Thallium

Post-transition metal · atomic number 81

81204.38TlThalliumSolidPost-transition metal

Thallium is a chemical element with symbol Tl and atomic number 81. This soft gray post-transition metal is not found free in nature. When isolated, it resembles tin, but discolors when exposed to air.

Properties of Thallium

Element category	Post-transition metal
Atomic number	81
Atomic mass	204.38 u
Phase at STP	Solid
Group	13
Period	6
Block	p
Electron configuration	[Xe] 4f14 5d10 6s2 6p1
Electrons per shell	2, 8, 18, 32, 18, 3
Electronegativity (Pauling)	1.62
Melting point	577 K (304 °C)
Boiling point	1746 K (1473 °C)
Density	11.85 g/cm3
Discovered by	William Crookes

← Mercury · Periodic table · Lead → Pb

About the post-transition metal group

Softer, lower-melting metals beside the transition block — aluminium, tin, lead — abundant in cans, foils, solders and pigments.

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🧪 Reactive nonmetal

11 elements: Elements that readily gain or share electrons — the building blocks of water, air, living tissue and most organic chemistry.

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6 elements: Full outer electron shells make these gases almost inert: they barely react, glow in discharge lamps and are used to create unreactive atmospheres.

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9 elements: Softer, lower-melting metals beside the transition block — aluminium, tin, lead — abundant in cans, foils, solders and pigments.

🧪 Metalloid

7 elements: In-between elements that behave as semiconductors: silicon, germanium and boron underpin electronics, glass and solar cells.

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