Naming ions and ionic compounds (article)

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Learn how to name positive ions (cations), negative ions (anions), and ionic compounds involving main group elements.

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. For example, the element sodium loses one electron to become a cation:

Element name	Symbol	Protons	Electrons	Charge
sodium	$Na$ ‍	$11$ ‍	$11$ ‍	$0$ ‍
sodium ion	$Na^{+}$ ‍	$11$ ‍	$10$ ‍	$1 +$ ‍

Similarly, when a neutral atom gains one or more electrons, the result is that the atom becomes an anion—an ion with a net negative charge. For example, the element chlorine gains one electron to become an anion:

Element name	Symbol	Protons	Electrons	Charge
chlorine	$Cl$ ‍	$17$ ‍	$17$ ‍	$0$ ‍
chloride ion	$Cl^{-}$ ‍	$17$ ‍	$18$ ‍	$1 -$ ‍

Main group elements (the elements in groups 1, 2, and 13-18 of the periodic table) typically form ions of only one charge. The diagram below shows the common charge of ions in different groups.

Note: Hydrogen is somewhat unusual in that it readily forms both cations and anions. Most elements form only one or the other.

Naming cations

How do we name main group cations? In general, the cation name is the same as the element name, but with the word "ion" added to the end.

For example, the alkali metals in group 1 on the periodic table tend to form cations with a $1 +$ ‍ charge. The following table shows how we name the ions for the first four elements in this group:

Element	Ion symbol	Ion name
hydrogen	$H^{+}$ ‍	hydrogen ion
lithium	$Li^{+}$ ‍	lithium ion
sodium	$Na^{+}$ ‍	sodium ion
potassium	$K^{+}$ ‍	potassium ion

In speech, we may also refer to a hydrogen cation simply as " $H$ ‍-plus". Similarly, a sodium cation can be called "a sodium ion" or " $Na$ ‍-plus".

The same rule applies to all other elements that typically form cations of one particular charge. This includes the alkaline earth metals in group 2 and a few other metals you may be familiar with, shown in the table below:

Element	Ion symbol	Ion name
magnesium	$Mg^{2 +}$ ‍	magnesium ion
calcium	$Ca^{2 +}$ ‍	calcium ion
strontium	$Sr^{2 +}$ ‍	strontium ion
aluminum	$Al^{3 +}$ ‍	aluminum ion
silver	$Ag^{+}$ ‍	silver ion

Why don't we need to include a charge in the names of these ions? For example, why can we say "calcium ion" instead of "calcium-two-plus ion"? Since these main group elements usually form ions of only one charge, which can be predicted from the periodic table, the charge is implied.

So far, we've 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 and several metals in groups 13-17, however, can form cations of various charges. For instance, iron is often found as both the ${Fe}^{2 +}$ ‍ and ${Fe}^{3 +}$ ‍ 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.

For metals that are polyvalent, we need to specify the magnitude of the charge on the ion. We do this by adding Roman numerals to the name, which indicate the charge of the cation. For example, ${Fe}^{2 +}$ ‍ is written as "iron(II)", and ${Fe}^{3 +}$ ‍ is written as "iron(III)."

In speech, we have to call ${Fe}^{2 +}$ ‍ "iron two-plus" or "iron two" because simply referring to it as an "iron ion" will not give enough information to specify the cation charge.

Naming anions

To name main group anions, we take the root of the element's name and add -ide to the end. The following table shows how this suffix is used to name anions of various elements:

Element name	Ion name	Ion formula
hydrogen	hydride	$H^{-}$ ‍
chlorine	chloride	${Cl}^{-}$ ‍
bromine	bromide	${Br}^{-}$ ‍
iodine	iodide	$I^{-}$ ‍
oxygen	oxide	$O^{2 -}$ ‍
sulfur	sulfide	$S^{2 -}$ ‍
nitrogen	nitride	$N^{3 -}$ ‍
phosphorus	phosphide	$P^{3 -}$ ‍
carbon	carbide	$C^{4 -}$ ‍

Let's use the oxygen anion as an example. In writing, it is shown as the symbol $O^{2 -}$ ‍ or written as "oxide". In speech, we'd typically refer to it as an "oxygen ion", "oxide", or " $O$ ‍-two-minus"

Since we can predict the charge of main group anions by their location on the periodic table, it's usually not necessary to specify their charge in the name. For example, it is implied that an oxide ion has a $2 -$ ‍ charge. We don't need to say "oxide-two-minus".

