# How to write a chemical equation (with pictures)

A good way to think of a chemical reaction is the process of baking cookies. You mix the ingredients (flour, butter, salt, sugar and eggs), you bake them and you see that they become something new: cookies. In chemical terms, the equation is the recipe, the ingredients are the "reactants" and the cookies are the "products." All chemical equations look similar to "A + B C (+ D..)", where each variable is an element or a molecule (a collection of atoms linked by chemical bonds). The arrow represents the reaction or change that takes place. In order to write the equations, there are a number of important rules that you should know.

## Steps

### Part 1 of 3: Writing Chemical Formulas of Covalent Compounds

#### Step 1. Memorize the prefixes for the number of atoms

When naming compounds, the Greek prefixes are used to indicate the number of atoms present in each element. In covalent compounds, the first element is fully written while the second element is suffixed with "uro" (or "gone" when the element is oxygen). However, when you type the compound name, this order is reversed and the second element is written first. For example, diphosphorus trisulfide has a chemical formula of P2S3. Here are the prefixes for the numbers of atoms from 1 to 10:

• 1: mono-
• 2: di-
• 3: tri-
• 4: tetra-
• 5: penta-
• 6: hexa-
• 7: hepta-
• 8: octa-
• 9: nona-
• 10: deca-

#### Step 2. Write the chemical symbol for the first element

After writing the name of a compound, you must identify the elements that compose it and know their chemical symbols. The first element that is written in the formula is the "middle name" of the compound. Use the periodic table to find the chemical symbol for the element.

### For example: dinitrogen hexafluoride. The first element is nitrogen and the chemical symbol for nitrogen is N

#### Step 3. Add the number of atoms as a subscript

To identify the number of atoms present in each element, you simply have to look at the element's prefix. Memorizing Greek prefixes will help you write chemical formulas quickly without having to search for anything.

• For example: dinitrogen has the prefix "di", which means 2. Therefore, there are two nitrogen atoms present.
• Write "dinitrogen" as N2.

#### Step 4. Write the chemical symbol for the second element

The second element is the "first name" of the compound and will follow the first element. For covalent compounds, the name of the element will have the suffix "uro" (or "gone" in case the element is oxygen) instead of the usual ending of the element.

### For example: dinitrogen hexafluoride. The second element is fluoride. Since "fluoride" already ends in "uro", just leave it as is. The chemical symbol for fluoride is F

#### Step 5. Add the number of atoms present as a subscript

As you did with the first element, identify the number of atoms present in the second element by reading the prefix. Using this prefix, write the number of atoms as a subscript to the right of the chemical symbol.

• For example: hexafluoride has the prefix "hexa", which means 6. Therefore, there are 6 fluoride atoms present.
• Write "hexafluoride" as F6.
• The final chemical formula for dinitrogen hexafluoride is N2F6.

#### Step 6. Practice with some examples

When you are just starting to learn chemistry, there is a lot of memorization involved. It is a bit like learning a new language. The more examples you use to practice, the easier it will be to figure out chemical formulas in the future and learn the language of chemistry.

• Sulfur dioxide: SO2
• Carbon tetrabromide: CBr4
• Diphosphorus pentoxide: P2OR5

### Part 2 of 3: Writing Chemical Formulas of Ionic Compounds

#### Step 1. Identify the chemical symbols for cations and anions

All chemicals have what can be called a first and last name. The first name is the anion (negative ion) while the last name is the cation (positive ion). Cations are written as the name of the element while anions are the name of the element with the suffix "uro" (or "gone" in the case of ions containing oxygen).

• You can find the chemical symbol for each element on the periodic table.
• Unlike covalent compounds, Greek prefixes are not used to indicate the number of atoms in each element. You have to balance the charges of the elements to determine the atoms.
• For example, lithium oxide is Li2OR.

#### Step 2. Recognize the polyatomic ions

Sometimes the cation or anion is a polyatomic ion. These are molecules that have two or more atoms with ionic groups. There is no good trick to remembering them, you just have to memorize them.

