Isotopes & Mass Number
Chemistry: AQA A Level, topic 1.1.2
Mass Number and Proton Number
An atom can be identified by its mass number and its proton number.
Mass number
- The mass number of an atom is the number of protons plus the number of neutrons.
- It is often given the symbol A.
Proton number
- The proton number of an atom is the number of protons in its nucleus.
- It is often given the symbol Z.
Ion Formation
Atoms can have electrons added or removed. This generates ions.
Positive ions
- A positive ion has more protons than electrons.
- Positive ions are generated by removing electrons from a neutral atom.
- Positive ions are often called 'cations'.
- This term comes from electrochemistry - cations are attracted to the cathode.
Negative ions
- A negative ion has fewer protons than electrons.
- Negative ions are generated by adding electrons to a neutral atom.
- Negative ions are often called 'anions'.
- This term comes from electrochemistry - anions are attracted to the anode.
Calculating charge
- Ion charge = number of protons − number of electrons.
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Overall charge
- The overall charge of any chemical substance will be zero. So cations will always be accompanied by anions in solid or in solution.
Isotopes
Atoms are defined as having a particular proton number. But this definition says nothing about the mass number.
Isotopes
- Isotopes are atoms of the same element that have the same proton number, but a different neutron number. This means that the atoms have different numbers of neutrons.
- Important examples of different isotopes are the radioactive 40K and the stable 39K.
- 40K is the reason bananas are slightly radioactive.
- The superscripted numbers indicate the mass number of the element.
Effect on chemistry
- Chemistry is about the largely electrostatic interactions between nuclei and electrons.
- Neutrons have no electric charge and so changing the mass number of a species very rarely affects its chemistry.
- This means that it is very difficult to chemically distinguish isotopes.
Test yourself on Isotopes & Mass Number
What is the correct definition of mass number?
How are ions formed?
Which of the following are true of cations?
What is the charge of a neutron?