How to Write Ionic Compounds
Identify a binary compound., Look up the two elements on a periodic table., Find the charge of each ion., Balance the charges., Write your answer as a chemical formula., Simplify if possible.
Step-by-Step Guide
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Step 1: Identify a binary compound.
The simplest type of ionic compound is made from exactly two elements, one metal and one non-metal.
Their name is always written as two element names, plus the
-ide suffix attached to the second name.Examples of simple binary ionic compounds include potassium oxide and sodium phosphide.
If the "-ide" suffix doesn't follow a single element name, see instructions for polyatomic ions.
For example, "oxide" is a simple oxygen ion, but "hydroxide" and "peroxide" are polyatomic. -
Step 2: Look up the two elements on a periodic table.
The first word of the name refers to the metal ion (the positively charged cation).You can find this element on the left side of the periodic table.
The second word, ending in
-ide, refers to a non-metal ion (the negatively charged anion).
Find this on the right side of the periodic table .
For example, potassium oxide is a combination of potassium (chemical symbol K, atomic number 19) and oxygen (O, atomic number 8).
Notice that the
-ide ending is not part of the element name.
You're just looking for an element with the same beginning (in this case ox-). , In these simple compounds, it is easy to predict the charge of each ion.
Each element in a particular column of the periodic table always forms an ion with the same charge:
The Group 1 elements Li, Na, K, Rb, and Cs gain a charge of 1+ (written simply as +).
The Group 2 elements Mg, Ca, Sr, and Ba gain a charge of 2+.
The Group 13 element Al gains a charge of 3+.
The Group 15 elements N and P gain a charge of 3-.
The Group 16 elements O and S gain a charge of 2-.
The Group 17 elements F, Cl, Br, and I gain a charge of 1- (written as
-). , Electrical forces hold together ionic compounds, pulling the positive and negative ions together.
Taken as a whole, the ionic compound is electrically neutral, meaning it has a total charge of zero. (If it had a different charge, it would pull in another atom.) Find the number of atoms of each element that combine to "cancel out" each other's charge and make a neutral compound.For example, potassium oxide is made up of potassium ions K+{\displaystyle K^{+}} and oxygen ions O2−{\displaystyle O^{2-}}.
This means that two potassium ions (total charge 2+) balance out the charge of one oxygen ion (charge 2-).
Here's a shortcut: the first ion's charge (ignoring + or
-) tells you the number of atoms of the second ion, and vice versa.
For example, aluminum fluoride is made of Al3+{\displaystyle Al^{3+}} and F−{\displaystyle F^{-}} ions.
The charge of Al3+{\displaystyle Al^{3+}} is three, so there are three F−{\displaystyle F^{-}} atoms.
The charge of F−{\displaystyle F^{-}} is one, so there is one Al3+{\displaystyle Al^{3+}} atom. , Write the two chemical symbols in the same order they appear in the compound's name (metal, then non-metal).
After each chemical symbol, write the number of atoms as subscript (like this{\displaystyle _{like\ this}}).
If there is exactly one atom of an element, you do not need a number.
There is no need to write the charges.
For example, potassium oxide has two potassium atoms and one oxygen atom.
The chemical formula is K2O{\displaystyle K_{2}O}.
Aluminum fluoride has one aluminum atom and three fluorine atoms.
The chemical formula is AlF3{\displaystyle AlF_{3}}. , Ionic compound formulas are always written with the minimum number of atoms required.
If you could balance out the charges with fewer atoms, rewrite the formula.
This is the same process as reducing fractions.
For example, barium sulfide is made of Ba2+{\displaystyle Ba^{2+}} and S2−{\displaystyle S^{2-}} ions.
Using the shortcut above, the barium ion's charge (2) is equal to the number of sulfur ions, and the sulfur ion's charge (2) is equal to the number of barium ions.
This gives us the formula Ba2S2{\displaystyle Ba_{2}S_{2}}.
However, you don't need this many atoms to balance the charges.
Write the two numbers as a fraction and simplify: 2 barium atoms2 sulfur atoms=11{\displaystyle {\frac {2\ barium\ atoms}{2\ sulfur\ atoms}}={\frac {1}{1}}}, so the correct formula is BaS{\displaystyle BaS}. -
Step 3: Find the charge of each ion.
