<P> The number of protons in the nucleus defines to what chemical element the atom belongs : for example , all copper atoms contain 29 protons . The number of neutrons defines the isotope of the element . The number of electrons influences the magnetic properties of an atom . Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules . The ability of atoms to associate and dissociate is responsible for most of the physical changes observed in nature and is the subject of the discipline of chemistry . </P>
<P> The number of protons in the nucleus defines to what chemical element the atom belongs : for example , all copper atoms contain 29 protons . The number of neutrons defines the isotope of the element . The number of electrons influences the magnetic properties of an atom . Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules . The ability of atoms to associate and dissociate is responsible for most of the physical changes observed in nature and is the subject of the discipline of chemistry . </P>
smallest unit that consist of at least one atom
smallest unit that consist of at least one atom
molecules
molecules