The Nucleus (Edexcel GCSE Chemistry: Combined Science)

• Atoms are extremely small with a radius of about 1 x 10^-10 metres
• The central nucleus contains protons and neutrons only which are packed close together in a small region of space
• The radius of the nucleus is about 10 000 times smaller than that of the atom, so it is an extremely small region of space compared to the overall size of the atom
• This means that rather than being evenly spread out throughout the atom, virtually all of the atom's mass is concentrated inside the nucleus
• Electrons have a much smaller mass than protons and neutrons (1 proton has the same mass of around 1840 electrons) and move in the space outside the nucleus in orbits

• You need to know the following terms to describe the properties and characteristics of atoms

Atomic Structure Key Terms Table

• The atomic number and mass number are represented by writing them next to the symbol of the element
• By convention the mass number is usually written as a superscript and the atomic number as a subscript

Conventions for showing atomic number and mass number