In an atom the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge. • All atoms of a particular element have the same number of protons. The number of protons in an atom of an element is called its atomic number. • The total number of protons and neutrons in an atom is called its mass number. • Atoms can be represented as shown in this example: [the mass number is written as a superscript and the atomic number as a subscript, both to the left of the chemical symbol of the element]. • Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element. • Atoms turn into positive ions if they lose one or more outer electron(s). • Students should be able to relate differences between isotopes to differences in conventional representations of their identities, charges and masses.