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Step 4: Balance the charges.
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Step 5: Write your answer as a chemical formula.
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Step 6: Simplify if possible.
Detailed Guide
The simplest type of ionic compound is made from exactly two elements, one metal and one non-metal.
Their name is always written as two element names, plus the
-ide suffix attached to the second name.Examples of simple binary ionic compounds include potassium oxide and sodium phosphide.
If the "-ide" suffix doesn't follow a single element name, see instructions for polyatomic ions.
For example, "oxide" is a simple oxygen ion, but "hydroxide" and "peroxide" are polyatomic.
The first word of the name refers to the metal ion (the positively charged cation).You can find this element on the left side of the periodic table.
The second word, ending in
-ide, refers to a non-metal ion (the negatively charged anion).
Find this on the right side of the periodic table .
For example, potassium oxide is a combination of potassium (chemical symbol K, atomic number 19) and oxygen (O, atomic number 8).
Notice that the
-ide ending is not part of the element name.
You're just looking for an element with the same beginning (in this case ox-). , In these simple compounds, it is easy to predict the charge of each ion.
Each element in a particular column of the periodic table always forms an ion with the same charge:
The Group 1 elements Li, Na, K, Rb, and Cs gain a charge of 1+ (written simply as +).
The Group 2 elements Mg, Ca, Sr, and Ba gain a charge of 2+.
The Group 13 element Al gains a charge of 3+.
The Group 15 elements N and P gain a charge of 3-.
The Group 16 elements O and S gain a charge of 2-.
The Group 17 elements F, Cl, Br, and I gain a charge of 1- (written as
-). , Electrical forces hold together ionic compounds, pulling the positive and negative ions together.
Taken as a whole, the ionic compound is electrically neutral, meaning it has a total charge of zero. (If it had a different charge, it would pull in another atom.) Find the number of atoms of each element that combine to "cancel out" each other's charge and make a neutral compound.For example, potassium oxide is made up of potassium ions K+{\displaystyle K^{+}} and oxygen ions O2−{\displaystyle O^{2-}}.
This means that two potassium ions (total charge 2+) balance out the charge of one oxygen ion (charge 2-).
Here's a shortcut: the first ion's charge (ignoring + or
-) tells you the number of atoms of the second ion, and vice versa.
For example, aluminum fluoride is made of Al3+{\displaystyle Al^{3+}} and F−{\displaystyle F^{-}} ions.
The charge of Al3+{\displaystyle Al^{3+}} is three, so there are three F−{\displaystyle F^{-}} atoms.
The charge of F−{\displaystyle F^{-}} is one, so there is one Al3+{\displaystyle Al^{3+}} atom. , Write the two chemical symbols in the same order they appear in the compound's name (metal, then non-metal).
After each chemical symbol, write the number of atoms as subscript (like this{\displaystyle _{like\ this}}).
If there is exactly one atom of an element, you do not need a number.
There is no need to write the charges.
For example, potassium oxide has two potassium atoms and one oxygen atom.
The chemical formula is K2O{\displaystyle K_{2}O}.
Aluminum fluoride has one aluminum atom and three fluorine atoms.
The chemical formula is AlF3{\displaystyle AlF_{3}}. , Ionic compound formulas are always written with the minimum number of atoms required.
If you could balance out the charges with fewer atoms, rewrite the formula.
This is the same process as reducing fractions.
For example, barium sulfide is made of Ba2+{\displaystyle Ba^{2+}} and S2−{\displaystyle S^{2-}} ions.
Using the shortcut above, the barium ion's charge (2) is equal to the number of sulfur ions, and the sulfur ion's charge (2) is equal to the number of barium ions.
This gives us the formula Ba2S2{\displaystyle Ba_{2}S_{2}}.
However, you don't need this many atoms to balance the charges.
Write the two numbers as a fraction and simplify: 2 barium atoms2 sulfur atoms=11{\displaystyle {\frac {2\ barium\ atoms}{2\ sulfur\ atoms}}={\frac {1}{1}}}, so the correct formula is BaS{\displaystyle BaS}.
About the Author
Harold Baker
Harold Baker has dedicated 5 years to mastering media and communications. As a content creator, Harold focuses on providing actionable tips and step-by-step guides.
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