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 world’s oldest tree is a 9,500-year-old Norwegian Spruce named 'Old Tjikko'.
Quote of the Day
"When they tell me I'm too old to do something, I attempt it immediately. "
~ Pablo Picasso
On This Day
1660 Charles II marched into London and was restored to the throne after 11 years in exile.