- Naming monatomic ions and ionic compounds
- Common polyatomic ions
- Polyatomic ions
- Naming ionic compound with polyvalent ion
- Worked example: Finding the formula of an ionic compound
- Predict the charge on monatomic ions
- Naming ionic compounds
- Find the formula for ionic compounds
- Naming ions and ionic compounds
Learn how to name monatomic ions and ionic compounds containing monatomic ions, predict charges for monatomic ions, and understand formulas.
Close up view of colorless sodium chloride crystals, which have the overall shape of a cube.
Atoms are electrically neutral because the number of protons, which carry a 1+ charge, in the nucleus of an atom is equal to the number of electrons, which carry a 1- charge, in the atom. The result is that the total positive charge of the protons cancels out the total negative charge of the electrons so that the net charge of the atom is zero. Most atoms, however, can either gain or lose electrons; when they do so, the number of electrons becomes different from the number of protons in the nucleus. The resulting charged species is called an ion.
Cations and anions
When a neutral atom loses one or more electrons, the total number of electrons decreases while the number of protons in the nucleus remains the same. The result is that the atom becomes a cation—an ion with a net positive charge.
The opposite process can also occur. When a neutral atom gains one or more electrons, the number of electrons increases while the number of protons in the nucleus remains the same. The result is that the atom becomes an anion—an ion with a net negative charge. We can illustrate this by examining some very simple cations and anions, those formed when a single hydrogen atom loses or gains an electron.
Note: Hydrogen is actually somewhat unusual in that it readily forms both cations and anions. Most elements much prefer to form only one or the other. In terms of its electron configuration, can you explain why hydrogen can form both cations and anions? Feel free to post in the comments at the end of the article!
A hydrogen cation, a hydrogen atom, and a hydrogen anion.
In the center column, we have a diagram of a single, neutral hydrogen atom. It contains one proton and one electron; thus, its net charge is zero. If hydrogen loses its electron, it forms the cation (left column). The cation has a net charge of 1+ from the one proton in the nucleus since there are no electrons to cancel out the positive charge. If neutral hydrogen gains an electron, it forms the anion (right column). The anion has a net charge of 1- because it has one extra electron compared to the total number of protons.
Concept check: A certain ion has 20 protons and 18 electrons. What kind of element is this ion, and what is its net charge?
Predicting charges on monatomic cations and anions
Did you know that you can use the periodic table to predict the charges certain elements will have when they ionize? This is a very convenient and powerful tool, so it's worth examining in some detail. The following figure summarizes the common charges for the elements in the eight main groups, or families, on the periodic table. Remember that periodic groups refer to columns on the periodic table, whereas rows are known as periods. Keep in mind that these charges only apply when these elements are found in ionic compounds since covalent compounds don't contain ions.
As a general rule of thumb, the main group elements will usually gain or lose electrons in order to get a full octet of valence electrons. By figuring out how many electrons an element is likely to lose or gain to reach a full octet, we can predict the charge on the ion. This requires first knowing how many valence electrons are in the neutral atom.
Tip: The number of valence electrons in the neutral atom is equal to the number in the s place in the new IUPAC group number.
Elements that form cations
For groups 1, 2, 13, and 14, the elements have one to four valence electrons as neutral atoms, and they will usually give away these valence electrons to become ions—carbon is sometimes an exception to this trend since it can also gain four electrons to form the anion. Since the resulting ion has fewer electrons than protons, the net charge on the ion is positive. The magnitude of the charge is equal to the number of electrons lost, which is equal to the number of valence electrons in the neutral atom.
For example, what if we wanted to predict the charge on an aluminum ion? Aluminum is in group 13, or IIIA. Since the group number, , has the number in the s place, we would predict the charge to be to give . We can also think about a neutral aluminum atom losing its three valence electrons to become , which has a full octet.