• There are only 3 polyatomic cations and they are ammonium (NH4+), hydronium (H3+) and mercury (I) (Hg22+). They all have a +1 charge.
• The rest of the polyatomic ions have negative charges ranging from -1 to -4. Some common ions are carbonate (CO32-), sulfate (SO42-), nitrate (NO3-) and chromate (CrO42-).

#### Step 3. Determine the valence charge of each element

The valence charge can be determined by looking at the position of the element on the periodic table. There are a few rules to keep in mind to help you identify loads:

• All elements in group 1 have a charge of +1.
• All elements in group 2 have a charge of +2.
• Transition elements will have Roman numerals to indicate load.
• Silver's charge is +1, zinc's is +2, and aluminum's is +3.
• The elements of group 17 have a charge of -1.
• Group 16 elements have a charge of -2.
• Group 15 elements have a charge of -3.
• Remember: when working with polyatomic ions, you only use the charge of the ion.

#### Step 4. Balance the positive and negative charges of the ions

Once you have identified the charge of each element (or polyatomic ion), you will use these charges to determine the number of atoms present in each element. The charge on the compound must equal zero, so you will add atoms to balance the charges.

• For example: lithium oxide. Lithium is a Group 1 element and has a charge of +1. Oxygen is a group 16 element and has a charge of -2. In order to balance the -2 charge of oxygen, you need two lithium atoms. Therefore, the chemical formula of lithium oxide is Li2OR.

#### Step 5. Practice with some examples

The best way to learn how to write the formulas is by practicing with lots of examples. Use examples from your chemistry book or find practice exercises online. Do as many as you can until you feel comfortable writing chemical formulas.

• Calcium Nitride - The symbol for calcium is Ca and the symbol for nitrogen is N. Calcium is a Group 2 element and has a charge of +2. Nitrogen is a group 15 element and has a charge of -3. To balance it, you need 3 calcium atoms (+6) and 2 nitrogen atoms (-6): Ca3N2.
• Mercury (II) phosphate: the symbol for mercury is Hg and phosphate is the polyatomic ion PO4. Mercury has a charge of +2 as indicated by the Roman numeral II next to it. Phosphate has a charge of -3. In order to balance them, you will need 3 mercury atoms (+6) and 2 phosphate molecules (-6): Hg3(PO4)2.

### Part 3 of 3: Determine the Products Given the Reactants

#### Step 1. Identify all the cations and anions in the reagents

In a basic double replacement equation, you will have two cations and two anions. The general equation takes the form of AB + CD AD + CB, where A and C are cations and B and D are anions. You must also determine the charges of each ion.

• For example: AgNO3 + NaCl?
• The cations are Ag+1 and Na1-. Anions are NO31- and Cl1-.

#### Step 2. Change the anions to develop the products

Once you have identified all the anions and their charges, rearrange them so that the first cation is now paired with the second anion and the second cation is now paired with the first anion. Remember the equation: AB + CD AD + CB.

• Remember to balance charges when forming new compounds.
• For example: AgNO3 + NaCl?
• Ag+1 now pairs with Cl1- to form AgCl.
• Na1- now pairs with NO31- to form NaNO3.

#### Step 3. Write the complete equation

After writing the products that will be formed in the equation, you can write the complete equation with products and reactants. Keep the reactants on the left side of the equation and write the new products on the right side with a + sign between them.

• For example: AgNO3 + NaCl?
• AgNO3 + NaCl AgCl + NaNO3

#### Step 4. Balance the equation

Once you have written the equation and have all the products and reactants, you need to make sure that everything is balanced. An equation is balanced only when you have the same number of atoms of each element on both sides.

• For example: AgNO3 + NaCl AgCl + NaNO3
• Count the number of atoms on each side: 1 Ag on the left side, 1 Ag on the right side; 1 N from the left side, 1 N from the right side; 3 O on the left side, 3 O on the right side; 1 Na from the left side, 1 Na from the right side; 1 Cl from the left side, 1 Cl from the right side.
• This equation is balanced because there are an equal number of atoms on both the left and the right of the equation.