1. Atom Number Tells us the element.

2. Covered in this Lesson This lesson will define atomic number and explain what happens if the number of protons in an atom changes. – The atomic number tells us what element we have.

3. The atomic number Henry Mosely found when the positive charge of an element increased by one you have a different element. Scientists concluded every element has a different positive charge.

4. The atomic number Every element has a different positive charge. Protons are the positively charged subatomic particles. Every element has a different number of protons The number of protons is the atomic number.

5. What does this mean? If an atom has a positive charge of 86, it has 86 protons and is the element radon. – Radon undergoes nuclear decay losing 2 protons. The new atom has a charge of positive 84 (84 protons) but it is no longer radon. – This element’s new atomic number is 84 and is the element polonium.

6. Identification of elements All elements can be identified by the positive charge in their nucleus. The positive charge comes from the number of protons. The number of protons is the atomic number The atomic number is unique for each element.