_________ is the reason behind most of your other horizontal trends.

Question

BlackTom AI · Accepted Answer

To tackle the statement, we first identify what the blank is asking about in the context of periodic trends. The claim is that a single concept underlies most of the horizontal (left–right) trends across a period.
A key concept that explains these horizontal trends is the effective nuclear charge (ENC). ENC represents the net positive charge that an electron experiences from the nucleus after accounting for shielding by inner electrons. As you move across a period from left to right, protons are added to the nucleus while the shielding effect from inner electrons remains relatively constant. This causes ENC to increase across the period.
Because ENC increases across a period, several observable horizontal trends follow. For instance, the increased attraction between the nucleus and the valence electrons pulls electrons in more tightly, resulting in a smaller atomic radius. It also raises the energy required to remove an outer electron, leading to higher first ionization energies. Additionally, the stronger attraction can make electron affinity more favorable for many nonmetals.
If we consider alternative factors, such as just nuclear charge alone, that would ignore the concomitant shielding effect and could mislead about the true pull on valence electrons. Similarly, attributing trends solely to atomic number without considering shielding would miss the nuanced increase in ENC across the period. Electron shielding is relatively constant across a period, whereas ENC increases due to the increasing number of protons, which is why ENC is the most coherent explanation for the horizontal trends observed.
Thus, the statement aligns with the central role of effective nuclear charge in governing the left-to-right (horizontal) periodic trends across a period, including atomic size, ionization energy, and related properties.

1A W25 Effective Nuclear Charge and Atomic radius (two videos)

_________ is the reason behind most of your other horizontal trends.

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