Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
The modern periodic table is based on atomic number. Elements in each period go from metals on the left to metalloids and then nonmetals on the right. Within groups, elements have similar properties.


Question: What is the modern periodic table based on?
atomic number
Atomic number.