Compounds and formulae
In this topic you will:
- learn about the differences between elements and compounds
- learn how to name compounds
- use symbols to represent compounds.
Key words
- bonding
- compound
- formula
- sodium chloride
What is a compound?
You have looked at elements in the Periodic Table. An element is made up of only one type of atom. Many substances are made up of more than one type of atom. If the different types of atom are joined tightly together, then the substance is a compound.
The chemical term for two atoms joining tightly together is bonding. In a compound, two or more different kinds of atom are bonded. For example, when sodium atoms bond with chlorine atoms, they form the compound sodium chloride.
Properties of elements and compounds
A compound is very different from the elements from which it is made. When two different elements are bonded, they completely lose the properties of the individual elements. The compound has totally new properties.
The first two photographs show the two elements sodium and chlorine. The third photograph shows the compound that is made when sodium and chlorine atoms bond together. This compound – sodium chloride – is not at all like either sodium or chlorine.
You may have eaten some sodium chloride today. Sodium chloride is common salt. You would not want to eat any sodium or chlorine, though.
Important Concept: Compound
A compound is a substance made when two or more different types of atoms are chemically bonded together. The properties of a compound are usually very different from the properties of the elements that make it.
Common Mistake
Some students think that a compound is just a mixture of elements. This is incorrect — in a compound, the atoms are chemically bonded, not just mixed.
Quick Check
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Because when sodium and chlorine atoms bond, they form a compound with new properties that are very different from the original elements. Sodium chloride is not reactive like sodium or poisonous like chlorine.
Questions
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- It is powdery
- It is not a metal
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- It is semi-solid
- It is not yellow
Naming compounds
Each compound has a chemical name. The chemical name usually tells you the elements that the compound is made from.
There are important rules to remember when naming compounds:
- If the compound contains a metal, then the name of the metal comes first in the name of the compound.
- If the compound contains a non-metal, the name of the non-metal is usually changed. For example, the compound made from sodium (a metal) and chlorine (a non-metal) is not sodium chlorine, but sodium chloride.
- When two elements form a compound the name often ends in ‘ide’.
Naming Rule
In compounds, the metal is named first and the non-metal's name is changed to end in “ide”. For example: sodium + chlorine = sodium chloride.
Quick Check
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Magnesium oxide
Questions
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Sodium and chlorine
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Hydrogen and sulfur
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Magnesium and oxygen
sulfur calcium
What is wrong with this name? Write the correct name for the compound.
Copper sulfate is a compound made up of copper, sulfur and oxygen.
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The metal name should come first. The non-metal name should be changed. Correct name: calcium sulfide.Questions
Some compounds contain two different elements, plus a third element – oxygen. These compounds often have names ending with ‘ate’. For example, a compound of calcium, carbon and oxygen is called calcium carbonate.
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Calcium, nitrogen, oxygen
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Magnesium, carbon, oxygen
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Lithium, sulfur, oxygen
Sometimes, the name of a compound tells you how many of each kind of atom are bonded together.
Carbon dioxide particles are made up of one carbon atom joined to two oxygen atoms. ‘Di’ means two.
Carbon monoxide particles are made up of one carbon atom joined to one oxygen atom. ‘Mon’ or ‘mono’ means one.
Particle diagrams
Particle diagrams, like those for carbon dioxide and carbon monoxide, show which atoms of which elements make up the particle.
It is easy to decide if a substance is a compound by looking at the particle diagram. If there are different kinds of atom bonded together, then it is a compound.
Carbon dioxide, water and methane are all compounds because their particles are made up of different kinds of atom. Oxygen is an element because the atoms in the particle are both oxygen atoms.
Important Concept: Compound or Element?
A compound is made of different types of atom joined together. An element contains only one type of atom. Particle diagrams help show this clearly.
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The oxygen (O₂) diagram represents an element because it contains only one type of atom: oxygen.
Using formulae
Every compound has a chemical name. For example, the compound of sodium and chlorine is sodium chloride. Some compounds also have an everyday name. 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 six compounds.
| Chemical name | Formula | What the compound contains |
|---|---|---|
| calcium oxide | CaO | one calcium atom bonded with one oxygen atom |
| carbon dioxide | CO₂ | one carbon atom bonded with two oxygen atoms |
| carbon monoxide | CO | one carbon atom bonded with one oxygen atom |
| hydrogen sulfide | H₂S | two hydrogen atoms bonded with one sulfur atom |
| calcium carbonate | CaCO₃ | one calcium atom, one carbon atom and three oxygen atoms bonded together |
| sodium hydroxide | NaOH | one atom of sodium, one atom of oxygen and one atom of hydrogen bonded together |
Be very careful reading the symbols of the elements. 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 the particle of the compound. If there is no number, it means there is just one atom of that element.
Understanding Subscripts in Formulas
The small number written below a symbol (like the ₂ in CO₂) tells you how many atoms of that element are in the molecule. No subscript means just one atom.
Quick Check
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Three oxygen atoms
Questions
K, O₂, NaCl, Al, Ca, CaCl₂, H₂
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Elements: K, O₂, Al, Ca, H₂
Compounds: NaCl, CaCl₂
Elements contain only one type of atom. Compounds are made of different types of atoms joined together.
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2 elements — sulfur and oxygen
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2 oxygen atoms
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Hydrogen and oxygen
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2 hydrogen atoms and 1 oxygen atom
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The “mono” prefix tells us there is only one oxygen atom. “Carbon oxide” would be unclear, since it doesn’t show the ratio of atoms.
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Magnesium oxide
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Sodium chloride
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Calcium chloride
Which two elements are do you think contained in all hydroxides?
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Oxygen and hydrogen
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Lithium hydroxide
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Three elements: lithium, oxygen and hydrogen