Atoms, Elements and Compounds
A compound is made of more than one element. The elements in a compound are chemically
bonded.
Elements are substances that are made up of only one type of atom. The periodic table
organises elements into groups of similar properties.
An atom is the smallest part of an element that can exist.
Carbon dioxide is one of the products formed when bicarbonate of soda and vinegar react.
Carbon dioxide has the formula CO2.
The formula shows us that carbon dioxide is made up of two elements. Each time that you see a
capital letter, it indicates a new element. Some elements have two or three letters in their formula,
the second and third letters will be lowercase.
C O2
oxygen
carbon
Sometimes an element symbol is followed by a subscript number. This tells us there is more than
one atom of the element immediately before the number.
C O2
The subscript 2 in this formula tells us that there are two oxygen atoms in a molecule of carbon
dioxide. There is no subscript number after the C, so we know there is only one atom of carbon.
Sodium acetate is another product of this reaction. It has the formula CH3COONa.
Not all of the similar atoms are grouped together in this formula. The capital O is written twice
which means there are two oxygen atoms in a molecule of sodium acetate.
2 atoms of carbon
1 atom of sodium
C H3 C O O Na
3 atoms of hydrogen
2 atoms of oxygen
1 of 2
Atoms, Elements and Compounds Questions
Sodium bicarbonate is the chemical name for bicarbonate of soda. It is a compound with the
formula NaHCO3.
How many elements are there in sodium bicarbonate? Use the periodic table to name each of the elements in sodium bicarbonate.
How many atoms are there in sodium bicarbonate? For each element in sodium bicarbonate, say how many atoms there are.
Acetic acid is the name of the solute in a solution of vinegar. It is a compound with the formula
CH3COOH.
How many elements are there in acetic acid? Use the periodic table to name each of the elements in acetic acid.
How many atoms are in acetic acid? For each element in acetic acid, say how many atoms there are.
2 of 2