## What's in an atom?

- Atoms contain a nucleus and a lot of empty space
- In the empty space, there are electrons
- Atomic number is determined by the number of protons in the nucleus
- Mass number (atomic mass) is determined by the number of protons PLUS neutrons in the nucleus

- The atomic number and mass number are written in atomic notation as:

## Examples

*How many protons are there inside an atom of Cl?*

According to the periodic table, chlorine is the 35th element, and so it has 35 protons.

*What is the atomic notation of an atom with 19 protons and 20 neutrons?*

19 protons = potassium = symbol "K"

Mass number = Protons + Neutrons = 19 + 20 = 39

Therefore the atomic notation is:

## Video Lesson

I have created this video about it if you are looking for a brief review.