Elements that form anions
For groups 15 through 17, the charge is usually negative because these elements are more likely to gain than lose electrons. The charge on the ion is therefore equal in magnitude to the number of electrons gained to reach a full octet of eight valence electrons. Mathematically, we can calculate the magnitude of the charge by subtracting the number of valence electrons in the neutral atom from eight. We can also use the periodic table to count how many columns to the right we need to go to reach the noble gases, group 18, where each adjacent column counts as one electron that needs to be gained to reach the full octet.
If we use these guidelines to predict the charge on a sulfur ion, which is in group 16, we predict that the magnitude of the charge is since sulfur has six valence electrons. We can also find the number of valence electrons by checking sulfur's group number, group 16, which has a in the s place. That means that a neutral sulfur atom will need to gain two electrons to reach a full octet of eight electrons. Therefore, we predict that the most common charge on a sulfur ion will be 2-.
Concept check: What ionic compound would you predict to form in a reaction between potassium metal and liquid bromine?
Now that we know that many common elements take on predictable charges, let's consider how to name the ions. We'll first look at the alkali metals—the elements in group 1 on the periodic table. From the figure above, we can see that the alkali metals tend to form cations with a 1+ charge. Thus, these cations include , , , , and so on. Naming these types of cations requires no special rule. For instance, we can refer to a hydrogen cation, , simply by calling it "-plus" or a "hydrogen ion". Similarly, a sodium cation, , can be called "-plus", "sodium plus", or most commonly, a "sodium ion". Note that it is unnecessary to say "a one plus sodium ion", because it is understood that a sodium ion usually has a 1+ charge.
The same logic also applies to all other elements that typically form cations of one particular charge. For instance, the alkaline earth metals, group 2, form cations with a charge of 2+: , , , etc. While we often refer to an ion such as as "magnesium two-plus", we could also simply say "magnesium ion", since it's understood what the charge on a magnesium ion is.
Note: The discussion in this section is mainly for naming cations by themselves, and the naming convention will be slightly different when the cation is part of an ionic compound. The naming of ionic compounds will be discussed separately below!
Elements that form multiple types of cations
So far, we have considered elements that typically form cations of one particular charge. For example, the alkali metals and the alkaline earth metals usually form 1+ ions and 2+ ions, respectively. Most transition metals, however, can form cations of various charges. That is why the d-block of the periodic table figure above has been labeled "variable charges". Iron, for instance, is often found as both the and cations, and sometimes other charges as well. Thus, iron is polyvalent, which literally means "many valued"—it is able to form cations of different charges.
On left, glass vial containing chromium(II) chloride, a grey-green powder and, on right, glass vial containing chromium(III) chloride, a bright purple powder.
For metals that are polyvalent, we need to specify the magnitude of the charge on the ion. For instance, we have to call "iron two-plus" or "iron two" because simply referring to it as "iron ion" will not give enough information to specify the type of cation. Most of the transition metals—those metals in the center d-block of the periodic table—are polyvalent. Since they can form cations with different charges, those charges must be specified when naming the ions and when naming compounds containing those ions.
In ionic compounds, the magnitude of the charge for a transition metal cation is usually included using Roman numerals in parentheses after the name of the metal, such as for chromium(II) chloride which contains . Naming ionic compounds containing transition metal cations will be discussed in more detail in a separate section below.
Naming monatomic anions
Most often, when we name monatomic anions, we add the suffix -ide to the end of the element's name. Because we can predict the charge on simple cations and anions based on an element's group number, it is not necessary to specify the magnitude of the charge on an anion most of the time. The following table shows how the suffix applies to naming anions of various elements:
|Element name||Ion name||Ion formula|
Formulas and naming of basic ionic compounds
Now that we've seen the naming conventions for cations and anions, we can discuss how they apply to naming simple ionic compounds made up of monatomic ions. The following guidelines can be used for naming ionic compounds:
- Always name the cation before the anion; in the chemical formula, the cation will always appear first as well.
- When naming the cation within an ionic compound, we don't include the word ion or the charge unless it is a polyvalent cation. That means we only have to name the element that the ion came from (see Example 2 below).
