Question

The set representing the correct order of first ionization potential is: (a) \(\mathrm{K}>\mathrm{Na}>\mathrm{Li}\) (b) \(\mathrm{Br}>\mathrm{Mg}>\mathrm{Ca}\) (c) \(\mathrm{B}>\mathrm{C}>\mathrm{N}\) (d) \(\mathrm{Ge}>\mathrm{Si}>\mathrm{C}\)

Short answer

The correct order is in option (b): Br > Mg > Ca.

Step-by-step solution

Understand Ionization Potential

Ionization potential (energy) is the energy required to remove an electron from an atom in gaseous state. Generally, ionization potential increases across a period and decreases down a group in the periodic table.

Analyze Option (a)

The order given in option (a) is K > Na > Li. In the periodic table, these elements belong to Group 1 and move from top (Li) to bottom (K). Ionization potential decreases down the group, which means the correct order should be Li > Na > K, hence this option is incorrect.

Analyze Option (b)

The order given in option (b) is Br > Mg > Ca. Br (Bromine) is a halogen in Group 17, while Mg and Ca are in Group 2. Comparing groups, halogens have higher ionization potentials than alkali earth metals. Therefore, Br having higher ionization potential than Mg and Ca is reasonable. Mg and Ca belong to the same group with Ca below Mg, indicating ionization potential: Br > Mg > Ca is correct.

Analyze Option (c)

The order given in option (c) is B > C > N. Moving left to right in a period increases ionization energy, so the typical order is N > C > B. This option is incorrect because nitrogen should have the highest ionization potential.

Analyze Option (d)

The order given in option (d) is Ge > Si > C. These elements belong to Group 14, and the ionization potential decreases down the group, making it C > Si > Ge. Thus, this order is incorrect.

Key concepts

Periodic Table Trends

The periodic table is an organized chart of elements that reveals several repeating patterns, known as periodic trends. One such trend is the ionization potential, which is the energy required to remove an electron from an atom.
As you move across a period from left to right, the ionization potential typically increases. This is because the nuclear charge increases, pulling electrons closer to the nucleus and making them harder to remove.
In contrast, as you move down a group, the ionization potential usually decreases. The outer electrons are farther from the nucleus due to additional electron shells, reducing the nuclear pull and making it easier to remove electrons.

Ionization Energy Order

Ordering elements by their ionization energy can help understand their chemical behavior. For example, elements with higher ionization energies usually resist losing electrons and are more likely to gain instead.
To determine the correct order, it's crucial to apply the periodic table trends. Elements across a period generally increase in ionization energy, while those down a group decrease. For instance, in option (a), the correct order for lithium, sodium, and potassium should be Li > Na > K, following the trend within Group 1.
Always consider periodic trends when determining ionization energy order, as it must reflect the underlying principles of atomic structure.

Group and Period Trends

Group and period trends in the periodic table are essential for predicting and understanding elemental properties.
Within a group, elements have similar configurations of outer electrons, so they show predictable changes in physical and chemical properties, like a decrease in ionization potential as you move from top to bottom.
Across a period, elements gain more protons in the nucleus, causing their ionization energy to increase. For example, in option (c), the sequence should be N > C > B, which follows the increase in ionization potential from left to right in a period.

Alkali and Halogen Comparison

Comparing alkali metals (Group 1) to halogens (Group 17) shows distinct differences in ionization energies. Alkali metals like lithium, sodium, and potassium have relatively low ionization energies. This is because they have a single electron in their outermost shell that they can lose easily.
Halogens, in contrast, possess higher ionization energies as they are one electron short of a complete valence shell, so they strongly resist losing electrons.

• This is exemplified in option (b) where bromine (a halogen) has a higher ionization potential than magnesium and calcium, which belong to the alkali earth metals group.
• The comparison highlights how the distinct electron configurations between these groups dictate their ionization potentials.

Understanding the differences lets us predict their chemical reactivity and bonding tendencies.