1.4.6—Factors influencing the ionisation energies

For a first-ionisation-energy comparison, identify the electron removed in each species and check four linked factors: nuclear charge, distance from the nucleus, shielding by inner shells, and spin-pair repulsion.

State how each factor changes the attraction: greater nuclear charge tends to increase ionisation energy, whereas greater distance or shielding tends to reduce it. Pairing can lower the value because two electrons repel within one orbital. Then decide which effect dominates for the actual comparison.

Use the sub-shell and pairing evidence to explain exceptions to a general period trend. A 3p electron can be easier to remove than a 3s electron because it is higher in energy, while a paired 3p electron can be easier to remove than an unpaired counterpart; these explain the Mg/Al and P/S-type dips supported by the SME.

Do not list four factors without linking them to the electron being removed, and do not claim that proton number alone decides the result. Keep this comparison method separate from the definition of first ionisation energy, the across-period overview and the successive-ionisation jump method.