- Any ionic compound will have a net charge of zero. Another way of saying this is that cations and anions must always combine in such a way so that their charges cancel.
- The number of cations and anions in the formula should be written as the lowest possible integer value. For example, the formula for sodium chloride is , not or some other multiple of , even though the charges would still add up to zero.
Let's look at a few more examples:
Example 1: Finding the chemical formula from the name
What is the chemical formula of potassium chloride?
Remember that potassium is a group 1 element that forms a 1+ ion. Chloride, by definition, is an anion that has formed from an atom of chlorine. Since chlorine is in group 17, it will form a 1- anion. Because their charges are equal and opposite, there will be one ion for every one anion, and the chemical formula will be . Remember that subscripts are not used when there is only one atom/ion of a particular type.
Example 2: Finding the name from the chemical formula
What is the name of the ionic compound ?
Magnesium, , is a group 2 element that will form 2+ cations. Because it usually forms cations of only one type, we don't need to specify its charge. We can simply refer to the cation in the ionic compound as magnesium. Phosphorus, , is a group 15 element and therefore forms 3- anions. Because it is an anion, we add the suffix -ide to its name to get phosphide as the name of the ion. Therefore, the name for the compound is magnesium phosphide.
Naming ionic compounds with polyvalent cations
Recall from our earlier discussion that if an element can form more than one type of cation, we have to specify the charge on that cation. The magnitude of the charge for a transition metal cation is usually indicated using Roman numerals in parentheses after the name of the metal—this is also called the systematic name of the ion. The following table lists some of the most common ions for polyvalent metals. The systematic name is included for all ions. For some ions, the common or trivial name is also given. The common or trivial names are somewhat old fashioned nowadays, but they're still used in some places, so they're helpful to know. Notice that the ions of lesser charge take the suffix -ous in the common name; ions of higher charge take the -ic suffix. For example, ferrous chloride () is the name of , while ferric chloride () is understood to contain .
|Element||Common ions formed||Systematic name||Common (trivial) name|
Using this table as a reference, let's look at how to name ionic compounds containing polyvalent metals.
Example 3: Naming compounds containing polyvalent cations
What is the name of the compound ?
When naming ionic compounds that contain transition metals, we first need to determine the charge on the transition metal cation. We can deduce this charge by first calculating the charge contributed by the anion, whose charge we already know for sure.
We recognize that is a group 17 halogen, so it forms the chloride anion . We can see from the chemical formula that there are four chloride ions in the compound. The total negative charge contributed by the four chloride ions is calculated below:
In order for the compound to be electrically neutral, the lead cation must be . This is because the 4+ charge on this ion will exactly cancel the net 4- charge contributed by the four chloride ions.
Therefore, the name of is lead(IV) chloride.
Cations are positively charged ions formed when neutral atoms lose electrons; anions are negatively charged ions formed when neutral atoms gain electrons. It is possible to predict the charges of common monatomic ions by looking at the group numbers on the periodic table. However, many of the transition metals are polyvalent, which means they can form cations of multiple charges. When naming these cations or compounds containing these cations, it is necessary to specify their charge.
Cations and anions combine to form ionic compounds. Ionic compounds are named with the cation first and the anion last. The same convention is used when writing their chemical formulas. Ionic compounds must be electrically neutral. Therefore, the cations and anions must combine in such a way that the net charge contributed by the total number of cations exactly cancels the net charge contributed by the total number of anions.
Want to join the conversation?
- Can you provide me an example of ionic compound where Hydrogen is the anion or the negatively charged section of the bond. Thanks(41 votes)
- In a chemical reaction with an alkali metal and hydrogen, the hydrogen atom will always form the anion as hydrogen can from both cations and anions, but alkali metals can only form cations. In this case, the alkali metal gets a 1+ charge, and the hydrogen gets a 1- charge.
