Thorium
Atomic number 90 means that every Thorium nucleus contains 90 protons. That proton count is the element's identity: change it and you no longer have Thorium.
Start with the nucleus
Atomic number 90 means that every Thorium nucleus contains 90 protons. That proton count is the element's identity: change it and you no longer have Thorium.
Thorium (Th) is chemical element 90. It belongs to the actinoids, 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 energy, alloys 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 90 means that every Thorium nucleus contains 90 protons. That proton count is the element's identity: change it and you no longer have Thorium.