Chlorine
Atomic number 17 means that every Chlorine nucleus contains 17 protons. That proton count is the element's identity: change it and you no longer have Chlorine.
Start with the nucleus
Atomic number 17 means that every Chlorine nucleus contains 17 protons. That proton count is the element's identity: change it and you no longer have Chlorine.
Chlorine (Cl) is chemical element 17. It belongs to the halogens, and its reference state at room temperature is a gas. Those basic facts are a useful starting point for its chemistry and applications.
In practice, it turns up most often in water treatment, PVC and chemical synthesis.
See the atom
Conceptual visual: electron shells are a learning aid, not a literal picture of quantum orbitals.
What this number means
Atomic number 17 means that every Chlorine nucleus contains 17 protons. That proton count is the element's identity: change it and you no longer have Chlorine.