The magnitude of this quantity equals the energy of the highest occupied molecular orbital.
The magnitude of this quantity equals the energy of the highest occupied molecular orbital. This property for silver is lower than one might expect, because it deviates from the aufbau principle to have a filled 4d subshell. For a hydrogenic atom, this quantity equals nuclear charge squared times 13.6 electron-volts. One analog of this quantity for solids is called the work function. Given this property's similarity to electron affinity, it is unsurprising that it increases towards the top right of the periodic table. For 10 points, give this name for the amount of energy required to remove an electron from a gas-phase atom. 
The magnitude of this quantity equals the energy of the highest occupied molecular orbital.
first ionization energy [or ionization potential] 