The relative atomic mass is the mean mass of an atom compared to 1/12 of the mass of a carbon-12 atom.
The relative formula mass (RFM or Mr) is the sum of the relative atomic masses of all the atoms in a chemical formula.
%mass of an element in a compound = (relative mass of element in compound ÷ RFM of compound) × 100

Common misconception

Pupils may use the atomic number rather than mass when calculating the relative formula mass. Also, the number of atoms can be missed for the percentage of an element in a compound.

Emphasise that atomic mass is the larger number for each element on the periodic table and encourage pupils to use the key present to help them identify the mass.

Keywords

Relative atomic mass - RAM is the mean mass of an atom relative to 1/12th the mass of an atom of carbon–12.
Relative formula mass - (RFM or Mr) is the sum of the relative atomic masses of all the atoms in a chemical formula.

Model many worked examples, especially with compounds containing multiple atoms of particular elements.

Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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6 Questions

Q1.

Which statement accurately describes the atomic number of any given element?

the atomic number represents the number of protons + neutrons

the atomic number represents the number of neutrons

the atomic number represents the number of protons + electrons

Correct answer: the atomic number represents the number of protons

Q2.

True or false: The atomic mass number of an element is listed on the periodic table.

True

Correct answer: False

Q3.

Match each term with its correct definition.

Correct Answer:Atomic Number,the number of protons in the nucleus of an atom

the number of protons in the nucleus of an atom

Correct Answer:Atomic Mass Number,sum of the number of protons and neutrons in the nucleus of an atom

sum of the number of protons and neutrons in the nucleus of an atom

Correct Answer:Relative Atomic Mass,average mass of an element's isotopes, based on their abundances

average mass of an element's isotopes, based on their abundances

Q4.

Isotopes of the same element have the same atomic number because they contain the same number of protons. However, they have different relative atomic masses due to varying numbers of .

Correct Answer: neutrons, Neutrons

Q5.

If an element has an atomic number of 8 and an average atomic mass of 16, which element is it?

Correct Answer: oxygen, O, o, Oxygen

Q6.

The number is always a whole number because it represents the fixed number of protons in the nucleus of an atom.

Correct Answer: atomic, Atomic

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6 Questions

Q1.

What does the relative atomic mass (RAM or Ar) compare the mean mass of an atom to?

1/12 of the mass of an oxygen-16 atom

the mass of an electron

Correct answer: 1/12 of the mass of a carbon-12 atom

the mass of a proton

Q2.

Order the following steps to calculate the RFM of a compound correctly. Start withe periodic table.

1 - use the periodic table to find the relative atomic masses of each type of atom

2 - identify the number of each type of atom in the formula

3 - calculate the number of atoms × the relative atomic mass value for each element

4 - add up the total relative atomic masses of all the atoms

Q3.

What is the relative formula mass (RFM or Mr) of water (H₂O)?

Correct answer: 18

Q4.

Calculate the RFM of carbon dioxide (CO₂)

Correct Answer: 44

Q5.

To calculate the percentage by mass of an element in a compound, you need:

Correct answer: the RFM of the compound and the RAM of the element

the atomic number of the element

the total number of electrons in the compound

the number of protons in the element

Q6.

What is the percentage by mass of hydrogen in water (H₂O)? Give your answer to four significant figures.

Correct Answer: 11.11 %, 11.11