You need a CPU (central processing unit), which is the so-called brain of the computer. It processes instructions given to it by software, such as your word processing or accounting programs. A computer also needs a place to store data. That's usually a magnetic hard drive.
A CPU
A Motherboard
RAM
A Storage device
And a Power Supply
If you're referring to the elements?
Silicon, Gold, Silver, Copper, Aluminum(ium) Germanium, and perhaps various other elements.
