An atom has 3 basic particles that it is composed of.  They are protons, neutrons, and electrons.  The protons are located in the nucleus of the atom and have an atomic mass of about 1 amu (atomic mass unit).  They are positively charged.  How many protons an atom has is called the atomic # of the atom and atoms on the periodic table are arranged according to their atomic #s.  Calcium, for example, has an atomic # of 20, meaning that it has 20 protons. 

     Neutrons are the other atomic particle that are found in the nucleus of the atom.  They are also about 1 amu, but they do not have a charge.  Together the number of protons + the number of neutrons = the atomic mass of the element.  Calcium has an atomic mass of 40.  So to figure out how many neutrons there are:

  Protons + Neutrons = atomic mass

     20     + Neutrons = 40

so subtract: 40-20= 20 Calcium has 20 neutrons.  

The number of neutrons will not always equal the number of protons.   

      The last atomic particle is the electron.  The electrons are very small compared to the protons and neutrons (1/1840 amu) and are negatively charged.  They orbit around the nucleus.  In a neutral atom, the number of electrons will always be equal to the number of protons.  So, calcium has an atomic # of 20, which means it has 20 protons and 20 electrons. 

These are the 3 basic particles that make up an atom.