Thallium
Atomic number 81 means that every Thallium nucleus contains 81 protons. That proton count is the element's identity: change it and you no longer have Thallium.
Start with the nucleus
Atomic number 81 means that every Thallium nucleus contains 81 protons. That proton count is the element's identity: change it and you no longer have Thallium.
Thallium (Tl) is chemical element 81. It belongs to the post-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 electronics, optics 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 81 means that every Thallium nucleus contains 81 protons. That proton count is the element's identity: change it and you no longer have Thallium.