Read the nuclide symbol and count the particles in atoms, ions and isotopes. If you slip up, the feedback tells you exactly which number you mixed up.
The rules
- Atomic number (bottom number) = number of protons. It tells you which element it is.
- Mass number (top number) = protons + neutrons. Electrons are not counted because their mass is tiny.
- Neutrons = mass number − atomic number.
- In an atom, electrons = protons. Atoms have no overall charge.
- Ions form when atoms lose or gain electrons. A positive ion has lost electrons. A negative ion has gained electrons. The protons and neutrons do not change.
- Isotopes are atoms of the same element with different numbers of neutrons. They have the same atomic number but different mass numbers.
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