What is the lower bottom number the periodic table mean again?

The number above the symbol is the atomic mass (or atomic weight). This is the total number of protons and neutrons in an atom. The number below the symbol is the atomic number and this reflects the number of protons in the nucleus of each element's atom. Every element has a unique atomic number.

Explanation:

Atomic number means the total number of protons present in an element. On the other hand, atomic mass means the sum of total number of protons and neutrons present in an atom.

In a periodic table, number at the top of symbol of each element represents atomic number. On the other hand, lower bottom number represents the atomic mass of respective element in each block of the periodic table.

Hence, we can conclude that atomic mass is the lower bottom number in the periodic table.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.