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
In 1956 John Wayne starred as Genghis Khan in The Conqueror. It was filmed in Utah, downwind of the US government's nuclear tests in Nevada, and about half the cast and crew developed cancer as a result.
Quote of the Day
"Blessed are the peacemakers "
~ James I & VI's personal motto
On This Day
1517 Martin Luther sent his 95 Theses to the Archbishop of Mainz, effectively starting the Protestant Reformation. Legend has it that he also posted them on the door of a church in Wittenberg.
1922 The Italian Fascist Party leader, Benito Mussolini, was appointed prime minister of Italy. King Victor Emmanuel III had refused to allow the former liberal prime minister, Luigi Facta, to declare martial law following the 30,000-strong blackshirt March on Rome, thus leaving the way open for Il Duce.
1926 Escapologist Harry Houdini died of a ruptured appendix, which may or may not have been caused by several punches to the stomach.
1940 The Battle of Britain, in which the RAF defended Britain from concerted attacks by the Luftwaffe, ended. With the Germans unable to gain air superiority over the British, they were unable to launch their planned invasion of Britain, Operation Sea Lion.
1941 Mount Rushmore was completed after 14 years of work.
1984 Indira Gandhi, India's only female prime minister to date, was assassinated by her own bodyguards.