7.1) Choose the correct word from the list to match each description.
atom                     compound                element
group                    molecule                   period
a: A substance made up of only one type of atom.
b: The tiniest particle of an element that can exist.
c: A group of atoms bonded tightly together.
d: A substance made of more than one kind of atom bonded together.
e: A column in the Periodic Table.
f: A row in the Periodic Table.

7.2) aGive the symbols for the following elements.
magnesium                 oxygen                     hydrogen                  
calcium                        boron
b: Name the elements with these symbols.
C              Na            K              Cl               Si
c: Explain why scientists use symbols for the elements.
d: Explain why some symbols - for example, Cl and Si-have two letters.

7.3) a: Suggest a name for a compound containing potassium and chlorine.
b: Which elements are present in the compound aluminium sulfate?
c: Suggest a name for a compound containing calcium, carbon and oxygen.
d: What is the difference between a molecule of carbon dioxide and a molecule of carbon monoxide?

7.4) Here are the formulae of four substances.
$MgS{O_4}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{H_2}O\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{N_2}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{O_2}$
a: Name the element that occurs in three of these substances.
b: Name the element present in these substances that is a metal.
c: Give the chemical symbols for magnesium and hydrogen.
d: List the types and numbers of the atoms in one molecule of ${H_2}O$.

7.5) The following chemicals are either elements or compounds. Copy the table and tick one box for each substance, to show which it is. The first one has been done for you.

7.6) The diagrams show the atoms in four different substances. Each circle represents an atom.

Copy and complete the table, to show which diagrams represent elements and which diagrams represent compounds.