an element is - it is easier to lose one electron than to lose three, so generally elements with only one outer electron are more reactive than those with three. As well as the number of outer electrons, the position of these electrons is very important. For example, the group one metals (the leftmost column of the periodic table) all have one outer electron, but they all have different reactivities. This is because the outer shell of each element is a different distance away from the nucleus. The further away an electron is from the nucleus, the easier it is to remove due to the attractive force from the nucleus being weaker. So, potassium is more reactive than sodium, because the outer shell of potassium is further away from the nucleus than the outer shell of sodium.
The reactivity series of metals
The most common reactivity series you will interact with is the reactivity series of metals:
Remember, the most reactive element is at the top of the list, so potassium is the most reactive and gold is the least. A good way to remember the order of a reactivity series is to make a memorable sentence with the first letters of each element. For example: Purple Slugs Like Chasing Me Around Zoos In Colourful Shorts, Gosh! This technique is called a mnemonic, and can be used to remember all sorts of useful things. Try making your own mnemonic for this reactivity series, and see if you can think of one for remembering anything else, such as the order of the planets in our solar system.
We can observe the reactivity of metals by conducting various experiments. The most simple type of experiment is to just place the metals into acid and watch how quickly each reacts - the more reactive metals will react faster. Most metals react with water to produce a salt and hydrogen gas, so the speed of the reaction can be judged by how quickly bubbles of hydrogen form. However, this isn't always the most effective method of recording the different reactivities of metals, since a difference in the speed of the bubbles might not always be obvious. Fortunately, metals often exist in compounds, such as copper sulfate or aluminium oxide, and a more reactive metal will displace a less reactive metal from its compounds in (drumroll please) a displacement reaction. For example, magnesium is more reactive than copper. So, when grey magnesium is stirred into blue copper sulfate solution, the copper is displaced and we end up with orange copper in a clear magnesium sulfate solution. Importantly, there is no reaction the other way round - if we stir copper powder into magnesium sulfate solution, nothing will happen because copper is less reactive than magnesium. The reactivity series for a set of metals can be deduced via a series of displacement reactions. By mixing each metal with every other in the set, we can observe which metals displace which and therefore which are more reactive. A metal must be above all the metals it displaces in the reactivity series, and below all those that it doesn't. So, the metal that displaces all the other metals from their compounds must be the most reactive, and the metal which displaces all the metals except the most reactive must be the second, and so on and so forth until we reach the least reactive metal, which doesn't react with any of the other compounds.
Serious series work
Having laid out the science behind reactivity series, we have seen that there is a lot more to them than simply listing elements. To begin with, to understand what an element's reactivity is we have had to look to its atomic structure - it wants to have a full outer shell of electrons, and the ease with which it can gain/lose electrons to make that full outer shell defines its reactivity. We then introduced ourselves to the reactivity series of metals, along with a fun technique for remembering the correct order (which may or may not contain purple slugs!). To top it all off, we have learned how to construct the reactivity series of a particular set of metals by conducting a series of displacement reactions with the metals and their compounds. As so often seems to be the case, while the end product may look quite simple, the science behind it is far from it!
Get hold of our Exam Revision Guide and let’s turn your exam experience into a success story 😀 → https://www.studysquare.co.uk/pdf
Liam Bradbury is an MPhys graduate from the University of Edinburgh. Although from a physics background, he loves all areas of science, and has spent far longer looking at viruses under microscopes than any physicist should.