Lead (Pb)

Lead is soft, dense, cheap, easy to melt and essentially permanent, which is why humanity has used it for 8,000 years and why the consequences last. Tetraethyl lead was added to petrol from 1923 until it was phased out over sixty years, and the sharp fall in children's blood lead after the ban is one of the best-documented public health improvements on record. There is no known safe level of lead exposure for a child, and the effect is on cognitive development rather than acute illness, which is exactly why it took so long to act on.

Properties

Atomic number

82

Symbol

Pb

Standard atomic weight

207.2 u

Category

Post-transition metal

Group

14

Period

6

Block

p-block

Electron configuration

[Xe] 6s² 4f¹⁴ 5d¹⁰ 6p²

Electrons per shell

2, 8, 18, 32, 18, 4

Melting point

600.61 K — 327.5 °C — 621.4 °F

Boiling point

2,022 K — 1748.8 °C — 3179.9 °F

Density

11.342 g/cm³

Electronegativity

2.33 (Pauling)

Oxidation states

+2, +4

Discovered

Known since antiquity

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p²

Noble gas shorthand

[Xe] 6s² 4f¹⁴ 5d¹⁰ 6p²

Shells

K: 2 · L: 8 · M: 18 · N: 32 · O: 18 · P: 4

What it is used for

• Lead-acid car batteries — around 85% of all lead used
• Radiation shielding in X-ray rooms and radiotherapy
• Roof flashing, cames in stained glass and sound-deadening sheet
• Diving weights, wheel balance weights and fishing sinkers
• Lead-tin solders in a few remaining exempt applications

Hazards

• A cumulative neurotoxin with no safe exposure level, especially in children
• Absorbed by ingestion of dust or paint flakes, and by inhaling fume when soldering or melting
• Old paint, old plumbing solder and shooting ranges are the common sources

IUPAC publishes an unusually wide interval, [206.14, 207.94], because lead is the endpoint of three different radioactive decay chains and its isotope mix depends on the uranium and thorium in the source rock; 207.2 is the conventional value. Electronegativity is quoted here as 2.33, the CRC value; some tables give 1.87 for lead(II) and 2.33 for lead(IV).

Neighbours in the table

Group 14

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Lead sits in group 14 with carbon, silicon, germanium, tin, flerovium.