Atoms and Isotopes Atoms are basic particles from which matter is built. They have a small nucleus consisting of protons and neutrons (with one exception, which is hydrogen.) Around the outside they have electrons, which orbit the nuclei. The particles that make up atoms have different relative masses. PARTICLE RELATIVE MASS CHARGE Proton 1 1+ Neutron 1 0 Electron 1/1840 1- All atoms of an element have the same number of protons. This is called the Atomic Number. The number of protons combined with the number of neutrons is called the Mass Number. An example of a simple atom is Helium: Neutron (neutral) Neutrons have the Proton (positive) same mass as protons. Atoms have the same number of protons as electrons. This makes the atom neutral. Electron (negative) Same number as protons, they have a mass of almost nothing. Atoms in the periodic table are all written in the following way: Mass Number 4 He (protons + neutrons) Symbol Atomic Number (protons) 2 Isotopes The number of protons in an element, defines which element that atom is. However some atoms of the same element can have different numbers of neutrons. This is what isotopes are. They have the same chemical properties as they have the same number of protons and electrons. They stand out from normal elements as they have different numbers of atomic numbers to mass numbers: Chlorine has 2 isotopes: Isotopes have the same reactions as the usual type of a certain element, as they have the same electron configuration. Atoms and isotopes are similar in that they create atoms with the same reactions and chemical properties, however they are different because they have for instance different atomic masses.