Science 8th grade
UNIT 7: Elements and compounds 7.5 Formulae
Science 8th grade
UNIT 7: Elements and compounds 7.5 Formulae
Particle models
It is easy to decide if a substance is a compound by looking at a particle diagram or model. If there are different kinds of atoms bonded together, then it is a compound.
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Carbon dioxide, water and methane are all compounds because their molecules are made up of different kinds of atoms. Oxygen is an element because the atoms in the molecule are both oxygen atoms.
Using formulae
Every compound has a chemical name. For example, the compound of sodium and chlorine is called sodium chloride.
Some compounds also have everyday names. For example, sodium chloride is also known as common salt.
Every compound also has a formula. The formula contains the symbols of the elements that are bonded together in the compound.
The table shows the chemical names and formulae of five compounds.
| What the compound contains | Formula | Chemical name |
| one calcium atom bonded to one oxygen atom | CaO | calcium oxide |
| one carbon atom bonded to two oxygen atoms | $C{O_2}$ | carbon dioxide |
| one carbon atom bonded to one oxygen atom | CO | carbon monoxide |
| two hydrogen atoms bonded to one sulfur atom | ${H_2}S$ | hydrogen sulfide |
| one calcium atom, one carbon atom and three oxygen atoms bonded together | $CaC{O_3}$ | calcium carbonate |
Be very careful reading the symbols of the elements. For example, you do not want to confuse the symbol for carbon, C, with the symbol for calcium, Ca.
The little number written below and to the right of some symbols tells you how many atoms of each element are found in one molecule of the compound. If there is no number, that means there is just one atom of that clement.
Questions
$A+I$: 1) Which of these substances are elements, and which are compounds? Explain your answer.
$K\,\,\,\,\,\,\,\,\,\,\,\,\,{O_2}\,\,\,\,\,\,\,\,\,\,\,\,\,NaCl\,\,\,\,\,\,\,\,\,\,\,\,\,Al\,\,\,\,\,\,\,\,\,\,\,\,\,Ca\,\,\,\,\,\,\,\,\,\,\,\,\,CaC{l_2}\,\,\,\,\,\,\,\,\,\,\,\,\,{H_2}$
$A+I$: 2) The formula for sulfur dioxide is $S{O_2}$.
a: How many different elements are combined in sulfur dioxide?
b: How many atoms of oxygen are combined with each atom of sulfur?
$A+I$: 3) The formula for water is ${H_2}O$.
a: Which two elements are combined in water?
b: What does the formula tell you about the numbers of cach kind of atom that are combined together?
4) The compound with the formula CO is called carbon monoxide. Suggest why it is not simply called 'carbon oxide.
$A+I$: 5) Suggest the names of the compounds with these formulae:
a: MgO
b: NaCl
c: $CaC{l_2}$
Hydroxides
There is one more kind of compound that you need to know about. These are the hydroxides.
You may remember learning about sodium hydroxide and potassium hydroxide when you studied acids and alkalis. Hydroxides are alkalis, Sodium hydroxide and potassium hydroxide are both strong alkalis. When they dissolve in water, they form solutions that can neutralise acids.
The formula for sodium hydroxide is NaOH. The formula for potassium hydroxide is KOH.
Questions
6) Which two elements are contained in all hydroxides?
7) What is the name of the compound with the formula LiOH?
$A+I$; 8) How many different elements are combined together in LiOH?