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 (a.k.a. salts). The following guidelines are used for naming ionic compounds:

Always name the cation before the anion. The cation will appear before the anion in the chemical formula, too.
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 $NaCl$ ‍, not ${Na}_{2} {Cl}_{2}$ ‍ or some other multiple of $NaCl$ ‍, even though the charges would still add up to zero.

Let's look at a few examples:

Example 1: Finding the chemical formula from the name

What is the chemical formula of potassium chloride?

Potassium ( $K$ ‍) is in group 1, so it forms a cation with a $1 +$ ‍ charge. Chloride is, by definition, an anion formed from an atom of chlorine ( $Cl$ ‍) . Chlorine is in group 17, so chloride has a $1 -$ ‍ charge. The potassium and chloride ions have equal and opposite charges, so they will join in a 1:1 ratio, forming $KCl$ ‍.

(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 ${Mg}_{3} P_{2}$ ‍?

Magnesium ( $Mg$ ‍) is in group 2, so it forms a cation with a $2 +$ ‍ charge. Because it is the cation in the compound, we put it first in the compound’s name and refer to it as just magnesium. (Notice we remove the word "ion" since it is now part of a compound).

Phosphorus ( $P$ ‍) is in group 15, so it forms anions with a $3 -$ ‍ charge. Because it is the anion in the compound, we put it second in the compound’s name and refer to the anion by its name, phosphide.

Therefore, the name for the ionic compound ${Mg}_{3} P_{2}$ ‍ is magnesium phosphide.

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.

For some ions, the common name is also given. The common names are somewhat old fashioned nowadays, but they're still used in some cases. 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 ( ${FeCl}_{2}$ ‍) is the common name of iron(II) chloride and contains the ${Fe}^{2 +}$ ‍ ion. Ferric chloride ( ${FeCl}_{3}$ ‍) is the common name for iron(III) chloride and contains the ${Fe}^{3 +}$ ‍ ion.

Element	Common ions formed	Systematic name	Common name
chromium	${Cr}^{2 +}$ ‍	chromium(II)	chromous
	${Cr}^{3 +}$ ‍	chromium(III)	chromic
cobalt	${Co}^{2 +}$ ‍	cobalt(II)
	${Co}^{3 +}$ ‍	cobalt(III)
copper	${Cu}^{+}$ ‍	copper(I)	cuprous
	${Cu}^{2 +}$ ‍	copper(II)	cupric
iron	${Fe}^{2 +}$ ‍	iron(II)	ferrous
	${Fe}^{3 +}$ ‍	iron(III)	ferric
lead	${Pb}^{2 +}$ ‍	lead(II)
	${Pb}^{4 +}$ ‍	lead(IV)
tin	${Sn}^{2 +}$ ‍	tin(II)	stannous
	${Sn}^{4 +}$ ‍	tin(IV)	stannic

Note: Roman numerals are only needed for polyvalent elements. Elements which form ions of only one charge do not require Roman numerals.

The following transition metals (groups 3-12) do not require a Roman numeral in their name since they only form one cation:

Element	Ion symbol	Systematic name
scandium	$Sc^{3 +}$ ‍	scandium
zinc	$Zn^{2 +}$ ‍	zinc
silver	$Ag^{+}$ ‍	silver
cadmium	$Cd^{2 +}$ ‍	cadmium

Using this table as a reference, let's practice naming an ionic compound containing a polyvalent metal.

What is the name of the compound ${PbCl}_{4}$ ‍?

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 $Cl$ ‍ is a group 17 halogen, so it forms the chloride anion ${Cl}^{-}$ ‍. We can see from the chemical formula ${PbCl}_{4}$ ‍ that there are four chloride ions in the compound. The total negative charge contributed by the four chloride ions is calculated below:

$Total charge from anions = 4 \times (1$ ‍ $-$ ‍ $) = 4$ ‍ $-$ ‍

In order for the compound to be electrically neutral, the lead cation must be ${Pb}^{4 +}$ ‍. 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 ${PbCl}_{4}$ ‍ is lead(IV) chloride.

Try it: Names and formulas of ionic compounds

Problem 1

What is the name of the compound ${SrF}_{2}$ ‍?

