# Introduction to chemical formulae

### Introduction to chemical formulae

#### Lessons

• A chemical symbol is a one or two letter symbol used to refer to any element listed in the Periodic Table.

• They are used not just because they are 'shortcut' ways of writing the element’s name, but because the chemical symbol of separate elements can be combined to make chemical formulae.

• Chemical formulae describe more complicated chemicals called molecules, which are chemicals made of more than one atom combined together, and compounds, which are chemicals made of more than one type of atom (elements!) combined together.

• Chemical formulae for chemical compounds obey Proust’s law, or the law of definite proportions: A chemical formula shows the ratio of each element (measured by mass) in a chemical compound. This ratio is always true regardless of how the chemical was prepared.

• The advantage of chemical formulae over written names of chemicals is that they can specify the exact number of each atom in that chemical, as a ratio of all the elements it’s made of. For example: magnesium hydroxide, Mg(OH)$_2$ and sodium hydroxide, NaOH. Here, the chemical formulae clarifies the ‘hydroxide’ term as the first compound has two hydroxide groups, and the second has only one.

• Be careful with capital letters in chemical formulae. The second letter of chemical symbols are always lower case, so a capital letter always shows a new element symbol. For example: CO and Co are two completely different chemicals. CO is a compound of carbon (C) and oxygen (O) called carbon monoxide, whereas Co is symbol for the metal element Cobalt.

• Numbers in $_{subscript}$ are used to specify how many of the atom are present in a molecule of the chemical. Do not confuse this with $^{superscript}$ which is not used in chemical formulae. For example: Na$_2$O is a compound that is made of two atoms of sodium and one atom of oxygen.

Brackets ( ) followed by numbers in chemical formulae are used to show that every atom contained in the brackets is present in that quantity. For example: the chemical formula Mg(OH)$_2$ shows that, as a ratio, this compound contains two of both O and H atoms for every one Mg atom.

• If there is no number in $_{subscript}$ written after a chemical formula, it means that only one of that type of atom is present in this chemical.

• When writing simpler chemical formulae, the metal (and hydrogen) atoms are normally written first, followed by non-metal atoms or groups of atoms in a formulae.

In this lesson, we will learn:
• What chemical symbols and formulae are, and how to read them correctly.
• Why they are used in chemistry and their advantage.
• How they are used to describe more complex chemicals.
• 1.
Introduction to Chemical formulae
a)
What is a chemical formula?

b)

c)
Why we use chemical formulae (Law of Definite Proportions).

d)
Mistakes to avoid when using chemical formulae.

• 2.
Recall how to read molecular formulae of simple compounds
Study the following chemical formulae:
HCl$,$H$_2$SO$_4,$HNO$_3.$H$_3$PO$_4. ,$Ca$_3$(PO$_4$)$_2$
For each chemical formula, identify the chemical elements it shows and the number of atoms of each element in the compound.

• 3.
Apply your knowledge to write the chemical formulae of simple compounds and molecules.
Write the chemical formulae for:
a)
Oxygen gas, a chemical molecule made of two oxygen atoms.

b)
Hydrogen gas, a chemical molecule made of two hydrogen atoms.

c)
Hydrochloric acid, a compound made of one atom of hydrogen and one atom of chlorine.

d)
Water, a compound made of two atoms of hydrogen and one atom of oxygen.

e)
Sodium Chloride, a compound made of one atom of sodium and one atom of chlorine.

f)
Potassium hydroxide, a compound of a potassium atom chemically combined with a hydroxide ‘ion’ which has the formula ‘OH’