Lutetium is more abundant than silver. Only two isotopes are known. One is the stable Lu-175 and the other is the radioactive Lu-176 which comprises about 2.5% of natural lutetium. The fact that Lu is #71 tells us it has 71 protons and thus (175-71) 104 neutrons. The radioactive Lu-176 has one extra neutron which must be the source of its instability.
The half-life of Lu-176 is about 38 billion years. A long half-life means a slow decay. Which means the stuff just isn’t that radioactive. Isotopes with short half-lives are more potent, that is, they put out particles at a faster rate. Lu-176 is used with its daughter product hafnium (#72) to date meteorites.
There’s a synthetic Lu-177 that is used in radiation therapy for brain tumors.
Lutetium is grouped with the lanthanoids (the rare earths) but it is also a d-block or transition metal. Its position on the long form of the periodic table shows it to be in column 3:
Here’s a link to the source. Click on the image to enlarge it.
You usually see the short form of the periodic table which clips out the two rows of lanthanoids and actinoids (the mauve ones in the image) and sticks them down below. This is so the table can fit on an 8-1/2 by 11-inch notebook sheet!
Lutetium is difficult to separate from other similar metals. It is produced commercially only as a by-product as there are no known lutetium-dominant minerals. Like the rest of the lanthanoids it has no known biological role.