Quick Guide to Writing Chemical Formulas

Quick Guide to Writing Chemical Formulas
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A chemical formula contains information about chemical substances and helps understand what these substances are made of. Simply put, a chemical formula is like a recipe for a dish that contains the ingredients. To explain what elements participate in chemical processes and to illustrate their proportions, you can write a formula. This way, a formula serves as a symbolic representation of a certain compound. Learn more about the rules of writing chemical formulas from this guide prepared by experts from College Writers.

Chemical formulas contain numbers and letters. Every symbol represents a certain element. All the elements are made of different atoms that are connected to each other with the chemical bond. Letters and numbers represent the types of atoms and their number in elements. Chemical formulas also contain arrows, which represent chemical reactions between different elements. Two-side arrows mean that these reactions can happen both backward and forward.

An Atom: What Is It?

It’s one of the smallest units that make an element. Everything is made of something, and molecules are made of atoms. Bound together, atoms form molecules and so everything in the universe. Every substance consists of tiny atoms, and types of atoms determine the features of a substance. This makes atoms extremely important in chemistry.

How to Present Atoms in Formulas

Every element has its symbol, name, and valency. Symbols are used in equations to describe elements. A single atom is written as a symbol, such as O, which is a symbol for Oxygen. Dalton was the first scientist who used symbols for atoms. He also noted that every element has its atomic mass.

Chemical Formulas

Chemical equations illustrate the way elements interact and show numbers of atoms in different elements. Symbols also determine the types of atoms. Subscript numbers show how many atoms are in an element. For example, the formula of water shows that this substance contains two Hydrogen atoms and one atom of Oxygen: H2O.

When writing chemical formulas, you should follow a few main steps. First of all, let’s figure out how to use prefixes.

Prefixes for Numbers of Atoms











When writing formulas for acids, keep in mind that they contain an ion attached to hydrogen and therefore require a “-hydro” prefix. Covalent compounds should be written as molecular formulas because every compound is a separate molecule. In this case, you should write the first covalent compound completely and add a suffix “-ide” to the second element. For example, you should write “nitrogen dioxide.”

Using Symbols

To write chemical formulas, you should know what symbols correspond to different elements. For example, you can check the periodic table to find a symbol for the necessary element. Let’s say, you need to write a formula for lithium nitride. The first element is lithium, which corresponds to the “Li” symbol. You should also include the number of atoms in a compound: Li3N. There are many different prefixes of Greek origin that you need to memorize. For example, dioxide refers to two atoms of oxygen.

Write the second element in the same way. In the covalent compounds, this element should contain the suffix “ide,” such as Dinitrogen hexafluoride. In this case, the second element is Fluorine, and its symbol is F.

To understand the number of atoms in a compound, you can analyze the Greek prefixes. Let’s take a look at Dinitrogen hexafluoride. The prefix “hexa” corresponds to the number 6, which means that this compound contains six fluorine atoms. Although greek prefixes may seem difficult to remember, the truth is that you won’t have problems with them if you have enough practice writing chemical equations.

For example, Calcium oxide (CaO) doesn’t have prefixes in the element names so you can see that there is one atom of each element. Sulfur dioxide (SO2) contains the prefix “di,” which means that there are two atoms of oxygen. Diphosphorus pentoxide (P2O5) contains two atoms of phosphorus and five atoms of oxygen.

Formulas for Ionic Compounds

Some compounds include ions. Positive ions are called Cations, and negative ions are called Anions. When writing ionic compounds, use regular element names for positive ions and the suffix “ide” for negative ones. Unlike covalent compounds, these compounds don’t have Greek prefixes, so if you want to understand the number of atoms, you should balance the charges of different elements. For instance, Potassium sulfide includes Potassium (K), and the oxidation state of Potassium is +1. Sulphur (S) is charged negatively because it has the “ide” prefix. According to the element table, its oxidation state is -2. Therefore, the formula will look like this: K2S. Two atoms of Potassium will balance the negative charge of Sulfur.

Polyatomic Ions

Polyatomic ions are anions and cations in molecules that have several ionic groups. There’s no simple method for remembering them because they have no systematic structure. Here’s what polyatomic ions look like:

  • Ammonium: NH4+
  • Hydronium: H3+
  • Mercury (I): Hg22+

All of these ions have a +1 charge, except for mercury, where two atoms give it a 2+ charge. Other polyatomic ions have negative charges. The charge may vary from -1 to -4. For example, sulfate (SO42-) and nitrate (NO3) are charged negatively.

How to Determine the Valence Charge

You can understand the valence charge of different elements if you search for them in the element table. Elements from the group 1 have a +1 charge. Elements from the group 2 have a +2 charge. Transition elements have their charges indicated by Roman numerals in parentheses. Elements from the groups 17, 16, and 15 are charged 1-, 2-, and 3-, respectively. Silver, zink, and aluminum are charged 1+, 2+, and 3+, respectively.

When working with compounds that contain polyatomic ions, you should only use charges of complete polyatomic ions, instead of individual ions.

Balancing Charges of Ions

As we’ve already mentioned above, the number of atoms is determined by charges of elements from a polyatomic ion. To balance the charges, you should add atoms so that both elements will be charged equally. For example, Lithium oxide has two elements. Lithium is charged +1 because it’s in the group 1 of the periodic table. Oxygen is in the group 16, and its charge is 2-. To balance these charges, you should add another lithium atom so the formula will be Li2O.

Formulas for Products

A double replacement equation will have two cations and anions. Let’s say, you have a formula that looks like this: AB + XY → AY + XB. A and X are cations, while B and Y are anions. Let’s replace letters in this formula with real formulas. For example, the left side of the formula may look like this: AgNO3 + NaCl. In this case, NO3 and Cl are anions, and Ag and Na are cations. Let’s place the products on the right and the reactants on the left. Here’s the formula: AgNO3 + NaCl → AgCl + NaNO3.

In addition, both products and reactants may be in different states. There are three states of matter: solid (s), liquid (l), and gas (g). Therefore, you should also mention the state of each compound:

SnO2(s) + 2H2(g) → Sn(s) + 2H2O(g).

The more formulas you write, the easier it will be for you.

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