How to Write Chemical Formulas

A chemical formula is something like a recipe that contains different ingredients and makes an item. For example, Cocoa Butter, Chocolate Liquor, Sugar, Lecithin and a flavoring agent makes a delicious item called Chocolate. Similarly, a compound is made of several elements and an element is made of atoms.

So if you want to write a compound that shows its elements and their proportions, you have to write it in a formula. So it’s like a symbolic presentation of a compound using the letters and numbers.

Each symbol represents an element, and an element is made of atoms bound together by the chemical bond. The letters represent the element and the number represents the number of atoms of an element.

The formulas also contain the arrows, where one-way arrow denotes the reaction of an element towards the other element. If there is an arrow showing two sides, it means the reaction can happen in any of the forward or backward. The lines and symbols are used to from diagrams.

What is an atom?

It’s the smallest or the single unit of that makes an element collectively. To understand an atom, take an example of a wall. A wall is made of bricks. So if you consider a wall as a compound, a brick can be considered as an atom. The bricks, bound by the mixture of cement and sand, make a wall.

Basically, everything in the universe is made of atoms. Like there can’t be a wall without the bricks, there can’t be anything without the atoms. So an atom is the smallest component of a chemical compound, it is an important part of it.

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How atom is presented in a formula

An element generally has three things, that is its name, its valency, and a symbol. And a symbol is used in writing an equation of a chemical compound as it refers to that particular element. In an equation or a formula, a single atom is denoted by a symbol, i.e. A1, which is an Aluminium atom.

Each element is made of an atomic mass and this was theorized by Dalton. He was the first scientist who started using symbols for a chemical element.

What is a chemical formula?

A Chemical equation or formula, as described at the beginning of the article, is a symbolic way of displaying the elements and the number of atoms in an element. The symbol determines the type of atom or the element.

In order to show how many atoms an element is having in a formula, we have to use the number in subscript. For example, sulfate formula is SO42-. To make it simple, if you have to show Hydrogen, you can use H. If you have to write the formula of water which is made of two atoms of Hydrogen and one atom of Oxygen, you have to write it as H2O. Here H2 represents two atoms of Hydrogen and O represents one atom of Oxygen in the formula.

How to write a formula

There are some crucial steps and rules that you have to consider to write a formula. They are:

Prefixes to write the number of atoms

The number of atoms for each element in the formulas is written using Greek prefixes in naming compounds. There are 1 to 10 prefixes, i.e.;

  1. Mono-
  2. Di-
  3. Tri-
  4. Tetra-
  5. Penta-
  6. Hexa-
  7. Hepta-
  8. Octa-
  9. Nona-
  10. Deca-

The acids contain one ion attached to hydrogen, so naming acids requires “hydro-” as a prefix.

When there are covalent compounds, they are to be written as molecular formulas. This is because each compound is a separate molecule. The first element of a covalent compound is written completely, but the second element is written with a suffix “ide” in the end. For example, nitrogen dioxide.

Write the first element in a symbolic way

You should be knowing the symbols for the elements when you are writing a compound. You can refer the periodic table to find out the symbol of an element, for example, Br element is a symbol for Bromine. When you write the first element, it is like the first name of the compound.

For example, if you are writing the name of Lithium nitride (Li3N), the first element is Lithium. The chemical symbol of this element is Li.

Now to show the number of atoms in the equation of Lithium nitride, you have to use a number in subscript, i.e. Li3. You can identify the number of atoms in a compound, you have to memorize the Greek prefixes. If you memorize the prefixes, you can know the number of atoms, i.e. Dioxide means 2 atoms of Oxygen.

Write the second element

Like the first element, that can be called a first name of the compound, the second element can be called the last name. The element name will end with the suffix ide for the covalent compounds. For example Dinitrogen hexafluoride. In this compound, the second element is Fluorine and the chemical symbol of this element is F.

Same as the first element, you can identify the number of an atom from the second element name if you memorize the Greek prefixes. For example, if you are writing a formula for Dinitrogen hexafluoride, you can write the number of atoms for Fluorine by checking its Greek prefix.

The second element in this compound is hexafluoride, and the prefix is hexa. In Greek, hexa means 6, so this compound contains 6 atoms of fluorine. To write this element in a symbolic way, you can write it as F6.

This way, by writing the first and the second element using their symbols and number of their atoms, the complete chemical formula of Dinitrogen hexafluoride is N2F6.

This may seem complex and difficult initially. You may also have issues with memorizing the Greek prefixes. But as you practice more writing the formulas, you will become familiar with the element symbols and prefixes and how to write them. Here are some more examples of chemical equations. Check them to improve your understanding of the language of chemistry.

Compound: CaO – Calcium oxide

Note that both calcium and oxygen have one atom each. You can see that there is no prefix in the element names. Also, note that there is no subscript as both elements have single atoms in this compound.

