Here's how to break down the information and determine the number of protons:

* Atomic Number (9): This directly tells you the number of protons in an atom. So, this atom has 9 protons.

* Mass Number (18): This represents the total number of protons and neutrons in the atom's nucleus.

* Charge (-1): This indicates that the atom has gained one electron. A neutral atom has an equal number of protons and electrons.

Therefore, the atom has 9 protons.

Let me know if you have any other questions about atomic structure!