Re: Why is the density of francium unkown?

Density is defined as mass per unit volume, or density = m/v. So we need to take two measurements for anything we want to find the density for (its mass, and its volume).

But for francium, we have a problem. It doesn't exist long enough for us to take any GOOD measurements - that is, any measurements that we know to be accurate.

First, we know that francium has 33 isotopes. Isotopes are atoms with the same number of protons but different numbers of neutrons. These isotopes don't hang around long enough before they turn into another element (we call this nuclear decay). The longest lived isotope of francium has a half life of only 22 minutes. This means that in 22 minutes, half of the francium in a sample that started out as pure francium has decayed. So if we have a sample that we would like to weigh, it will be impure because by the time we can weigh it, some of the francium has already decayed into another element.

Francium is also extremely rare. It can be found in uranium deposits or created in the laboratory by bombarding the element thorium with protons. But at any given time there are no more than 20 to 30 grams of francium in the earth's crust! This would make it difficult to find enough francium to weigh. In fact, no one has ever prepared enough (pure) francium for us to weigh.

So we cannot find the density of francium until we can isolate a completely pure sample to weigh and measure the volume.

Here are two web pages that you might want to check out (courtesy of a colleague at www.madsci.org): http://pearl1.lanl.gov/periodic/elements/87.html
http://www.webelements.com/webelements/elements/text/key/Fr.html

Hope this helps,
-Dan