An isotope of an element has a slightly different number of particles to other isotopes of the element. So, the most common isotope of oxygen is oxygen-16, and it has 16 nucleons (both protons and neutrons) in its nucleus. Oxygen-18 has 18 nucleons.
Fact of the Day
The Wehrmacht lost more than 400,000 soldiers during the first two months of Operation Barbarossa.
Quote of the Day
"Time is a great teacher, but unfortunately it kills all its students. "
~ Hector Berlioz, composer
On This Day
1534 Nun Elizabeth Barton was executed at Tyburn for prophesying against Henry VIII and his marriage to Anne Boleyn.
1653 Oliver Cromwell dissolved the Rump Parliament with the words: 'Let us have done with you. In the name of God, go!'
1889 Adolf Hitler, future leader of the Nazi Party and dictator of Germany, was born at Braunau am Inn in modern-day Austria.
1912 The author of the legendary novel Dracula, Bram Stoker, died aged 64, possibly of syphilis or overwork.
1964 BBC 2, the third British television station, was launched, but it was interrupted by a power cut at Battersea Power Station.
1968 Tory right-winger Enoch Powell delivered his infamous 'Rivers of Blood' speech to the General Meeting of the West Midlands Area Conservative Political Centre, criticising immigration from the Commonwealth.
1974 Catholic James Murphy, a petrol station owner from County Fermanagh, became the 1,000th victim of the conflict in Northern Ireland.
1999 Twelve students and a teacher were killed by gun-wielding fellow students at Columbine High School. The two perpetrators went on to commit suicide.