Ah ok Mxy.
Hmm, my guess is that nobody is going to get this one, so I will give it to Goalie.
The answer I was looking for was Technetium. This is element number 43 in the periodic table, but despite being so light it has in fact got no stable isotopes - all of them are radioactive to one degree or another. However it does occur naturally - in the cores of red supergiant stars, for example - so in that sense it's not entirely artificial.
After that there's promethium, element number 61, which also has no stable isotope so must be produced in labs. But again, this element has been observed in nature (although very rarely - in some stars, and at a concentration of about 4 parts in 1018 on the Earth) so again can't necessarily be said to be truly synthetic.
That honour goes instead to Einsteinium, which to date has never been observed in nature and is thus the "lightest" purely artificial element there is. The next lightest element, Californium, and all the actinides/-oids below it, are usually produced in labs but do (or are expected to) occur in trace amounts on Earth.
So...
You can go next Goalie.
