81 Tl Thallium 204.38 u

Thallium

Period
6
Group
13
Block
p-block
Phase (STP)
Solid

Thallium is a soft, highly toxic post-transition metal. It is used in electronics, optics, and was historically used as a rat poison.

About Thallium

Atomic number 81 places thallium in the sixth period of the p-block, where the configuration [Xe] 4f14 5d10 6s2 6p1 leaves three electrons outside a filled 5d shell. The relative atomic mass is 204.38 and the Pauling electronegativity 1.62. It is a soft post-transition metal, solid at standard temperature and pressure, sitting in group 13 directly beneath indium. Acute toxicity governs how the element is handled: thallium compounds were once marketed as rat poison, a use later withdrawn across most jurisdictions. Present-day demand comes instead from electronics and from optical components, where the heavy, polarisable thallium ion gives useful transmission behaviour. The combination of low mechanical hardness and high toxicity makes it one of the more hazardous of the heavy main-group metals.

Physical Properties

Atomic Mass 204.38 u
Phase at STP Solid

Electronic Properties

Electronegativity 1.62

Electron Configuration

Configuration [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹

Electron Shell Configuration

Thallium electron shell configuration Tl 2e⁻ 8e⁻ 18e⁻ 32e⁻ 18e⁻ 3e⁻
Electron shell structure of Thallium — electrons per shell.
Shell Electrons
1 (K) 2
2 (L) 8
3 (M) 18
4 (N) 32
5 (O) 18
6 (P) 3

Position in Periodic Table

Thallium position in the periodic table Tl

Block & Period

Block

s p d f

Period

1 2 3 4 5 6 7

Periodic Table

Lanthanides Actinides

More Post-Transition Metal

13 Al Aluminum 31 Ga Gallium 49 In Indium 50 Sn Tin 82 Pb Lead 83 Bi Bismuth 84 Po Polonium

Frequently Asked Questions

What is Thallium?
Thallium (Tl) is a chemical element with atomic number 81. It is a solid at standard temperature and pressure, classified as a post-transition metal.
What are the key properties of Thallium?
Thallium has an atomic mass of 204.38 u.
What is the electron configuration of Thallium?
The electron configuration of Thallium is [Xe] 4f14 5d10 6s2 6p1. It is in the p-block of the periodic table.
What is the electronegativity of Thallium?
Thallium has an electronegativity of 1.62 on the Pauling scale.