1. The periodic table and the atom part 2
Lesson 8

2. Atomic Number and Atomic massZ
Element Name
Element Symbol
Atomic Mass A

3. Atomic Number: (Z)
The number of protons in an atom defines what element it is.  For example carbon atoms have six protons, hydrogen atoms have one, and oxygen atoms have eight.  The number of protons in an atom is referred to as the atomic number of that element. The number of protons in an atom also determines the chemical behavior of the element.

4. Atomic Symbol:
The atomic symbol is one or two letters chosen to represent an element ("H" for "hydrogen," etc.).  These symbols are used internationally.  

5. Atomic Mass: (A)
The atomic mass is the average mass of an element in atomic mass units .  Though individual atoms always have an integer number of atomic mass units, the atomic mass on the periodic table is stated as a decimal number because it is an average of the various isotopes of an element. The average number of neutrons for an element can be found by subtracting the number of protons (atomic number) from the atomic mass. 

6. How to calculate the number of Neutrons
The number of neutrons does not always equal the number of protons. We can find the number of neutrons by using the formula
A – Z = N

7. A= Atomic mass
Z= Atomic Number (number of protons)
N= Number of Neutrons
Example: How many Neutrons does K have?
A = 39 , Z = 19, N= ?
N= A-Z = = 20 Neutrons