Caesium
Atomic number 55 means that every Caesium nucleus contains 55 protons. That proton count is the element's identity: change it and you no longer have Caesium.
Start with the nucleus
Atomic number 55 means that every Caesium nucleus contains 55 protons. That proton count is the element's identity: change it and you no longer have Caesium.
Caesium (Cs) is chemical element 55. It belongs to the alkali 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 atomic clocks, drilling fluids and scientific research.
See the atom
Conceptual visual: electron shells are a learning aid, not a literal picture of quantum orbitals.
What this number means
Atomic number 55 means that every Caesium nucleus contains 55 protons. That proton count is the element's identity: change it and you no longer have Caesium.