Final answer:
An atom with an atomic number of 9 and a mass number of 19 in the electron configuration of 2-6-1 is in an excited state because it has an electron in the third shell, indicating it has absorbed energy and transitioned to a higher energy level than the ground state configuration of 2-7.
Step-by-step explanation:
An atom with an atomic number of 9 and a mass number of 19 typically represents fluorine. Normally, a ground state fluorine atom would have an electron configuration of 2-7, meaning 2 electrons in the first shell and 7 in the second shell. However, the configuration provided of 2-6-1 indicates an excited state because one of the electrons from the second shell has absorbed enough energy to jump to the third shell, which is not its lowest energy state.
The principal quantum numbers indicate the energy levels of the atomic orbitals (shells), with the first shell being the lowest energy level, followed by the second, and so on. In its ground state, the second energy level (shell) of fluorine would be filled with 7 electrons, while the third would have none because fluorine has a total of 9 electrons. The configuration of 2-6-1 suggests that one electron has moved to a higher energy level than it would normally occupy in a ground state, hence the atom is in an excited state.
The electron configuration of a ground state fluorine atom is typically 1s²2s²2p⁵. In contrast, the excited state mentioned with a configuration of 2-6-1 has an electron that has leaped from the second shell to the third, after absorbing energy in some form, likely a photon. When an electron transitions from a lower energy level to a higher one, it is because it has absorbed energy, which is indicated by a positive value in energy change. When the excited electron returns to the ground state, it releases this energy, often in the form of photon emission.