Mendeleev
Periodic table · chemistry · physics
Element story

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.

Keep exploring

→ Thallium · Atomic structure · Periodic trends

81TlThallium
Thallium · Tl · Z = 81

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.

Understanding the element

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.

Where do we encounter it?

In practice, it turns up most often in electronics, optics and scientific research.

What its position in the table tells us

Thallium sits in period 6, group 13 and the P block. That location is useful because neighbouring elements often share trends in valence electrons, oxidation states and bonding behaviour.