Atomic Number, Mass Number and Isotopes

All atoms contain the three fundamental particles of matter, the proton, electron, and neutron.  But how do the atoms differ from one another?  Each element has its unique number of protons that serve as the identity of the element.  We can identify the number of protons of an element by knowing the atomic number of that element.  Atomic number (Z) is the number of protons in the nucleus of each atom of an element.  In a neutral atom the number of protons is equal to the number of electrons, therefore atomic number gives the number of protons and the number of electrons of an atom.  Example, Oxygen has an atomic number of 8, this means oxygen atom has 8 protons and 8 electrons.

Mass Number (A) is the total number of protons and electrons in the nucleus of an atom.  All atomic nuclei contain both protons and neutron.
 
                            Mass number  =   number of protons  +  number of neutrons

We can determine the number of neutrons if we know the atomic number and mass number of and element. The number of neutrons can be determine by subtracting the atomic number to the mass number. Example the element nitrogen, it has atomic number of 7 and mass number of 14.  Therefore, there are 7 protons and 7 neutrons.   How do we arrive at that answer?  Since the atomic number is 7 the number of protons is also seven.  To get the number of neutrons simply subtract 7 to 14 to get 7 (14 - 7 = 7).

In writing the atomic notation the atomic number is written on the lower left portion of the chemical symbol and the mass number on the upper left corner of the chemical formula.  As shown below:

Example of atomic notation of element are the isotopes of Uranium as shown below: 

As you can see above:  the element Uranium has different mass number.  Is that possible?  The atomic notation above is an example of  isotopes of uranium.  What is an isotope?  Isotopes are atoms of the same element having the same atomic number but different mass number.  It means that the number of protons are the same but different number of neutrons.  Other example of isotopes are the isotopes of hydrogen: the protium, deuterium, and the tritium. 

Sample Problem:
Determine the number of protons, electrons and neutrons of the following:
1.  Oxygen, Z= 8, A=17
2.  Hg, Z=80, A=199
3.  Hg, Z=80, A=200
4.  Cu, Z=29, A=63
5.  Fe, Z=26, A=56

Answer:
1. Oxygen has 8 protons and 9 neutrons
2.  Mercury-199 has 80 protons and 119 neutrons
3.  Mercury-200 has 80 protons and 120 neutrons
4.  Copper has 29 protons and 34 neutrons
5.  Iron has 26 protons and 30 netrons