What are the periodic trends for atomic number and atomic mass?
The atomic number equals the number of protons in an atom. The atomic number is what the periodic table is based on. So the atomic number increases with each element. The atomic mass is the sum of the protons and neutrons. As the atomic number increases it causes an increase in the atomic mass. However sometimes there is not a corresponding increase in the number of neutrons.
An example of this is the mass of Argon atomic number 18 has a mass of 40 because it has 22 neutrons. Potassium number 19 has a mass of 39 because it has only 20 neutrons.