Help is science, if you don't know the answer just don't put anything /:

Light

Explanation:

Light is a good example of invisible particles. We cannot see light particles aka photon. We only know the light is present by the objects we see. Because we can only see objects when light reflects from them on our eyes.

[I could only think of one example if I can remember one more I'll edit it down]

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.

nacl or sodium chloride is an ionic compound. this is because an ionic compound pertains to a compound that consists of positively and negatively charged ions. in this case, na (sodium) has a tendency to lose one electron when it reacts with with other atoms, so the resulting sodium ion contains 11 protons and 10 electrons, thus making it positively charged (na+), when chlorine reacts, it usually gains one electron to form an ion called chloride, thus making it negatively charged ( since the ions are oppositely charged, they attract.