It was Lavoisier who produced the first fairly reliable list of elements. However, it contained substances which had not, up to that time, yielded up their true elements, such as magnesia. Only with the discovery of the electric battery by Volta did the means become available to split magnesia (actually magnesium oxide). The metal magnesium was obtained for the first time by Humphry Davy, using electrolysis. Davy also became famous for releasing the elements potassium, sodium, calcium, strontium and barium from their compounds, none of which had ever been seen before. All of these metals react vigorously or violently with water to produce alkaline solutions. They are much more reactive than metals such as copper, lead, tin and iron, which humans had learnt to extract from the rocks by the process of smelting, in which the rock was heated strongly with charcoal or coke. However, the vast majority of elements are stable. Bismuth, which has the atomic number of 83, is the last element in the list with a stable nucleus. Beyond 83, all elements are unstable (radioactive). The atomic number of an element is the number of protons in the nucleus.