Electron configuration
Ionization of the transition metals

In chemical environments, configurations can change even more: Th as a bare ion has a configuration of 5f, yet in most Th compounds the thorium atom has a 6d configuration instead. Mostly, what is present is rather a superposition of various configurations. For instance, copper metal is poorly described by either an 3d 4s or an 3d 4s configuration, but is rather well described as a 90% contribution of the first and a 10% contribution of the second. Indeed, visible light is already enough to excite electrons in most transition metals, and they often continuously "flow" through different configurations when that happens (copper and its group are an exception).
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