All matter is made up of atoms (which are basic particles)
Atoms have a small nucleus consiting of protons (+ charged ) and neutrons (neutral). The nucleus is surrounded by electrons (-)
The mass number (or nucleon number) of an element is the total number of protons and neutrons in the nucleus of an atom.
The atomic number (or proton number) of an element is the number of protons (or electrons) in the nucleus of an atom.
Isotopes
All atoms of a particular element have the same number of protons. The number of protons defines the element.
However, some atoms of the same element can have different numbers of neutrons - these are called isotopes. oxygen has three isotopes oxygen-16, oxygen-17 and oxygen-18
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