Zirconium
Atomic number 40 means that every Zirconium nucleus contains 40 protons. That proton count is the element's identity: change it and you no longer have Zirconium.
Start with the nucleus
Atomic number 40 means that every Zirconium nucleus contains 40 protons. That proton count is the element's identity: change it and you no longer have Zirconium.
Zirconium (Zr) is chemical element 40. It belongs to the transition metals, and its reference state at room temperature is a solid. Those basic facts are a useful starting point for its chemistry and applications.
In practice, it turns up most often in nuclear reactors, ceramics and alloys.
See the atom
Conceptual visual: electron shells are a learning aid, not a literal picture of quantum orbitals.
What this number means
Atomic number 40 means that every Zirconium nucleus contains 40 protons. That proton count is the element's identity: change it and you no longer have Zirconium.