Bishop James Ussher, a 17th-century Irish cleric, for example, calculated that creation occurred in 4004 B. There were many other such estimates, but they invariably resulted in an Earth only a few thousand years old.By the late 18th century, some naturalists had begun to look closely at the ancient rocks of the Earth.Scientists determined the Earth's age using a technique called radiometric dating.Radiometric dating is based upon the fact that some forms of chemical elements are radioactive, which was discovered in 1896 by Henri Becquerel and his assistants, Marie and Pierre Curie.giving an age for the Solar System and an upper limit for the age of Earth.
Following the development of radiometric age-dating in the early 20th century, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old.Radiometric dating is also used to date archaeological materials, including ancient artifacts.Different methods of radiometric dating vary in the timescale over which they are accurate and the materials to which they can be applied.All ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus.Additionally, elements may exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus.
(The nucleus of an atom is made up of protons and neutrons.) For example, the element carbon, which always has six protons in its nucleus, has three isotopes: one with six neutrons in the nucleus, one with seven, and one with eight.