Choose 1 answer:

(Choice A)
fluoride strontium
(Choice B)
strontium fluoride
See Also
Ionic Crystal Structures 9.1: Ionic Radii and Radius Ratios 5.5: Writing Formulas for Ionic Compounds Anion • Anion und Kation, Säurerestionen
(Choice C)
strontium fluorine
(Choice D)
sulfur fluorine

$Sr$ ‍ is the symbol for strontium, a group 2 element. Based on strontium's position on the periodic table, we predict it will form ${Sr}^{2 +}$ ‍ cations.

The name of the cation appears first in the names of ionic compounds. So, we know this compound's name will start with strontium.

$F$ ‍ is the symbol for fluorine, a group 17 element. Based on fluorine's position on the periodic table, we predict it will form $F^{-}$ ‍ anions, which are called fluoride.

The name of the anion appears second in the names of ionic compounds. So, we know this compound's name will end with fluoride.

Therefore, the name of the ionic compound ${SrF}_{2}$ ‍ is strontium fluoride.

Problem 2

What is the chemical formula for calcium bromide?

Choose 1 answer:

(Choice A)
${Ca}_{2} Br$ ‍
(Choice B)
$BrCa$ ‍
(Choice C)
${CaBr}_{2}$ ‍
(Choice D)
$CaBr$ ‍

Bromide is the name of the bromine anion. From bromine's position in group 17 of the periodic table, we know that the bromine anion has a $1 -$ ‍ charge. Therefore, the formula for bromide is ${Br}^{-}$ ‍ .

Calcium is a group 2 element. Based on calcium's position on the periodic table, we predict it will form a ${Ca}^{2 +}$ ‍ cation.

Next, we need to determine the ratio of ${Ca}^{2 +}$ ‍ and ${Br}^{-}$ ‍ in calcium bromide. These ions have unequal charges, so the ratio will NOT be 1:1.

We need one ${Ca}^{2 +}$ ‍ ion for every two ${Br}^{-}$ ‍ ions so the overall charge is zero:

$(2 +) + 2 \times (1 -) = (2 +) + (2 -) = 0$ ‍

The ratio of ${Ca}^{2 +}$ ‍ to ${Br}^{-}$ ‍ will therefore be 1:2. These values are reflected in the subscripts of the chemical formula.

So, the chemical formula for calcium bromide is ${CaBr}_{2}$ ‍.

Conclusion

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 main group ions by looking at the group numbers on the periodic table.

To name main group cations, we simply refer to them as the element's name + "ion". To name main group anions, we add the suffix -ide to the end of the element's name.

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 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.

Zumdahl, S. S., and S. A. Zumdahl. "Atomic Structure and Periodicity." In Chemistry, 290-94. 6th ed. Boston, MA: Houghton Mifflin Company, 2003.

