Given a scientific passage and an answer, generate a question for the given answer.

Passage: Carbon has four valence electrons, so it can achieve a full outer energy level by forming four covalent bonds. When it bonds only with hydrogen, it forms compounds called hydrocarbons. Answer: four
How many valence electrons does carbon have?

Passage: Nuclear changes occur with a simultaneous release of energy. Where does this energy come from? If we could precisely measure the masses of the reactants and the products of a nuclear reaction, we would notice that the amount of mass drops slightly in the conversion from reactants to products. Consider the following nuclear reaction, in which the molar mass of each species is indicated to four decimal places:. Answer: energy
Nuclear changes occur with a simultaneous release of what, and a slight drop in mass?

Passage: Ionic compounds are compounds in which positive metal ions and negative nonmetal ions are joined by ionic bonds. In these compounds, the chemical symbol for the positive metal ion is written first, followed by the symbol for the negative nonmetal ion. The video at the following URL gives a fuller explanation of ionic compounds and their formulas: http://www. youtube. com/watch?v=vscoYh6m46M . Answer: positive metal ion
The chemical symbol of which ion is written first in ionic compounds?