In   a  helium  atom,  the  nucleus
   contains  2 neutrons which have no electrical charge, and 2 positively
   charged protons.
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   Two electrons, which are negatively charged, move around the nucleus.
<P>
   The  atomic  number  of  an  element  is  the number of protons in the
   nucleus.
<P>
   The mass number (atomic mass) is the sum of protons plus neutrons.
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   Review:  <A ClAsS=ACT Href="http://media.pearsoncmg.com/bc/bc_campbell_biology_7/media/activities/load.html?2&amp;A" oNClick="popact('http://media.pearsoncmg.com/bc/bc_campbell_biology_7/media/activities/load.html?2&amp;A');txtcolor('atom1','gray');return false;" Id=atom1><iMg sRc="/images/tutorial.gif" aLt='' boRDer=0></A>