What makes the nuclei of our atoms stable? The atomic nucleus consists of a collection of protons and neutrons (collectively called Nucleons); for example, the typical oxygen nucleus has 8 protons, and 8 neutrons packed densely into a ball approximately of radius 2x10^-15 m (One billionth of a dust particle!). Since protons are (positively) charged particles, they repel each other. Neutrons act as fillers, and thus weaken this eletromagnetic force of repulsion, and allow the strong nuclear force (the force that binds the nucleons in a ball) to dominate, which makes the nucleus stable:).

A typical magical shell (Courtesy: SUNY Stony Brook)However, then the question becomes: how many neutrons is enough? Too less, and the nucleus will fall apart due to repulsive forces; too many, and the nucleus will be unstable as it tends to beta decay (process by which a neutron converts into a proton, and emits a electron). Therefore there is a optimal region, when the nucleus is most stable.
According to the nucleus shell model proposed in 1949, the nucleons inside the nucleus are arranged in shells. The nucleus is most stable when there are enough nucleons to fill a shell. This number is known as the magic number, are are represented by the sequence 2, 8, 20, 28, 50, 82, 126 and so on. Any nucleus containing these many nucleons are the most stable:):).
Now, nuclear physicists have created an isotope of silicon that contains twice as many neutrons as protons. Measurements made with silicon-42 - which contains 14 protons and 28 neutrons - will shed new light on the concept of "magic numbers" in nuclei.
The fact that silicon-42 is stable, was a surprising find. Paul Cottle and colleagues at Florida State University, Michigan State University, the Lawrence Berkeley National Laboratory and Surrey University in the UK produced the silicon-42 nuclei by crashing sulphur-44 nuclei into a beryllium target at the National Superconducting Cyclotron Lab (NSCL) at Michigan State University. The results show that the silicon-42 nucleus remains stable despite containing a large excess of neutrons. The data also suggest that the proton number 14 is semi-magic because it corresponds to a closed subshell, which means that the nucleus is also spherical :):).