Problem 66 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of first ionization potential is (a) \(\mathrm{F}>\mathrm{He}>\mathrm{Mg}>\mathrm{N}>\mathrm{O}\) (b) \(\mathrm{He}>\mathrm{F}>\mathrm{N}>\mathrm{O}>\mathrm{Mg}\) (c) \(\mathrm{He}>\mathrm{O}>\mathrm{F}>\mathrm{N}>\mathrm{Mg}\) (d) \(\mathrm{O}>\mathrm{F}>\mathrm{He}>\mathrm{N}>\mathrm{Mg}\)

Step-by-Step Solution

Answer

The correct order is (b) \( \mathrm{He} > \mathrm{F} > \mathrm{N} > \mathrm{O} > \mathrm{Mg} \).

Step 1: Understanding Ionization Potential

The first ionization potential (or ionization energy) is the energy required to remove the outermost electron from a neutral atom in its gaseous state.

Step 2: Analyze Trends in the Periodic Table

Ionization potential generally increases across a period (left to right) due to increasing nuclear charge which leads to stronger attraction to electrons. But it decreases down a group (top to bottom) due to increasing atomic size.

Step 3: Comparing Helium to Fluorine

Helium, despite being in Group 18, has the highest ionization potential because it is a noble gas with a full outer shell. Fluorine, though very electronegative, has a decreased ionization potential compared to Helium.

Step 4: Evaluate Nitrogen and Oxygen

Nitrogen has a higher ionization potential than Oxygen due to its half-filled p subshell providing extra stability. Removing an electron from Nitrogen is harder, thus a higher ionization energy is needed.

Step 5: Position Magnesium

Magnesium, being further down in a period and in Group 2, has a lower first ionization potential than all these elements because its electrons are more shielded and further from the nucleus.

Step 6: Identify the Correct Order

From analysis: the correct order seems to be Helium, Fluorine, Nitrogen, Oxygen, Magnesium based on those ionization principles.

Key Concepts

Periodic Table Trends

Understanding how ionization potential changes in the periodic table can help predict how easily an atom will lose an electron. As you move from left to right across a period, the ionization potential generally increases. This is because atoms hold their electrons more tightly due to the increasing positive charge in the nucleus. As a result, more energy is needed to remove an electron.
On the other hand, as you go down a group, the ionization potential tends to decrease. This occurs because electrons are further from the nucleus and are less tightly held, making it easier and requiring less energy to remove an electron. Alongside this, additional electron shells increase shielding, weakening the attraction between the nucleus and the outermost electrons.

Noble Gases

Noble gases like Helium are unique due to their complete outer electron shells, making them very stable and unreactive. This full shell configuration results in the highest ionization energies on the periodic table.
Since they are already stable, it requires a significant amount of energy to remove an electron from a noble gas. Helium, with only one electron shell that holds its electrons very close to its nucleus, has the highest ionization potential among all elements.

• Noble gases have full valence shells.
• Highest ionization energy due to stable configuration.
• Exceptionally unreactive.

P Subshell Stability

The stability of the p subshell plays a crucial role in determining an atom's ionization potential. Nitrogen is an interesting case where it has a half-filled p subshell that provides extra stability.
This half-filled shell arrangement makes nitrogen more stable than it would be if you attempted to remove an electron. It requires more ionization energy to disrupt this stable configuration.
In contrast, Oxygen has one more electron than a half-filled shell, making it slightly destabilized and requiring less energy to remove an electron compared to nitrogen.

• Half-filled p subshell increases stability (e.g., Nitrogen).
• Nitrogen has higher ionization energy than Oxygen.
• Disrupting stability requires extra energy.

Group 2 Elements

Group 2 elements, which are also known as alkaline earth metals, have relatively low ionization potentials. This is because the outer electrons are further from the nucleus compared to other elements within the same period and are more shielded by inner electrons.
Magnesium, a member of this group, demonstrates this property. With its electrons being further away and the shielding effect in play, it requires significantly less ionization energy to remove an outer electron compared to elements closer to the start of the period.
Despite being in the same period as elements like Fluorine and Neon, the ionization energy of Group 2 elements is much lower.

• Group 2 elements have electrons farther from the nucleus.
• Show strong shielding effects.
• Lower ionization energy compared to nearby period elements.