Energy-Mass States of Bound and Free Electrons: ECM Interpretation of Atomic Transitions, Thermionic Emission, and Photon Emission.

An Extended Classical Mechanics Interpretation: Energy-Mass States of Bound and Free Electrons. 

Soumendra Nath Thakur | ORCiD: 0000-0003-1871-7803 | Tagore's Electronic Lab, India | July 04, 2025

The total energy-mass of a free electron is equivalent to its rest mass energy, expressed as:

Eₜₒₜₐₗ = Mₑc² ≈ 0.511 MeV,

where Mₑ denotes the electron's rest mass.

Within an atom, an electron bound in the lowest energy orbital—the ground state (n = 1)—exhibits a significantly lower Eₜₒₜₐₗ due to the presence of negative electrostatic potential energy resulting from Coulomb interaction with the atomic nucleus. For a hydrogen atom, this energy level is quantized and is given by:

Eₙ = −13.6 eV for n = 1,

E₂ = −3.4 eV, E₃ = −1.51 eV, etc.

These values represent net bound-state energies, which are markedly lower than the energy-mass condition of a free, unbound electron at rest (0.511 MeV), emphasizing that atomic electrons possess lower Eₜₒₜₐₗ due to confinement.

In outer shells, the valence electron, often residing at the highest occupied energy level, possesses the greatest total energy relative to other bound electrons. Under thermal excitation, when sufficient energy is supplied (typically via heat), the electron may acquire enough kinetic energy to overcome the −Mᵃᵖᵖc² binding potential (where −Mᵃᵖᵖ denotes the negative apparent mass associated with electrostatic confinement), thereby escaping the atomic structure in a process known as thermionic emission.

During thermionic emission, the electron transitions from a bound state (Mᴍ < Mₑ) to a nearly free state, achieving:

ΔMᴍ = Mₑ − Mᴍ > 0,

accompanied by the displacement of −Mᵃᵖᵖ from the atomic system to the metallic boundary surface. This released electron becomes quasi-free and localized near the outer metallic surface, though not yet a completely free particle in vacuum.

In contrast, photoelectric emission occurs when incident photons of sufficient frequency (f) interact with valence electrons. If the photon energy hf ≥ |−Mᵃᵖᵖ|c², the electron overcomes its binding condition and is emitted from the material. Here, the interaction satisfies:

hf = ΔMᴍc² = −Mᵃᵖᵖc²,

highlighting the mass-energy equivalence of photon interaction with electron confinement energy.

Within atoms, when an electron transitions from a higher energy level (nᵢ) to a lower one (n𝒻), the energy difference is released as a photon:

ΔE = hf = Eₙᵢ − Eₙ𝒻,

signifying the conversion of potential and kinetic energy loss (−ΔPEᴇᴄᴍ and −ΔKEᴇᴄᴍ) into radiative output. This emission occurs only for bound electrons undergoing quantized transitions. In contrast, a truly free electron (Mᴍ = Mₑ) does not emit photons under motion in free space, as it lacks quantized energy states or orbital confinement.

Thus, under conservative dynamics, such as an electron moving within an electric potential, any gain in potential energy is reciprocated by a corresponding loss in kinetic energy, and vice versa:

−ΔPEᴇᴄᴍ = +ΔKEᴇᴄᴍ,

preserving total internal energy. During quantum transitions, the decrease in bound-state potential energy is manifest externally as photon emission—corresponding to the released hf, now separable from the atom as a radiative quantum of energy and mass.