Lesson 6, Topic 1
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# Atomic Number, Mass Number

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### Atomic Number (Z)

The atomic number of an element is the number of protons in one atom of that element. The number of protons is equal to the number of electrons. When the number of protons is not equal to the number of electrons, we have ions. This occurs by increasing or decreasing the number of electrons in a neutral atom.

### Mass Number (A)

The mass number of an element is the sum of the Protons and Neutrons.

Thus, the atoms of Hydrogen, Oxygen and Sodium can be written as:

$$\scriptsize _{1} ^{1} \textrm {H}$$, $$\scriptsize _{8} ^{16} \textrm {0}$$, $$\scriptsize _{11} ^{23} \textrm {Na}$$

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