Atoms of the same element that have different numbers of neutrons are called isotopes. Most carbon on Earth exists as the very stable isotope carbon-12, with a very small amount as carbon-13.
Here’s an example using the simplest atom, hydrogen. Carbon-14 is an unstable isotope of carbon that will eventually decay at a known rate to become carbon-12.
Improved techniques now date the earliest stone structures at Stonehenge to about 2600 B. Sir Joseph Norman Lockyer, who co-discovered helium and founded the journal, Nature, wrote in 1901 that the Heel Stone section of Stonehenge "had been originally aligned with the summer solstice" and calculated that it was built in 1800 B. Further investigations have suggested that Stonehenge was an astronomical observatory, a place of worship and healing or perhaps a cemetery.
Whatever its exact history, origins or age, thousands each year flock to Stonehenge to welcome the sun on the summer solstice.
Renfrew (1973) called it 'the radiocarbon revolution' in describing its impact upon the human sciences.