Hello!
the atomic number is equal to the number of protons in the nucleus of an element's atom
because electrons are negative, neutrons are neutral, and protons are positive, you need the same number of protons as electrons to have an electrically neutral atom.
In this case, because there are 17 electrons, you need 17 protons to make the charge neutral
therefore, the atomic number is 17
this element is known as Chlorine
I hope this helps, and have a nice day!