MATTER one type of atom gold contains only gold atoms and neon contains only neon atoms other substances are mixtures of different kinds of atoms Atoms also join together chemically to form molecules Ions are elec trically charged atoms or groups of atoms All atoms are roughly the same size About 50 million atoms of solid matter lined up in a row would measure only 0 4 inch 1 centimeter Since atoms molecules and ions are very small the bulk matter of everyday life consists of large amounts of these components A glass of water for instance contains an extremely large amount about 8 x 1024 of water mole cules Each water molecule is in turn made up of two hydrogen atoms and one oxygen atom that have been combined chemically ATOMS Scientists long believed that atoms were ele mentary particles that had no discernible structure and could not be broken apart In fact the word atom comes from a Greek word meaning indivisible Since the end of the 19th century however it has been known that atoms are themselves made up of smaller particles It requires a great deal of energy to break an atom into its component parts 12
THE BUILDING BLOCKS OF MATTER Helium Carbon e e nucleus O proton O neutrone electron The positively charged nuclei of belium and carbon consisting of protons and neutrons are circled by negatively charged electrons Encyclopaedia Britannica Inc 13
MATTER Splitting the core of an atom involves nuclear reactions rather than ordinary chemical reactions PARTS OF THE ATOM Atoms are made up of three basic types of particle protons neutrons and electrons These particles as well as other particles smaller than atoms are known as subatomic particles Most of an atom consists of empty space Its mass is concentrated in its cen ter which is called the nucleus The nucleus consists of protons and neutrons The ordinary hydrogen atom is an exception it contains one proton but no neutrons As their names suggest protons have a positive electrical charge while neutrons are electri cally neutral they carry no charge Overall then the nucleus has a positive charge The number of protons determines which element is represented while the number of neutrons determines which iso tope of the element is represented Isotopes are atoms of the same element that vary in mass because of differing numbers of neutrons Circling the nucleus is a cloud of electrons which are negatively charged 14
THE BUILDING BLOCKS OF MATTER ELECTRICAL CHARGE AND IONS It is the electrical forces in an atom that hold the atom together Because opposite electric charges attract each other there is an attrac tive force between the negatively charged Bohr atomic model of a nitrogen atom electron orbits electron proton neutron Electrons can circle the mucleus of an atom only in particular orbits of fixed size and energy The Danish physicist Niels Bohr proposed this model of the atom in the early 20th century to explain how electrons can have stable orbits Encyclopaedia Britannica Inc 15 AM
