2.2: Atomic and Mass Number

Atomic Number, Mass Number, Isotopes

Atomic Mass: Protons + Neutrons

 Atomic Number: Protons = Electrons    

• Protons NEVER change. A proton is the "ID Number" that identifies each element.
• Electrons can change when they lose or gain in order to become "isoelectric" with a noble gas.
• Neutrons can also be different since different weights of the same element can be found on earth also called isotopes.

Exercise:

Determine the number of protons, electrons and neutrons of the following:

Neutrons = Atomic Mass - Protons = 138 - 56 = 82 

Protons = Electrons = 56 

Isotope: Atoms of the same element (same atomic number) but have a different number of neutrons

                              Protons =                     6                    6     

                              Electrons =                  6                    6      

                              Neutrons =                  6                    8           

Atomic Mass in Periodic Table: Atomic mass is a weighted average of all isotopes of that element as it occurs in nature.

Exercise:

Three isotopes of silicon occur in nature. Si-28 (92.23%) with an atomic mass of 27.97693 amu; Si-29 (4.68%), which has an atomic mass of 28.97649 amu; and Si-30 (3.09%), which has an atomic mass of 29.97377 amu. Calculate the atomic weight of silicon.

Step 1: Write down the data and multiply each atomic mass by its percentage:

Si-28 =    = 25.8031 amu


Si-29 =       = 1.3561 amu


Si-30 =       = 0.926189 amu


Step 2: Add all the resultant values:

25.8031     amu +
  1.3561      amu
  0.926189 amu
____________

28.0854 amu