1.4.3—The trends in ionisation energies across

Across a period, first ionisation energy generally increases. Proton number rises while added electrons enter the same principal shell, so shielding changes relatively little and the outer electron experiences a stronger overall attraction to the nucleus.

The rising nuclear charge pulls the outer electron closer, while atomic radius generally decreases across the period. The stronger attraction makes the outer electron harder to remove, so more energy is required; the trend is general rather than perfectly smooth.

Explain a small dip from the electron arrangement, not from a fall in nuclear charge. A p electron can be higher in energy than an s electron, as for Al compared with Mg, or paired p electrons can repel each other, as for S compared with P; either makes removal slightly easier.

When reading a period trend, compare the relevant outer-electron sub-shell, distance, shielding and spin-pairing evidence together. Do not claim a perfectly straight increase or explain every dip only by atomic number; detailed successive-ionisation jumps and group deductions belong to the neighbouring objectives.