In a hydrogen atom there is only one fundamental interaction: the Coulomb attraction between the single electron and the positively charged nucleus.

In any multi‑electron atom, all of the following additional interactions appear that hydrogen does not have:

  1. Electron–electron repulsion
    • Each electron is negatively charged, so every pair of electrons repels via Coulomb force.
 * This mutual **repulsion** makes the potential energy more complicated than the simple one‑electron Coulomb potential of hydrogen.
  1. Shielding (screening) of nuclear charge
    • Inner (core) electrons partially block or “shield” the full positive charge of the nucleus from outer (valence) electrons.
 * As a result, outer electrons feel an effective nuclear charge ZeffZ_{\text{eff}}Zeff​ that is smaller than the actual nuclear charge ZZZ.
  1. Orbital‑dependent effective attraction
    • In hydrogen, all orbitals with the same principal quantum number nnn (like 2s, 2p, 2d) have the same energy because energy depends only on nnn.
 * In multi‑electron atoms, because of shielding and electron–electron repulsion, s, p, d, f orbitals at the same nnn penetrate the electron cloud differently and feel different ZeffZ_{\text{eff}}Zeff​, so they have different energies (e.g., E2s≠E2pE_{2s}\neq E_{2p}E2s​=E2p​).
  1. Many‑body correlation of motion
    • In hydrogen, the electron moves only in the field of the nucleus; the SchrĂśdinger equation has an exact analytical solution.
 * In multi‑electron atoms, each electron’s motion is correlated with the others because they are constantly repelling each other, leading to complex “electron correlation” effects that cannot be captured by treating electrons as completely independent.
  1. Consequences for spectra and energy levels
    • Hydrogen‑like atoms show energy levels that depend only on nnn, giving simple line series (Lyman, Balmer, etc.).
 * Multi‑electron atoms exhibit splitting of levels within the same shell (e.g., 3s, 3p, 3d all different), modified ionization energies, and richer spectral patterns because of electron–electron repulsion, shielding, and orbital‑dependent ZeffZ_{\text{eff}}Zeff​.

So, the key forces/interactions that are present in a multi‑electron atom but absent in a one‑electron hydrogen atom are electron–electron Coulomb repulsion, shielding of the nucleus, and the resulting many‑body correlation of electron motion, which together break the simple hydrogen‑like energy degeneracy and structure.

Meta description (SEO style)
Learn what forces or interactions are present in a multi‑electron atom that are not present in a hydrogen atom with just one electron, including electron–electron repulsion, shielding, and their impact on energy levels and spectra.

Information gathered from public forums or data available on the internet and portrayed here.