Under normal atmospheric conditions and at room temperature what is the physical state of CO2

What does a numerical subscript following an element in a chemical formula mean?

Which of the following statements is true about energy quantization at the atomic level? electrons in the outermost orbits are the most stable. electrons in all the orbits around the nucleus have the same amount of energy. electrons in the orbit closest to the nucleus have the least amount of energy. electrons absorb or release the same amount of energy independent of the energy levels.

Ahelium balloon contains 16.9 l of helium at stp. how many atoms of helium are in the balloon