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Information valid for Tl-204 unless otherwise noted

Basic Information

Symbol: Tl
Atomic Number: 81
Mass: 204.3833
Melting Point: 303.5 C
Boiling Point: 1457.0 C
Number of Protons/Electrons: 81
Number of Neutrons: 123
Classification: Other Metals
Crystal Structure: Hexagonal
Density @ 293 K: 11.85 g/cm3
Color: bluish

Atomic Structure

[Bohr Model of Thallium]
Number of Energy Levels: 6

  1. First Energy Level: 2
    Second Energy Level: 8
    Third Energy Level: 18
    Fourth Energy Level: 32
    Fifth Energy Level: 18
    Sixth Energy Level: 3


Isotope Half Life
Tl-200 1.08 days
Tl-201 3.03 days
Tl-202 12.23 days
Tl-203 Stable
Tl-204 3.78 years
Tl-205 Stable
Tl-206 4.2 minutes
Tl-207 4.77 minutes
Tl-208 3.05 minutes
Tl-209 2.2 minutes
Tl-210 1.3 minutes


Date of Discovery: 1861
Discoverer: Sir William Crookes
Name Origin: From the Greek word thallos (young shoot)
Uses: rat and ant poisons, detecting infrared radiation
Obtained From: crookesite, hutchinsonite, lorandite