Shells
Electrons are construed to swirl in orbital layers – shells – defined by their relative quantum energy state. The energy ranges of shells can overlap. Generally, more energetic electrons move in orbitals farther from an atom’s nucleus. But electrons can store some energy before getting so excited that they feel obliged to migrate to another shell.
Like an electronic Matryoshka doll, a shell may have subshells. While it is commonly stated that same-shell electrons have the same energy, that is an approximation. But electrons in the same subshell are equally energetic.
Shell layering is related to atomic spectral lines (electron energy level changes). These energetic relations are a product of quantum interactions and are an hd phenomenon. Electrons in different shells exchange status information to favor a coherent spin alignment of all electrons in their atom.
As atom size increases, more electrons whiz about it. Some electrons in heavier elements attain velocities approaching the speed of light. This increases relativistic mass, causing certain inner orbitals to contract and stabilize; which, in turn, destabilizes outer orbitals and provokes their expansion.
(More specifically, electron velocity is correlated with its effective nuclear charge, which is the net positive charge experienced by an electron in any atom with multiple electrons.)
The relativistic effects of electrons in heavier atoms contribute to their attributes. Gold has its characteristic amber color because of it. Mercury is a liquid at room temperature owing to the relativistic speed of its electrons.
Electron orbitals have a wavelike existence. Their position at any point in time is only a probability.
Today, instead of thinking of electrons as microscopic planets circling a nucleus, we now see them as probability waves sloshing around the orbits like water in some kind of doughnut-shaped tidal pool governed by Schrödinger’s equation. ~ American physicist James Trefil
