To answer your question, let's look at the structure of an atom. Atoms are made of particles called protons, neutrons, and electrons. Positively charged protons team up with neutrons, which have no charge, to form the core of an atom, the nucleus. Electrons create a negatively charged cloud around the nucleus. It's the number of protons that determines the name of each element. Finally, elements with different numbers of neutrons are called isotopes.

Protons have the same charge, so they repel each other. However, the dense nucleus is held together by what is called the strong force, which is very powerful. The nucleus likes to organize itself according to certain rules. If it's unhappy with its combination of protons and neutrons, it will eject particles, usually changing into a different element. In other words, it's radioactive! A similar situation occurs when you try to carry as many marbles in your hands as possible. You might start out with a bunch, but sooner or later you'll drop a few until you have a good grasp on the rest.

Technetium is unique because it has 43 protons and isn't stable with any number of neutrons. However, the elements next to Tc on the periodic table have a nice, stable combination of protons and neutrons. Tc would rather be like its neighbors. The 'half-life' is a measure of how long a radioactive element will last. After one half-life has passed, only half of the element is left. The longest-lived isotope of Tc has a half-life of 4,200,000 years, but the earth is over 1000 times older than that! That's why it's all gone today.

Where does it all go? It converts one neutron into a proton by ejecting an electron from the nucleus, becoming element 44, ruthenium.

A special manmade isotope of Tc is extremely useful in science and medicine today. The nucleus gives off extra energy by releasing high-energy light, called a gamma ray. The diagnosis of many illnesses is made possible by this isotope. Doctors inject Tc and use the gamma rays as tracers inside the body to image bones and several organs, including the brain and heart. It's like getting an X-ray from the inside out!