energy required to remove 1 mole of electrons from each atom in 1 mole of gaseous atoms from 1 mole of gaseous 1+ ions
second ionisation energy
energy required to remove 1 mole of electrons from each ion in 1 mole of gaseous +1 ions to form 1 mole of gaseous +2 ioms
orbital
a region in an atom that can hold up to 2 electrons with opposite spins
periodicity
trends in element properties with increasing atomic number
hund's rule
when electrons fill orbitals they occupy them singly before they pair up
pauli exclusion principle
electrons within the same orbital must have opposite spins
aufbau principle
as atomic number increases, the electrons are added to the orbitals in order of increasing orbital energy until all electrons are accommodated
shielding
a decrease in the nuclear charge experienced by an outer shell electron caused by electron-electron repulsion between the outer shell electron and electrons from adjacent quantum shells
