How can it be single characters for new chemical elements like 鈽 and 鈾? I found that most chemical elements (ones I checked) are denoted by single character in Chinese. This is understandable for historical elements, but how it can be for new elements like Uranium and Plutonium? Does this means, that chinese people can invent new characters nowadays?
You're right. We did invent a bunch of characters for chemical elements in 20th century. 

We made it just one character by combining different radical(偏旁) together,  but people can associate them with the English words simply by their pronunciation.

You can read more about it from this Wiki article: Chemical elements in East Asian languages