The most stable nuclear configuration for all elements is 20 protons and 30 neutrons which is the element lead. All elements lighter than this are formed during the fusion process and these reactions give off energy as they become heavier. All elements heavier than lead are created using fission and require energy to create. The heavy ones are created in the explosion of the stars as they die where there is a lot of excess energy. After the heavier elements have been created they will eventually become lighter elements again by radioactive decay when they give off the energy used to create them.

(The Great Courses – The Physics of History)