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

[Q]: 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
[A]: How many valence electrons does carbon have?

[Q]: Passage: outermost layer of the sun, made up of a plasma that extends millions of kilometers into space. Answer: plasma
[A]: What is the outer layer of the sun made up of?

[Q]: Passage: Nuclear symbols are used to write nuclear equations for radioactive decay. Let's consider an example. Uranium-238 undergoes alpha decay to become thorium-234. (The numbers following the chemical names refer to the number of protons plus neutrons. ) In this reaction, uranium-238 loses two protons and two neutrons to become the element thorium-234. The reaction can be represented by this nuclear equation:. Answer: nuclear symbols
[A]:
What are used to write nuclear equations for radioactive decay?