The atomic radius of magnesium is approximately 150 picometers.
More Info: A picometer is an incredibly small unit of measurement within the metric system-one picometer is equal to one trillionth of a meter. It's almost impossible for the human mind to appreciate such small measurements. For instance, a single human hair is around 100 micrometers thick, yet this is still one million times larger than the atomic radii of Magnesium!
Particles and Quantum Mechanics
The true radius of magnesium is somewhere between 150-160 picometers. When most people imagine an atom, they are picturing the planetary model: there is a nucleus made of round protons and neutrons and orbiting around the nucleus are the electrons, like planets orbiting the sun. This model is useful, but it doesn't tell the whole story. In reality, the particles in quantum mechanics are not discrete little billiard balls-they're more like clouds.
That's because the mathematics used in quantum mechanics is the same type of mathematics used to produce probabilities: quantum mechanics can only give a probability of where an electron might be found. Therefore, a real atom is much harder to imagine: instead of billiard balls orbiting around each other, an atom is more like a cloud with a definite shape. For instance, one shape that quantum mechanics predicts is a figure-eight-scientists can only say for sure that the electrons are somewhere within that figure-eight shape. The reason it's so difficult to give exact answers in quantum mechanics is that matter behaves so strangely at such small scales!