A hydrogen atom consists of a single proton in its nucleus and one electron orbiting around the nucleus. The electron in a hydrogen atom occupies the lowest energy level or shell, which is known as the 1s orbital.
The electronic structure of an atom is determined by the distribution of electrons in various energy levels or shells. Each shell can hold a specific number of electrons, and electrons tend to occupy the lowest available energy level first before moving to higher energy levels.
In the case of hydrogen, the 1s orbital is the lowest energy level available, and it can hold a maximum of two electrons. However, since hydrogen has only one electron, it occupies the 1s orbital, and there are no other lower energy levels to fill.
The distribution of electrons in an atom follows the principles of the Aufbau principle and the Pauli exclusion principle. The Aufbau principle states that electrons fill the lowest energy levels first before moving to higher energy levels. The Pauli exclusion principle states that no two electrons within an atom can have the same set of quantum numbers, which means that each electron must have a unique combination of quantum numbers (such as principal quantum number, azimuthal quantum number, magnetic quantum number, and spin quantum number).
In summary, a hydrogen atom has only one electron in its outer shell (the 1s orbital) because it has only one electron overall, and the 1s orbital is the lowest energy level available for electron occupancy.