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Step-by-Step Explanation
Step 1: Understanding Ionization Enthalpy
Ionization enthalpy (IE) is the energy required to remove an electron from an isolated, gaseous atom. In other words, it measures how strongly an atom holds onto its outermost electron.
Step 2: General Trend in a Period
As we move from left to right across a period in the periodic table:
The nuclear charge increases because the number of protons in the nucleus increases.
The electrons are added to the same principal energy level, so shielding does not increase substantially.
Hence, the effective nuclear charge experienced by each electron generally increases.
As a result, electrons are held more tightly, and the ionization enthalpy usually increases across a period.
Step 3: Ionization Enthalpy in the 3rd Period
In the third period (Na to Ar), the first ionization enthalpies roughly follow the order:
$ \text{Na} < \text{Al} < \text{Mg} < \text{Si} < \text{S} < \text{P} < \text{Cl} < \text{Ar} $
Numerically in electron volts (eV), this can be seen as:
Na $\approx 5.14\,\mathrm{eV}$
Al $\approx 5.99\,\mathrm{eV}$
Mg $\approx 7.65\,\mathrm{eV}$
Si $\approx 8.15\,\mathrm{eV}$
S $\approx 10.36\,\mathrm{eV}$
P $\approx 10.49\,\mathrm{eV}$
Cl $\approx 12.97\,\mathrm{eV}$
Ar $\approx 15.76\,\mathrm{eV}$
From this order, one can see that Mg has a higher ionization enthalpy than Na and Al but lower than Si, P, S, Cl, and Ar.
Step 4: Identifying X, Y, and Z
According to the question:
“Magnesium’s first ionization energy is smaller as compared to that of elements X and Y”
→ X and Y must lie to the right of Mg in the period (with higher IE than Mg). Argon and chlorine both have higher ionization enthalpies than magnesium.
“But higher than that of Z.”
→ Z must be an element to the left of Mg in the period (with lower IE than Mg). Sodium has a lower ionization enthalpy than magnesium.
Thus, the elements X, Y, and Z that fit these conditions are:
X = Argon (IE higher than Mg)
Y = Chlorine (IE higher than Mg)
Z = Sodium (IE lower than Mg)
Step 5: Conclusion
The correct set of elements is:
Argon, Chlorine, and Sodium.
