206Characteristics
Lead is a bright and silvery metal with a very slight shade of blue in a dry atmosphere. Upon contact
with air, it begins to tarnish by forming a complex mixture of compounds depending on the
conditions. The color of the compounds can vary. The tarnish layer can contain significant amounts
of carbonates and hydroxycarbonates. It has a few characteristic properties: high density, softness,
ductility and malleability, poor electrical conductivity compared to other metals, high resistance to
corrosion, and ability to react with organic chemicals.
Various traces of other metals change its properties significantly: the addition of small amounts of
antimony or copper increases hardness and improves the corrosion reflection from sulfuric acid for
lead. A few other metals also improve only hardness and fight metal fatigue, such as cadmium, tin,
or tellurium; metals like sodium or calcium also have this ability, but they weaken the chemical
stability. Finally, zinc and bismuth simply impair the corrosion resistance (0.1% bismuth content is
the industrial usage threshold). In return, lead impurities mostly worsen the quality of industrial
materials, although there are exceptions: for example, small amounts of lead improve the ductility
of steel.
Lead has only one common allotrope, which is face-centered cubic, with the lead–lead distance
being 349 pm. At 327.5 °C (621.5 °F), lead melts; the melting point is above that of tin (232 °C,
449.5 °F), but significantly below that of germanium (938 °C, 1721 °F).[6] The boiling point of lead
is 1749 °C (3180 °F), which is below those of both tin (2602 °C, 4716 °F) and germanium (2833
°C, 5131 °F). Densities increase down the group: the Ge and Sn values (5.23 and 7.29 g•cm−3,
respectively) are significantly below that of lead: 11.32 g•cm−3.
A lead atom has 82 electrons, having an electronic configuration of [Xe]4f^14 5d^10 6s^2 6p^2. In its
compounds, lead (unlike the other group 14 elements) most commonly loses its two and not four
outermost electrons, becoming lead(II) ions, Pb2+. Such unusual behavior is rationalized by
considering the inert pair effect, which occurs because of the stabilization of the 6s-orbital due to
relativistic effects, which are stronger closer to the bottom of the periodic table. Tin shows a weaker
such effect: tin(II) is still a reducer.
The figures for electrode potential show that lead is only slightly easier to oxidize than hydrogen.
Lead thus can dissolve in acids, but this is often impossible due to specific problems (such as the
formation of insoluble salts). Powdered lead burns with a bluish-white flame. As with many metals,
finely divided powdered lead exhibits pyrophoricity. Toxic fumes are released when lead is burned.
Isotopes
Lead occurs naturally on Earth exclusively in the form of four isotopes: lead-204, -206, -207, and -
- All four can be radioactive as the hypothetical alpha decay of any would be exothermic, but
the lower half-life limit has been put only for lead-204: over 1.4×10^17 years. This effect is, however,
so weak that natural lead poses no radiation hazard.
Three isotopes are also found in three of the four major decay chains: lead-206, -207 and -208 are
final decay products of uranium-238, uranium-235, and thorium-232, respectively. Since the
amounts of them in nature depend also on other elements' presence, the isotopic composition of
natural lead varies by sample: in particular, the relative amount of lead-206 varies between 20.84%
and 27.78%.
Aside from the stable ones, thirty-four radioisotopes have been synthesized: they have mass
numbers of 178–215. Lead-205 is the most stable radioisotope of lead, with a half-life of over 10^7
years. 47 nuclear isomers (long-lived excited nuclear states), corresponding to 24 lead isotopes,
have been characterized. The most long-lived isomer is lead-204m2 (half-life of about 1.1 hours).