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nucleus an atomcentral region which is made up of protons and neutrons proton a subatomic particle that has a positive charge and that is found in the nucleus of an atom neutron a sub atomic particle that has no charge and that is found in the nucleus of an atom electron a subatomic particle that has a negative electric charge Valence Electron an electron that is found in the outermost shell of an atom and that determines the atom's chemical properties group a vertical column of elements in the periodic table ion an atom radical or molecule that has gained or lost one or more electrons and has a negative or positive charge atomic number the number of protons in the nucleus of an atom mass number the sum of the numbers of protons and neutrons in the nucleus of an atom isotope an atom that has the same number of protons an other atoms of the same element do but that has a different number of neutrons metal an element that is shiny and that conducts heat and electricity well nonmetal an element that conducts heat and electricity poorly and that does not form positive ions in an electrolytic solution semiconductor an element or comound that conducts electric current better than an insulator but not as well as a conductor halogen one of the elements of Group 17 of the periodic table noble gas an unreactive element of Group 18 of the periodic table mole the sibase unit used to measure the amount of a substance whose number of particles is the same as the number of atoms of carbon in 12 g of carbon 12 Avogadro's constant equals 6.02x1023; the number of particles in 1 molar mass the mass in grams of 1 mol of a substance alpha particle a positively charged atom that is released in the disintegration of radioactive elements and that consists of two protons and two neutrons beta particle a charged electron emitted during certain types of radioactive decay, such as beta decay fission the process by which a nucleus splits into two or more fragments and releases neutrons and energy fusion the process in which light nuclei combine at extremely high temperatures forming heavier nuclei and releasing energy gamma ray the high energy proton emitted by a nucleus during fission and radioactive decay half-life the time required for half of a sample of a radioactive substance to disintergrate by radioactive decay or by natural processes nuclear chain reaction a continuous series of nuclear fission reactions radioactivity the procewss by which an unstable nucleus emits one or more particles or energy in the form of electromagnetic radiation