Atomic Number
81
Element 81
Thallium (Tl)
Thallium is a chemical element with symbol Tl and atomic number 81. This soft gray post-transition metal is not found free in nature. It is highly toxic and was historically used in rat poisons and insecticides.
Atomic Mass
204.38 u
Category
Post-Transition Metal
Period
6
Group
Group 13: boron group
Phase
solid
Density
11.85 g/cm3
Electron Shells
2, 8, 18, 32, 18, 3
Atomic Structure
Protons: 81. Electrons: 81. Neutrons: Varies by isotope.
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p¹
Why Thallium Matters
Thallium helps connect atomic number 81, post-transition metal behavior, period 6 trends, and electron shell structure. Its solid phase, atomic mass of 204.38 u, and electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p¹ make it useful for comparing bonding patterns across the periodic table.
Discovery and Occurrence
Thallium was discovered 1861 by William Crookes. Thallium is a chemical element with symbol Tl and atomic number 81. This soft gray post-transition metal is not found free in nature. It is highly toxic and was historically used in rat poisons and insecticides.
Safety and Handling Context
Thallium is presented here for learning and comparison. Pure elements and laboratory compounds can have hazards that depend on dose, form, and setting.
Related Elements
Frequently Asked Questions
- What is the atomic number of Thallium?
- Thallium has atomic number 81, which means a neutral atom has 81 protons and 81 electrons.
- What is the chemical symbol for Thallium?
- The chemical symbol for Thallium is Tl.
- What category is Thallium in?
- Thallium is classified as a post-transition metal and sits in period 6 of the periodic table.
Explore Thallium in 3D
Use the interactive table above to rotate atomic models, compare periodic trends, and move from Thallium to neighboring elements without losing your place.