Compound: SO2 – Sulfur dioxide

This compound is made of two elements, i.e. Sulfur and Oxygen. Note that the first element i.e. Sulfur is having one atom and the second element, i.e. oxygen, is having two atoms. You can see Di, which means two in Greek. Note how the number of atoms is written.

Compound: P2O5 Diphosphorus pentoxide

This compound is made of Phosphorus and Oxygen. You can see that both the elements are having Greek prefixes, i.e. Di in Diphosphorus and Pent in Pentoxide. In Greek, Di means two and Penta means 5. So this component is made with two atoms of phosphorus and five atoms of oxygen. The number of atoms of both elements is written in subscript.

How to write formulas for ionic compounds

Some compounds have ions. Ions are of two types, i.e. positive ions which are called Cations and the negative ions which are called Anions.

Chemical symbols for cations and anions

As you know from the previous sections of this article, the chemical compounds have elements which can be called first name and last name. Same way, in ionic compounds, there are two types of ions, i.e. cations and anions. In the ionic formulas, the cations are written by the element name while the anions are written with “ide” as the suffix at the end of the element name.

If we talk about the ions, the cations are the first names. They are positive ions and are written first. The anions are called the last names. They are negative ions and are written as the second element in a compound.

In ionic compounds, the Greek prefixes are not used like covalent compounds. So you can’t say how many atoms an element contains in an ionic compound. In this type compounds, in order to know how many atoms are there, you have to balance the charges of elements.

For example Potassium sulfide which is written as K2S.

This compound is made of Potassium and Sulfur. The symbol of Potassium as per the element table is K. It is in the first column of the table, so its oxidation state is +1. Sulfur is written with “ide” as it is the second element in the compound. Its symbol as per the element table is S and the oxidation state is -2.

In the ionic compound, the total positive charge should be equal to the total negative charge. So for this example, we need 2 atoms of potassium (K). It will balance the negative charge of Sulfur which is -2. We can write these all together as K2S.

Understanding polyatomic ions

Sometimes the formulas for molecules have two or more atoms with ionic groups. Such cations or anions are called as polyatomic ions. This type of ions are not having any systematic structure, so you can’t remember them with any trick. You just have to memorize them.

If we talk about the cation polyatomic ions, three are only three. They are as below.

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

All of these three ions contains a +1 charge. But mercury have 2 atoms bonded together and they create 2+ charge and each mercury cation contains a 1+ charge.

All the remaining polyatomic ions contain negative charges. They are ranging from -1 to -4, for example, nitrate (NO3-) or NO3 Charge that contains a charge of -1. Another good example of negative charges is sulfate charge is -2, i.e. (SO42-).

Determining the valence charge

You can determine the valence charge of an element by looking at its position in the element table. There are also some rules which can help you identify the valence charge if you keep in mind. They are;

  • All the elements of group 1 have a charge of +1
  • All the elements of group 2 have a charge of +2
  • The charge of transition elements is indicated by Roman numerals in parenthesis.
  • The charge of silver is 1+, zinc has a 2+ and aluminum has a 3+ charge.
  • All the elements of group 17 have a charge of 1-
  • All the elements of group 16 have a charge of 2-
  • All the elements of group 15 have a charge of 3-

Keep in mind that you should use the charge of a complete polyatomic ion instead of the individual ions when you are working with a compound with polyatomic ions.

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Balancing the positive and negative charges of the ions

As you know now that the number of atoms of each element present in a compound is determined by the charge of each element or polyatomic ion. In order to balance the charges, you have equal them for both the elements of a compound by adding the atoms.

Take Lithium Oxide for example. This compound contains lithium and oxygen. But lithium is from group 1 and hence have a +1 charge while oxygen is from group 16 and have a 2- charge. This has to be balanced, so you will need 2 atoms of lithium. This will balance 2- charge of oxygen and the formula of this compound is Li2O.

How to write formulas for product given reactants

There will be 2 cations and anions in a double replacement equation. For example AB + CD →AD + CB. Here cations are A and C and anions are B and D. Consider a real formula for an example, i.e. AgNO3 + NaCl, here Ag and Na are cations and NO3 and CI are anions.

Now the switch of the ions in the above formula, so it will look like AgCI + NaNO3. In this switch, the first cation and second anion are making a pair, and second cation and first anion are making a pair.

By putting the reactants on left and products on the right in the equation makes a full equation.

For example:  AgNO3 + NaCl → AgCI + NaNO3.

You should also mention the state of matter for both reactants and products in parenthesis. In a formula, (s) indicates solid, (l) indicates liquid and (g) indicates gas. You should mention the state after each element as shown in the example below.

SnO2(s) + 2 H2(g) → Sn(s) + 2 H2O(g)

Now you should be familiar enough to write chemical formulas. However, you can refer to more examples online and practice by writing more formulas yourself.

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