Actinium
Atomic number 89 means that every Actinium nucleus contains 89 protons. That proton count is the element's identity: change it and you no longer have Actinium.
Start with the nucleus
Atomic number 89 means that every Actinium nucleus contains 89 protons. That proton count is the element's identity: change it and you no longer have Actinium.
Actinium (Ac) is chemical element 89. 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.
Actinium sits in period 7, group 3 and the D block. That location is useful because neighbouring elements often share trends in valence electrons, oxidation states and bonding behaviour.
See the atom
Conceptual visual: electron shells are a learning aid, not a literal picture of quantum orbitals.
What this number means
Atomic number 89 means that every Actinium nucleus contains 89 protons. That proton count is the element's identity: change it and you no longer have Actinium.