The atomic number on the periodic table tells you the number of protons in the nucleus. Potassium (K) is number 19; so a potassium atom has 19 protons in the its nucleus.

Click on the link for the periodic table above, then look for the potassium (K) square, click there again, and you will get all the information you need for this element. It works in the same way for all the other elements in the table.