To understand why a cation is smaller than its parent atom, we need to consider the changes in the number of electrons and the effect of the nuclear charge on the remaining electrons when an atom loses electrons to form a cation.
Correct Option: B) It has fewer electrons but the same nuclear charge, increasing effective nuclear pull per electron
This option accurately describes the primary reasons for the size reduction. When an atom loses electrons to form a cation, the number of protons (nuclear charge) remains constant, but the number of electrons decreases. This results in a higher effective nuclear charge per electron, pulling the remaining electrons closer to the nucleus and reducing the overall size of the ion.
To determine why an anion is larger than its parent atom, we need to consider the changes in electron count and their effect on electron-electron repulsion and nuclear attraction.
Correct Option: A) Addition of electrons increases electron-electron repulsion and decreases effective nuclear charge per electron. This statement accurately describes the two primary reasons for the expansion of the electron cloud when an atom forms an anion.
The problem asks us to arrange isoelectronic species in increasing order of their ionic radii. Isoelectronic species are atoms or ions that have the same number of electrons. For isoelectronic species, the ionic radius is inversely proportional to the effective nuclear charge. A higher nuclear charge pulls the electron cloud more tightly, resulting in a smaller ionic radius.
Correct Option: C) Mg²⁺ < Na⁺ < F⁻ < O²⁻
To determine if a pair of species is isoelectronic, we need to count the total number of electrons in each species. Isoelectronic species are atoms or ions that have the same number of electrons.
B) Na⁺ and Cl⁻ — Na⁺ has 10 electrons, and Cl⁻ has 18 electrons. Since their electron counts are different, they are not isoelectronic.
To determine the smallest species among the given options (Mg, Mg²⁺, Al, Al³⁺), we need to compare their atomic/ionic radii. This comparison involves understanding trends in atomic size across a period and the effect of forming cations on atomic size.
B) Al³⁺ is the smallest species. It is a cation formed from Aluminium, and among isoelectronic species (like Mg²⁺ and Al³⁺), the one with the higher nuclear charge (more protons) will have the smallest radius.
The question asks for the definition of the first ionization enthalpy. We need to recall the precise definition of ionization enthalpy from chemistry principles.
C) Energy required to remove the most loosely bound electron from an isolated gaseous atom in its ground state
This option perfectly matches the standard, precise definition of the first ionization enthalpy. The process can be represented as:
\[ \text{X(g)} + \text{Ionization Energy} \rightarrow \text{X}^{+}(\text{g}) + \text{e}^{-} \]