10 5p 3 6.81 Valence shell electrons are all the s and p electrons beyond the next lowest noble gas configuration. A nodal plane is a stationary surface along which the matter wave is zero. It separates regions of space with positive amplitude from those of negative amplitude. 6.98 The size of all atoms is determined by the outermost electrons. With fewer nodes, the d's lie closer to the nucleus than the s's, shielding them from the growing attraction of the increasing protons as one traverses the table left to right.
More effective shielding means weaker changes.
That pairing forces the additional electron into the same spatial geometry occupied by another. The Hund's Rule energetic advantage of 15 P is thus lost to 16 S, and sulfur's IP is thus a tad greater than phosphorus's. Br is a larger atom than Cl, hence less attractive to the added electron since it'll end up further from the nucleus. So F is somewhat less hospitable to an added electron than is Cl.