By Ron B. Davis Jr., Georgetown University
When it comes to metallic elements and their chemistry, the loss of one or more electrons to form a positively charged ion—called, a cation—drives a great deal of their chemistry. For historical reasons, chemists call any loss of electrons, oxidation. And the more electrons an element loses, the more ‘oxidized’ it is said to be.
First Ionization Energy
Chemists can measure how readily an atom of a given element gives up its first electron, and becomes oxidized to a plus one cation. The energy needed to create this cation with a plus-one charge is what we call an element’s ‘first ionization energy.’
Not surprisingly, the elements with the lowest first ionization energies are the group-1 metals of the s-block. Since, the s-block metals are either one or two electrons over an octet, for the group one metals, losing just one electron takes them all the way home to that coveted octet.
On the other hand, for the group-2 metals, losing one electron gets them only halfway to their desired number of electrons.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Importantly, nuclear charge increases as we move left to right within a row in the periodic table. Therefore, these nuclei with greater positive charge cling more tightly to their negatively charged valence electrons, resisting oxidation more.
This helps to explain, at least in part, why so-called ‘noble metals’ like platinum, palladium, gold, are often found and used in their native metallic state. These metals do not tend to ‘rust’, or combine with oxygen at all, as that would mean giving up one or more of these tightly held electrons to oxygen atoms.
Lower Ionization Energies
In the case of metals known for their durability, such as titanium and chromium, we see their first ionization energies are much lower than gold. And that’s what’s interesting.
The reason that elements like titanium and chromium are valued as durable metals is precisely because those are metals that do react with oxygen to form oxide compounds.
The trick is that their oxide compounds are tough and hard, creating a very thin, passivating layer of oxide that protects the remaining metal beneath from further reaction.
These tough oxide layers form almost immediately when metals like titanium are molded and cooled, protecting the rest of the metal inside from exposure to oxygen.
What this means is that holding pure titanium metal or pure chromium metal in our hand is virtually impossible under normal conditions. What we are actually holding is pure titanium or chromium wrapped in a thin layer of protective metal oxide.
Iron, on the other hand, is located between the fast-oxidizers and the noble metals. Iron does go through the process of oxidation, of course, but it oxidizes more slowly and forms a soft, brittle oxide. This brittle oxide flakes off easily, exposing additional iron that can also be oxidized, repeating the process and causing more and more iron metal to corrode over time.
While first ionization energies produce a very distinctive trend within the table, it is, however, only a modest predictor of the real overall reactivity for many metals.
The Activity Series
A more practical way of considering oxidation in metals is to use what is known as the activity series. The activity series is a hierarchy of the most common metallic elements ranked by just how badly they wish to be oxidized to their most common oxidation state, losing whatever number of electrons necessary.
Thus, instead of just the first ionization energy, now we are considering whatever change it takes to get a metal to oxidize to its most stable positive ion. When we consider oxidation this way, we get a chart that roughly tracks with the first ionization energy trends, while giving us a more accurate way to compare how eager metals are to oxidize in the real world.
Levels of Reactivity
The activity series is broken into a few larger groups of elements with similar levels of reactivity. It is a hierarchy, meaning that elements near the top will react under any conditions listed below, as well as the conditions indicated for their level in the series.
The top group of the activity series is so easily oxidized that its members will react chemically with water, one of the most chemically stable substances on Earth. The water breaks down to produce hydrogen gas, and an oxidized metal ion results.
The next group down is a bit less reactive, requiring some heat to encourage the process of oxidation. These elements will react with water only when it is heated to steam.
Below those are a group of metals that will not react with water at all, but will react with acids, which contain hydrogen ions. This type of reaction also produces hydrogen gas. Here we find many familiar metals that can chemically react and weather over time when exposed to materials like rain water, which is weakly acidic. This also helps explain why drain cleaners are usually made of extremely basic materials. Pouring a strong acid into a plumbing system with lead or iron pipes can actually dissolve the pipe itself instead of the clog!
More Difficult to Oxidize
Next, in the series are hydrogen ions, the source of acidity as a reference point. This is followed by groups of metals that are less reactive than hydrogen ions, and progressively more and more difficult to oxidize. Unsurprisingly, this group contains elements like platinum and gold—elements we associate with having an enduring lack of reactivity.
Thus, the activity series clearly gives us a way to predict how easily oxidized common metallic elements will be in real-world environments, including how readily they will react with water, acid, and even each other.
Common Questions about the Process of Oxidation and the Activity Series
Nuclear charge increases as we move left to right within a row in the periodic table.
The activity series is a hierarchy of the most common metallic elements ranked by just how badly they wish to be oxidized to their most common oxidation state, losing whatever number of electrons necessary.
The top group of the activity series is so easily oxidized that its members will react chemically with water, one of the most chemically stable substances on Earth.