Chemistry Foundations

4.3.2 - Displayed Formulae and Molecular Formulae

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Displayed Formulae and Molecular Formulae

Earlier in the course, we learnt that we can use chemical formulae to represent particles, including atoms, ions and subatomic particles.

For example, the chemical formula for a sodium atom is:

Na

and the chemical formula for an oxide ion is:

O2-

We can also use chemical formulae to represent molecules. There are multiple different kinds of chemical formulae that can be used to represent molecules. On this page, we will learn about two of them: displayed formulae and molecular formulae.

A displayed formula is a drawing showing all the atoms and bonds

A displayed formula is drawing of a molecule in which chemical symbols are used to represent the atoms and lines are used to represent the covalent bonds.

For example, the image below is the displayed formula for a molecule called ethene:

Displayed formula of an ethene molecule. The atoms are represented by their chemical symbols and the bonds are represented by lines. There are two carbon atoms joined by a double bond (two lines). Each carbon atom is bonded to two hydrogen atoms, making four hydrogen atoms in total.

The displayed formula of an ethene molecule.

From the displayed formula, we can see that an ethene molecule has two carbon atoms joined by a double covalent bond, and each carbon atom is also joined to two hydrogen atoms by single covalent bonds.

In a displayed formula, every single atom and every single bond must be shown. For example, if a molecule has a carbon atom bonded to an oxygen atom bonded to a hydrogen atom, it must be written C-O-H. It is not enough to just write C-OH.

A molecular formula just shows how many atoms of each element there are

Unlike a displayed formula, a molecular formula does not show any of the bonds in the molecule. Instead it simply shows which elements are present and how many atoms of each element there are.

For example, the molecular formula for an ethene molecule is:

C2H4

Each element is represented by its chemical symbol (C for carbon and H for hydrogen). After each chemical symbol, a number is written to show how many atoms of that element are present (2 carbon atoms and 4 hydrogen atoms). Note that this number is written in small writing, low down on the line like this (this is called subscript).

If there is only one atom of a particular element present, then we do not write the number 1 after its chemical symbol. We simply write the chemical symbol with no number after it. For example, the molecule ammonia is made up of 1 nitrogen atom and 3 hydrogen atoms and its molecular formula is:

NH3

The fact that there is no number after the N means that there is one nitrogen atom in the molecule.

Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.

1/3

What is a displayed formula?

2/3

What is a molecular formula?

3/3

In a molecular formula, how do you show that only one atom of an element is present?

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