Re: How do chemists create manmade elements?

Hi Yan Ho. 

As you know, atoms (i.e. elements) are roughly made up of a heavy 
nucleus with neutrons and protons, and light electrons oribiting around 
it. Particles in the nucleus (protons and neutrons) are bounded by 
means of a force among them called the "Strong Nuclear Force" that only 
attracts particles when they are very close to each other.

Atoms are usually electrically neutral, thus they have the same number 
of protons (positive charges) as electrons (negative charges),  and 
what makes an element different from any other is precisely the number 
of protons (or electrons) it has, as the number of electrons determines 
chemical properties of the element.

The number of protons in an element's nuclei is called its "Atomic 
Number" (Z), and is the number which is used as the index in the 
Periodic Table of Elementes (Hydrogen: Z=1; Helium: Z=2; Lithium: Z=3; 
Berilium: Z=4 ...). In nature elements have been found with atomic 
numbers up to Z=92 (Uranium). A nucleus of this size or greater (almost 
a hundred protons and more than a hundred neutrons in it) is unstable, 
which means that it will explode spontaneously because nuclear forces 
aren't strong enough to keep such a number of protons and neutrons 
together (yes, you're right, this is radioactivity), and that's why 
there're no natural elements with Z>92, because they're short lived 
(atoms disappear by exploding nuclei).

In order to get a nucleus with Z>92 (an artificial element) you have to 
put together two existing nuclei with Z<92 to form a greater nucleus. 
This is roughly the way to create artificial elements: take two nuclei 
and put them together. This seems easy, but it is not easy at all. Let 
me explain it. 

"Strong Nuclear Force" only acts at very short distances, so you have 
to put the nuclei really close to each other.  As you know, particles 
with same charge repel, and nuclei are positively charged, as they 
have protons, thus if you try to get two nuclei closer and closer an  
extremely itense electric force will tend to keep them apart. In order 
to overcome this electrical force, nuclei must be accelerated to 
extremely high velocities (of the order of 10000 miles per second). 
Then nuclei collide and they could stick each other, forming a new 
nucleus of an artificial element.

In practice you make a gun of ions (ions are atoms which are not 
electrically neutral). Ions are accelerated by means of an electric 
field and fired towards a target made of some heavy element (say 
lead, uranium ...). Some of the ions join nuclei in the target, thus 
forming atoms of a new element. Sometimes you need to keep shooting the 
ion gun for a week to get a single atom of the new element (it 
is hard work).

I hope it explains what you wanted to know.

You have a wonderful webpage at Los Alamos National Laboratory website 
which explains how every element in the Periodic Table was discovered 
or created, and what are they used for:
 http://pearl1.lanl.gov/periodic

You could also check out WebElements.

Enjoy, ... and keep on thinking.