P2:Atoms and radiation

P2:Atoms and radiation

1. Structure of the atom

   • Atoms contain three sub-atomic particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus at the centre of the atom. The nucleus is very much smaller than the atom as a whole. The electrons are arranged in energy levels around the nucleus.

2. Properties of sub-atomic particles

   • The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral overall. Atoms can lose or gain electrons. When they do, they form charged particles called ions:
     • If an atom loses one or more electrons, it becomes a positively charged ion
     • If an atom gains one or more electrons, it becomes a negatively charged ion

3. Isotopes

   • Atomic number
     • The number of protons in the nucleus of an atom is called its atomic number: The atoms of a particular element all have the same number of protons. The atoms of different elements have different numbers of protons
   • Mass number
     • The total number of protons and neutrons in an atom is called its mass number

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