Atomic Structure The concept of the atom has existed since the 5th century BC, but it was not until the beginning of the 19th century that this concept was developed into a scientific theory. Almost as soon as the modern atomic theory was established, it was discovered that atoms were not the basic building blocks that were being sought. Instead, atoms consist of electrons that are bound to a tiny nucleus. The nucleus is made of neutrons and protons. For some time it seemed possible that these objects could be the fundamental building blocks, but in the 1960s it was found that the neutron and proton have str