Lanthanum
Atomic number 57 means that every Lanthanum nucleus contains 57 protons. That proton count is the element's identity: change it and you no longer have Lanthanum.
Start with the nucleus
Atomic number 57 means that every Lanthanum nucleus contains 57 protons. That proton count is the element's identity: change it and you no longer have Lanthanum.
Lanthanum (La) is chemical element 57. It belongs to the lanthanoids, 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 optical lenses, batteries and catalysts.
See the atom
Conceptual visual: electron shells are a learning aid, not a literal picture of quantum orbitals.
What this number means
Atomic number 57 means that every Lanthanum nucleus contains 57 protons. That proton count is the element's identity: change it and you no longer have Lanthanum.