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B Atoms Atoms Atomic symbols Atoms Atoms consist of a tiny nucleus containing positivelycharged protons and neutral neutrons, surrounded by a cloud of negatively-charged electrons. Electrons are almost 2000 times smaller than protons and neutrons, so almost all the mass of an atom is concentrated in its nucleus. Although there are only a few electrons, they move so fast they seem to occupy the whole volume of the atom. Atomic symbols Mass number Scientists summarise the most important data about an Atomic number atom by listing its atomic number and its mass number. 11 5 The atomic number is equal to the numbers of protons. The mass number give the total number of massive particles: the protons and neutrons. This atom has: This diagram shows that the electrons are found in shells. 11 – 5 = 6 neutrons 5 protons 5 electrons Inside atoms The nucleus, in the centre of an atom, contains protons and neutrons, while the electrons orbit the nucleus in shells. Particle Proton Neutron Mass 1 1 Charge +1 0 Nucleus Electron 1 1836 –1 Shells The atomic number is the number of protons in an atom. The mass number is the number of protons plus neutrons. 3B 1 Atom words Position Nucleus Isotopes Isotopes are atoms with the same number of protons but different numbers of neutrons. Isotopes have the same atomic number but different mass numbers. They have the same chemical properties because they have the same electron structure. However, because isotopes have slightly different masses some of their physical properties are different. 3B 2 How chemists changed the NZ wine industry End of chapter Proceed to next chapter