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© Courtesy of Brookhaven National Laboratory.

The mass number, A, of atoms is not entirely under our control, as this chart of nuclides shows. Each colored dot on the chart represents a nucleus either found in nature or created in the laboratory, with atomic number Z on the vertical axis, and mass number A (or N) on the horizontal axis. The color of the dot indicates the half-life of each nucleus: how long it lasts, on average, before decaying into another element. The darkest dots represent elements that we encounter in daily life, and the pink dots represent elements that last for less than 10-15 seconds. The choice of nuclei stable enough to use in experiments is quite limited. This chart is continually evolving. The element with Z = 118 in the upper-right-hand corner of the chart was discovered in 2006, and convincing evidence for a Z = 117 nucleus first appeared in 2010. The "island of stability" to the right of the pink and yellow region is the subject of active research. (Unit: 6)