The Technetium Day Thread

Welcome to the I’ve-already-forgotten-which-installment-this-is installment of the Elemental Open Threads series, my own personal journey into the heart of darkness that is committing to something with 118 entries. Today’s element is special, because it’s going to be our bridge into understanding radioactivity – and what makes elements elemental.

Atom_diagram

(Atoms don’t look exactly like this, but this shows the important parts in about the right places.)

Every atom is made of three things – protons (positively charged particles) and neutrons (non-charged particles), surrounded by a swarm of electrons (negatively charged particles). What type of element an atom is is determined by the number of protons. Hydrogen has one, helium two, and so on. But there are a number of isotopes possible for each element, and those are determined by the number of neutrons.

Protons don’t like being next to other protons for the same reason you can’t stick two magnets together at the ends with the same polarity. Neutrons are spacers; they help the protons be comfortable. It’s important for protons to be comfortable, because otherwise the atom wants to fall apart. When atoms want to fall apart, that’s called radioactivity.

However, the forces that hold atoms together are only effective over very, very short distances. The bigger an atom gets, the harder it is to stick together. Atoms are only stable (or stable-ish) at certain ratios. Graph individual isotopes out by how stable they are, with proton count on one axis and neutron count on the other, and you get this – a peninsula of stability in a sea of instability:

Table_isotopes_en.svg.png

(Note with interest the “island of stability” out at the end. That’s predicted in physics, but the elements that comprise it haven’t been made – or found – yet.)

So where does technetium come into play? At #43, technetium is the lightest element – the one with the fewest protons – for which there are no stable isotopes. That makes it stick out in the middle of the otherwise-reliable transition metal belt.

Periodic_Table_Stability_&_Radioactivity.png(In this periodic table from Wikipedia, elements are colored based on the stability of their longest-lived isotopes. Blue is stable, and the other colors, from green through purple, represent increasingly drastic radioactive potential.)

Technetium’s name represents its synthetic nature; it was the first element that was made, not discovered in the wild. This came after a long and fruitless hunt for a predicted element 43. The Periodic Table was, in fact, created decades before it was fully populated, and there was a long period of guesswork in filling the holes.

After numerous false discoveries, technetium was discovered in irradiated parts from a cyclotron in the 1930s. The element it had been made from is the element it wants to turn back into – molybdenum, element 42. Like most of the transition metals, it’s a silvery-white. Here is the one picture of it on Wikipedia:

Technetium.jpg

When I was a kid, my periodic table showed technetium (along with promethium and all the transuranics) as “man-made,” but that’s not entirely accurate. Technetium does occur in nature, rarely, as a product in the decay chains of other elements. Radioactivity is complex, and a decaying element may become many things before it finally reaches stability.

That changing quality is part of what makes radioactivity so insidious, but it can be harnessed for good, too. Technetium-99m (numbers after an element name indicate the isotope, and m means “metastable”) is used for medical procedures. Humanity’s battle with radioactivity has been hard-fought and tragic; the radium thread, when it comes, will be eye-opening. There is no accepted “safe” dose of radiation: a low dose has a low risk, but the risk is always there. We learned that the hard way. But there have been victories, and technetium-99m is one of them. The dangers in our world can be put to work for us.