Final answer:
The 2s orbital is at a higher energy level and is larger than the 1s orbital, which is at the first and lowest energy level. Both can hold up to two electrons, but due to its lower energy, the 1s orbital is filled first.
Step-by-step explanation:
The major difference between a 1s orbital and a 2s orbital is that the 2s orbital is at a higher energy level than the 1s orbital. While both the 1s and 2s orbitals are spherically symmetrical and can hold a maximum of 2 electrons, the 2s orbital is larger and has a principal quantum number, n, of 2, indicating that it occupies the second energy level in an atom. In contrast, the 1s orbital has a principal quantum number of 1 and is located at the first, lowest energy level.
As the principal quantum number increases, so does the size of the orbital and the energy, resulting in the 2s orbital having electrons that spend more time farther from the nucleus compared to those in the 1s orbital. The 1s orbital is always filled first due to its lower energy before electrons start filling the 2s orbital.