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rishinihalani
6 months agoPosted 6 months ago. Direct link to rishinihalani's post “How do you name cations? ...”
How do you name cations? If anions have their endings replaced with, "-ide", is there a different suffix for cations?
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(9 votes)
- William Crye
  5 months agoPosted 5 months ago. Direct link to William Crye's post “Naming cations typically ...”
  Naming cations typically involves using the name of the element followed by the word "ion" or simply the name of the element itself. Unlike anions, cations do not have a specific suffix like "-ide." They are named based on the name of the element from which they are derived. For example:
  Sodium cation: Na⁺
  Calcium cation: Ca²⁺
  Aluminum cation: Al³⁺
  So, for cations, the naming convention is simpler compared to anions, and no suffix like "-ide" is used.-hope this helps!
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  (11 votes)
hemidovcingiz183
a month agoPosted a month ago. Direct link to hemidovcingiz183's post “why does not acept Ca2Br ...”
why does not acept Ca2Br but everybody knows that H2O is correct
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(3 votes)
- TheReal3A
  a month agoPosted a month ago. Direct link to TheReal3A's post “Woah! `H₂O` is actually a...”
  Woah! H₂O is actually a covalent compound, not an ionic one.
  But if you viewed it as ionic, you can say the ions are H⁺ and O²⁻. To balance out the charge the compound is H₂O. (It's more accurate to say H⁺ and OH⁻ due to the phenomenon known as the self-ionisation of water; this also balances to H₂O.)
  Anyway, for calcium bromide, the ions are Ca²⁺ and Br⁻. To balance out the charge the compound is CaBr₂.
  Takeaway: Check your charges; don't mix them up! Either/both ions may be present multiple times to balance the charge in an ionic compound.
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  (9 votes)
miadaveen
3 months agoPosted 3 months ago. Direct link to miadaveen's post “How do you name polyatomi...”
How do you name polyatomic ions? Is there a different naming system just for them, or do you just have to memorize all the different names? I noticed that some of them have the suffixes "-ite" or "-ate" at the end, (eg: Nitrite, Sulfite; Carbonate, Chlorate), but what about others like Hydroxide and Peroxide?
Does anyone know if a system exists, and if so, how it works? 🤔
Thank you for your help! 🙏
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(3 votes)
- Infrared
  3 months agoPosted 3 months ago. Direct link to Infrared's post “As for -ates versus -ites...”
  As for -ates versus -ites, my personal observation is the ate goes to the ion with more oxygens and the ite is assigned to the ion with less oxygens.
  EXAMPLE: The nitrate (NO₃⁻) has three oxygen and nitrite (NO₂⁻) has only two. Sulfate has four oxygen and sulfite has three.
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  (4 votes)
viraj_sharma
2 months agoPosted 2 months ago. Direct link to viraj_sharma's post “I dont understand the Ca ...”
I dont understand the Ca Br chemical formula example they gave us in this article. I dont understand the formula and how you need 1 Ca2+ ion for every 2 Br- ion.
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(2 votes)
- Infrared
  2 months agoPosted 2 months ago. Direct link to Infrared's post “The calcium atom needs to...”
  The calcium atom needs to lose two electrons. The bromine atoms each only can receive one electron. So, the calcium donates one to each bromine.
  Comment on Infrared's post “The calcium atom needs to...”
  (5 votes)
jaewat
a month agoPosted a month ago. Direct link to jaewat's post “Is it possible for an ele...”
Is it possible for an element to have different anions? If so how would this work when naming it.
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(2 votes)
- Milkdromeda2b
  20 days agoPosted 20 days ago. Direct link to Milkdromeda2b's post “From what I have research...”
  From what I have researched, the answer is no as all the anions have designated main groups and - this is a bit complicated- but in reality, atoms have their main shells (n=2 e.g) but those shells consist of subshells, s( smallest subshell can hold 2) and the d subshell can hold 10 electrons. Transition metals can choose to have 2 or 8 electrons on their outer shell because they are in the d-block of the periodic table! Khan Academy also has an article about this! Hope this helps!
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  (3 votes)
auspiciousfaiza
a month agoPosted a month ago. Direct link to auspiciousfaiza's post “why do we write charge in...”
why do we write charge inside the bracket and sometimes outside the bracket?
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(2 votes)
- msalikkhan
  a month agoPosted a month ago. Direct link to msalikkhan's post “Charges written inside br...”
  Charges written inside brackets in Roman numbers are only used for cations and not for anions because their charge is already indicated in their name (for example chloride is a negative chlorine ion). Charges written outside brackets can be written for both anions and cations for example {Ca}^{2+} is a cation while {Cl}^- is an anion.
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  (2 votes)
semir Alexander
23 days agoPosted 23 days ago. Direct link to semir Alexander's post “How can you tell the char...”
How can you tell the charges in the example questions?
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(2 votes)
- TheReal3A
  19 days agoPosted 19 days ago. Direct link to TheReal3A's post “Look on the periodic tabl...”
  Look on the periodic table :)
  Remember that the charge of the ion version of the element is related to the element's group number, because of valence electrons.
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  (2 votes)
lolliepop0915
6 days agoPosted 6 days ago. Direct link to lolliepop0915's post “don't we need a periodic ...”
don't we need a periodic table to see if the compound is anion or cation?
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(1 vote)
- Richard
  5 days agoPosted 5 days ago. Direct link to Richard's post “Well if it’s an ionic com...”
  Well if it’s an ionic compound, it’s always neutral. It’s composed of a cation and an anion which cancel each other’s charge.
  If mean how do we find the charge of an atom, that’s just a matter of memorization.
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  (2 votes)
Volcannon1752
3 months agoPosted 3 months ago. Direct link to Volcannon1752's post “How do you tell the charg...”
How do you tell the charges in the example questions?
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(1 vote)

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