Monday, April 6, 2009

The Elements

Today our exploration of the Periodic Table of Elements enters the sixth period. The table is organized in vertical columns, called groups, and horizontal rows called periods. Elements in the same group have the same number of electrons in the outermost shell, while elements in the same period have the same number of shells.

Imagine the electron shells as something like Russian nesting dolls. The innermost shell is very small, and can only hold two electrons. Shells two and three are larger, and can hold up to eight electrons; shells four and five can hold as many as eighteen.
Atoms with a full outer shell are generally stable. Elements like cesium (in group I) are always looking to give one electron away, shedding its outermost shell to reveal the full shell underneath. Contrariwise, elements like chlorine (in group VII) have an almost full outer shell already; and will beg, steal, share, or borrow an electron to complete the set.

It is this instability, the desire to gain or lose electrons, that causes atoms to join together to form compounds, eventually giving rise to the large and complicated molecules necessary for life. If all the atoms were stable, like the noble gases, nothing would ever happen in the Universe, because the atoms would have no need to interact.

Caesium (Cesium in USA):
Atomic Symbol: Cs
Atomic Number: 55
Caesium was discovered in Heidelberg, Germany in 1860 by Gustav Kirchhoff and Robert Bunsen, who is best remembered for his invention of the Bunsen burner. It is a soft, shiny metal and a member of group I, also known as the alkali metals. When it is dipped into water, it reacts violently, igniting into colorful blue flames and emitting hydrogen gas.
When bombarded with microwaves, cesium atoms will vibrate at a very predictable rate. The US National Institute of Standards and Technology (NIST) uses a caesium atomic clock to determine the time, accurate to within one second every 30 million years.

Atomic Symbol: Ba
Atomic Number: 56
Barium sulfate is among the most insoluble of all salts, which means it safe to ingest and does not react with stomach acid. For this reason, it is given to people with digestive disorders so that its progress through the body can be followed by X-ray scans.
Its soluble salts are extremely toxic. Barium carbonate is used in rat poison because it dissolves in the stomach acid, causing vomiting, diarrhea, tremors, and paralysis.

In our next chemistry post, we'll discuss the Lanthanides: a group of elements which are also called the Rare Earths (although not all are rare). In the meantime, check out my previous posts on chemistry by clicking here.

For the chemistry enthusiast, there's an informative and entertaining book called Nature's Building Blocks: An A-Z Guide to the Elements, by John Emsley. Every element is fascinating in its own way, and this book has been a terrific help to me in researching these chemistry posts.

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