Isotopes (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Mass Number

What is the mass number of an atom?

• The mass number (or nucleon number) is the total number of protons and neutrons in the nucleus of an atom

  • The symbol for this number is A

• The nucleon number minus the proton number gives you the number of neutrons of an atom

• Note that protons and neutrons can collectively be called nucleons

• The atomic number and mass number for every element is on the periodic table

Diagram showing the notation used on the periodic table

Isotopes

What is an isotope?

• Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons

  • This means isotopes have the same atomic number but different mass numbers

• The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number

  • So, C-14 is the isotope of carbon which contains 6 protons and 6 electrons

  • But, the 14 signifies that it has 8 neutrons (14 - 6 = 8)

    • It can also be written as ¹⁴C

• Isotopes display the same chemical characteristics

• This is because they have the same number of electrons in their outer shells, and this is what determines their chemistry

• The difference between isotopes is their neutrons, which are neutral particles that add mass only

Table to show the structure of isotopes of hydrogen

The atomic structure and symbols of the three isotopes of hydrogen

Representation

• Atoms are represented using the following notation, showing the mass number and atomic number alongside the chemical symbol:

• For example:

• From this notation, the number of protons, electrons and neutrons can be worked out:

Number of protons = atomic number

Number of electrons = atomic number (for a neutral atom)

Number of neutrons = mass number − atomic number

• For ions, the electron count must be adjusted for the charge:

  • Negative ions

    • Have gained electrons

    • Add the charge value to the electron count

  • Positive ions

    • Have lost electrons

    • Subtract the charge value from the electron count