Final answer:
Elements with the smallest effective nuclear charge among elements 1–18 are hydrogen and helium, as they have the least number of shielding electrons which results in a low Zeff value.
Step-by-step explanation:
To determine which elements among 1–18 have the smallest effective nuclear charge (Zeff), we must consider the shielding effect of electrons and the atomic number (Z) for each element. The equation to calculate Zeff is Zeff = Z – S, where S represents the shielding constant. As per the concept of Zeff, elements with the highest number of core or inner-shell electrons will cause more shielding, thus reducing Zeff. Hydrogen, with an atomic number of 1 and no shielding electrons, has a Zeff of 1. Other elements in the first and second rows of the periodic table will have a higher nuclear charge but will also have more inner-shell electrons that provide shielding.
In the case of Helium (He), with a Z of 2 and no shielding electrons, its Zeff is 2. As we move to elements like Lithium (Li) and Beryllium (Be), even though their Z increases, the filled 1s² shell provides a shielding effect, therefore, they have lower Zeff values compared to the increase in their atomic numbers. Therefore, elements like hydrogen and helium have significantly lower Zeff compared to elements with higher atomic numbers such as neon, because they have fewer inner-shell electrons to shield the valence electrons from the nuclear charge.