Bohr said that the electron in a hydrogen atom could only occupy certain orbits n = 1, 2, 3, etc., with corresponding energy levels E₁, E₂, E₃, etc - Leaving Cert Chemistry - Question (b) - 2018

The term used for the condition of the hydrogen atom when its electron occupies the levels E₂, E₃, etc., is excited state(s). These states occur when the electron has absorbed energy and moved to a higher energy level.

The electron leaves the E₁ level when it acquires energy. This energy can come from various sources, such as heating, electric currents passing through the gas, or when the atom is irradiated with light, allowing the electron to absorb a photon.

The electron does not remain in the levels E₂, E₁, etc., because it is in a higher energy state which is unstable. Electrons tend to return to lower energy states, resulting in the emission of energy, often in the form of light.

The energy level that the electrons fall to, producing visible lines in the emission spectrum, is identified as E₁, which corresponds to the ground state of the hydrogen atom.

Modern atomic theory describes the behaviour of electrons by stating that they exhibit both wave and particle properties. This dual nature allows for a more comprehensive understanding of their interactions and locations around the nucleus.

Orbitals are defined as regions around the nucleus of an atom where there is a high probability of finding an electron. They are solutions to the Schrödinger wave equation and describe the spatial distribution of electrons rather than fixed paths.