Lithium hydride (LiH)
Sodium hydride (NaH)
Potassium hydride (KH)
Rubidium hydride (RbH)
Caesium hydride (CsH)
Francium hydride (FrH)(31 votes)
- When we say "Transition metals", what does "transition" mean?(32 votes)
- it's because they "have an incomplete inner electron shell and that serve as transitional links between the most and the least electropositive in a series of elements"
- Hydrogen's preference to form ions will ultimately be related to the energy associated with the 1s shell. Although, I thought an atom's stability decreased upon forming ions, and so, an atom of hydrogen, with no external influences ( no transfer of energy between it and anything else ) could not form an ion. Energy would first have to be given too, or taken away from hydrogen before any electronic reactions could take place.
Returning swiftly back to why hydrogen can easily form ions, well, for the cation, I think it's because only a small amount of energy is needed to remove the electron. A little bit of research led to me ionization energies! 1,312 Joules of energy needed to remove hydrogen's electron. I'm not sure what I can relate that number too though, is it a lot?
For the anion, I think it's because the nucleus has a strong positive charge, which will attract any passing electrons.
That's my answer, and it could all be complete nonsense.(29 votes)
- For reference, it would take 1 liter of water that is at 0 degrees celsius and at 1 atm pressure 418100 joules to boil.
Also, there are 4184 joules in a kilocalorie.
Humans require anywhere from 1600-3000 calories to function.(5 votes)
- hydrogen exists as H2 or H in the atmosphere?(9 votes)
- In particular, there are 7 elements that are called "diatomic," meaning that on their own they will almost always form molecules consisting of two atoms of that element. They are: bromine, iodine, nitrogen, chlorine, hydrogen, oxygen, and fluorine. You can remember them by the acronym Br-I-N-Cl-H-O-F (pronounced "brinkle-hoff").(18 votes)
- So if you had a magnet attached to some compound, could you change the overall charge of the compound?if so, wouldn't it also be true that you could magnetically remove electrons and/or protons, thus changing the compound itself?(8 votes)
- The magnet will not "steal" the electrons from other metallic items. The magnet will simply force all the electrons in the metal to one side (non-permanently) which is how they attract each other. Once you remove the magnet, all electrons go back to where they came from and the metal compound neutral again.(23 votes)
- When hydrogen loses an electron won't it just be a proton and no longer hydrogen?
Sorry if it's a stupid question(8 votes)
- In Example 3, PbCl4 is Lead (IV) Chloride, doesn’t Cl4 refer to tetrachloride?(15 votes)
- Yes. For example, CCl₄ is carbon tetrachloride. But that's how you name covalent compounds.
The name of an ionic compound is: name of metal ion + name of anion ( with no multiplying prefix).
So, PbCl₄ is lead(IV) chloride.(2 votes)
- what does the roman numerals mean? like Cobalt(III) sulfide, or lead (IV)(6 votes)
- The roman numerals show the amount of charge. Cobalt (III) has a charge of (+3) and Lead (IV) has a charge of (+4).(2 votes)
- Why is PbCl4 named lead (IV) chloride?
We said we specify the charge for transition metals, which Pb doesn't appear to be. Also, since Pb can only have 4+ or 4- ions, and it is named first, doesn't that automatically mean it will be the 4+ cation?(6 votes)
- Hello there,
in Example 2: Finding the name from the chemical formula
"Therefore, the name for the compound is magnesium phosphide."
Shouldnt it be magnesium ion phosphide or magnesium 2-plus phosphide?.. Im really new to this and it sounds weird but earlier in article its said that : "While we often refer to an ion such as Mg2+ as "magnesium 2-plus", we could also simply say "magnesium ion," "
or is "just" magnesium a good answer afteral ?l.(3 votes)
- When we name an ionic compound, the charge is not included unless that element forms multiple ions. For example, if I have Fe2O3, I have to call it iron(III) oxide because iron also forms an ion with a 2+ charge.
Magnesium only forms the ion with a 2+ charge, so you don't need to include it.(13 votes)