A very common mistake is to confuse the relative atomic mass with the mass number.
The relative atomic mass is the mean mass of 1 atom of an element on a scale in which one atom of 12C has a mass of exactly 12. It is a weighted average mass that takes into account the abundance and mass of all isotopes of an element. The relative atomic mass is, therefore, never an exact whole number.
Even the relative atomic mass of C is not exactly 12. This is because a very small amount (~1%) of C is 13C and other isotopes, the high resolution relative atomic mass of C is 12.011…
In contrast, the mass number of an atom is simply the number of protons plus the number of neutrons. It must always be a whole number as it relates simply the number of particles.
You will remember from GCSE that:
Atomic Number = Number of Protons
Mass Number = Number of Protons + Number of Neutrons
Charge = Number of Protons – Number of Electrons
This set of relationships means that if we know some of these quantities, we can usually work out the rest, and questions may need you to do this. The calculations are usually straightforward (it is just adding or subtracting whole numbers!) but it is easy to make a mistake if you rush this, especially when dealing with negative numbers and subtractions. Also, don’t forget that you’ll need to use the periodic table to find the atomic number of each element.
Here are a few examples:
Question 1: An atom has 10 protons, 12 neutrons and 10 electrons. Determine its atomic number, mass number and charge.
Atomic Number = 10 (i.e., Ne)
Mass Number = 10 + 12 = 22
Charge = 10 – 10 = 0
Question 2: A nitrogen ion has a mass number of 15 and a charge -3. Determine the number of protons, neutrons and electrons.
Atomic Number = 7 (i.e., N)
Number of Protons = 7
Number of Neutrons = 15 – 7 = 8
Number of Electrons = 7 – (-3) = 10
Question 3: An ion has a charge of -1, 18 electrons and a mass number of 35. Determine the atomic number, the number of protons and the number of neutrons
Number of Protons = 18 – 1 = 17 (i.e., Cl)
Atomic Number = 17
Number of Neutrons = 35 – 17 = 18
Question 4: An ion has the same number of protons and neutrons, 10 electrons and a charge of -2. Determine the atomic number, the number of protons, the number of neutrons and the mass number.
Number of Protons = 10 – 2 = 8
Atomic Number = 8 (i.e., O)
Number of Neutrons = 8
Mass number = 16