When a single atom becomes ionized, it forms a monatomic ion. A monatomic ion is a one-atom ion such as Ca2+ or S2-. The prefix “mono-” means one, so a monatomic ion is made up of one atom.
It is very important for chemists to be able to communicate information to each other. Therefore, a set of rules is used to name ions.
Let's Watch
Watch this video to learn how to name monatomic ions. As you watch the video, pay attention to how Roman numerals are used in the names of some cations.
You may want to use the study guide to follow along. If so, click below to download the study guide.
Now that we’ve gone over the complicated rules that dictate how ions form, we can learn the much simpler set of rules about how ions are named.
Let’s begin by looking at how we name the cations formed by s-block elements. All of these elements in the s-block have a very short path to that stable, noble electron configuration. And that’s that they just need to lose their one or two valence electrons. As a result, these elements form very predictable cations, and our system for naming these cations is very simple as a result: we just add the word “ion” to the element name.
For example, “Na one plus,” the element is sodium, and we just add the word ion, and we call this a sodium ion. Another example, “Be two plus,” the element is beryllium, so we call this a beryllium ion.
Now let’s look at all the other cations that can form. Many of these elements can form multiple ions; we call them polyvalent. As a result, our name for these cations needs to convey the charge that they carry. So, to name them, we add the charge, in roman numerals and parentheses, between the element name and the word “ion.”
Let’s look at a couple examples. “Pb two plus,” the element is lead, the charge is two, and then we add the word “ion,” and we call this a “lead two ion.” But what about “Pb four plus?” Well, the element is still lead, but now the charge is four, and then we add the word “ion,” and we call this one a “lead four ion.”
Lastly, let’s look at how we name anions. Just like with the s-block elements, these elements have a very short path to that noble configuration. And as a result, these elements also form very predictable ions. So, just like with the s-block elements, our name for these ions doesn’t need to convey charge. We just drop the end of the element name, and replace it with the syllable “ide.”
For example, “I one minus,” the element is iodine, and we just drop the end and replace it with “ide,” to give us “iodide.” Or another example, “N three minus,” the element is nitrogen. We drop the end,
replace it with “ide,” to get “nitride.”
Question
Why are Roman numerals used in naming cations formed by non-s-block elements?
Non-s-block ions are polyvalent, which means they can form ions with multiple charges. Therefore, their name must specify the charge in that instance by using a Roman numeral.
