The atomic number is the number of electrons present in an atom. We know that an atom has the same number of electron and protons. Therefore chlorine is the element that has 17 protons. Hence the element is Chlorine.
The number of electrons in an atom of element = 17
The number of protons in the atom = 17
The number of protons in the nucleus of the atom is equal to the atomic number (Z). Hence the atomic number of the element will be 17. The element with atomic number 17 is Chlorine.