Atoms are very small, having a radius of about 0.1 nm (1 x 10-10 m). The radius of a nucleus is less than 1/10 000 of that of the atom (about 1 x 10-14 m). Almost all of the mass of an atom is in the nucleus. The relative masses of protons, neutrons and electrons are: The sum of the protons and neutrons in an atom is its mass number. Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element. Atoms can be represented as shown in this example: Students should be able to calculate the numbers of protons, neutrons and electrons in an atom or ion, given its atomic number and mass number. Students should be able to relate size and scale of atoms to objects in the physical world.