Chemistry

An Overview about the Chemical Element Lead



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Lead is the classic example of a heavy metal. Dense, soft, and dull gray in color, it isn't anything terribly inspiring to look at. Yet it grabbed the attention of the now ancient alchemists for its very similar properties to gold, which was dense, soft, and yellow. Today we know that only a nuclear reaction could change one to the other, but back then a mere color change seemed quite workable. That was never achieved, of course, but making lead into colors is quite another matter. Lead-based paints were some of the earliest and most intense colors available. Lead salts have a variety of hues - from white to red to yellow to black - lead paints were widely used for generations. Sadly, it turned out that lead and lead salts are toxic. Worse, lead paints have a sweet taste, so young children were eagerly poisoning themselves, eating paint from the walls. Lead paint can still be found on walls of older homes, but it has mostly been phased out, covered or removed.

Lead's toxicity isn't only of concern to humans. Along with other heavy metals (like mercury), it can be leached from the soil and flushed into the water by acid rain. That lead is then taken in by fish and spread to any animals which consume them as well. In decades past, industrial enterprises also contributed significantly to lead in the environment. Environmental agencies work to regulate lead exposure now, and in responsible nations it is regarded as a hazardous material, its disposal carefully regulated.

Lead is still an important component for car batteries (and an important reason that they be recycled - not dumped in a landfill). It is commonly used in solders, precisely because it is soft and has a fairly low melting point (for a metal that is). Oddly enough, the "lead" in pencils is not lead at all, but a carbon compound called graphite. That we call it lead is a reminder of writing implements in days long gone that did use lead, which was soft enough to leave a mark when run along a surface.

Another historical reference to lead is found in the name of our modern plumbers. Plumbum was the Latin name for lead, and the plumbing in ancient Rome, as you may guess, was made out of lead. With lead pipes, of course, a small amount of lead wound up in the drinking water. Worse yet, the wealthy tended to have cups made of lead. When Nero (and other Emperors) had a tendency towards insanity, there's a very good chance that it had a lot to do with lead poisoning.

To list specific physical properties* of lead:

It has a melting point of only 327.46 degrees Celsius, and boils at 1749 Celsius. By comparison, iron doesn't even melt until 1538 Celsius - which means that one must be aware of toxic lead vapors when melting down scrap metal. This is a health hazard encountered by low-tech computer recycling operations (like many found in China), as unprotected workers work to recover the valuable metals, sucking down the noxious vapors constantly.

The density of lead is 11.3 g/cc, placing it among the heavier naturally occurring materials. (Thus the name "heavy metal" - and if you're looking for a connection between rock music and the element - try Led Zeppelin.)

Getting into the basic chemistry* of the element lead:

Lead has an average atomic mass of 207.2 amu. It bears 82 protons and 82 electrons. There are four isotopes of lead, having 122 (1.4%), 124 (24.1%), 125 (22.1%) and 126 (52.4%) neutrons, respectively. Lead-206 (the one with 124 neutrons) is the stable end-product of the radioactive decay of uranium.

The electronic configuration of lead is [Xe] 6s2, 4f14, 5d10, 6p2. As a result, it has two common oxidation states, forming ions with either +2 (loss of the two 6p electrons) or +4 charge (loss of both the 6p and 6s pairs).

*Numerical values found in the 82nd edition of the CRC Handbook of Chemistry and Physics.

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