Can iPad download whatsapp? If no why?

It is not compatible with the iPad, only with the iPhone - it needs full phone capability which the iPad hasn't got. Their support site states (http://www.whatsapp.com/faq/) :

We support all iPhone devices no matter how old or new, as long they are running iPhone OS 3.1 and up. Sorry, but there is no iPod or iPad support at the moment.

how come I have it on my ipod 1st generation?, now im trying to download it for the ipad and wont work =(

King_Penguin already gave you the answer that the Whatsapp developers have given. For any further information, you'll need to contact them.