Problem 123 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following $$ \begin{array}{ll} \hline \text { Column-I } & \text { Column-II } \\ \hline \begin{array}{ll} \text { (a) } \mathrm{C}>\mathrm{N} & \text { (p) More favourable (exothermic) } \\ \text { electron affinity. } \end{array} \\ \text { (b) } \mathrm{Se}>\mathrm{Br} & \text { (q) The higher first ionization energy. } \\ \text { (c) } \mathrm{Mg}>\mathrm{K} & \text { (r) The larger size } \\ \text { (d) } \mathrm{F}>\mathrm{Cl} & \text { (s) The higher electronegativity. } \\ & \text { (t) The higher number of valence } \\ \text { electrons. } \\ \hline \end{array} $$

Step-by-Step Solution

Answer

(a) - (p), (b) - (r), (c) - (q), (d) - (s).

Step 1: Understanding Electron Affinity

Electron affinity refers to the energy change when an electron is added to a neutral atom. More favorable (exothermic) electron affinity means more negative energy change, indicating it is easier for the atom to gain an electron.

Step 2: Match (a) C>N

Carbon ( C ) has a greater electron affinity compared to nitrogen because adding an electron to nitrogen would require it to pair up, which is less favorable due to electron-electron repulsion. Therefore, C>N matches with (p) More favourable (exothermic).

Step 3: Comparing First Ionization Energy

First ionization energy is the energy required to remove the first electron from an atom. Elements with higher electronegativity tend to have higher ionization energies.

Step 4: Match (b) Se>Br

Selenium ( Se ) and Bromine ( Br ) have different ionization energies. However, Se being in the higher period does not align for higher ionization energy. Instead, the size comparison for Se and Br makes Se larger. Thus, Se> Br matches with (r) The larger size.

Step 5: Comparing Atomic Size

Atomic size generally increases down a group and decreases across a period.

Key Concepts

Electron Affinity

Electron affinity is an important concept to understand in the context of periodic table trends. It refers to the energy change that occurs when an electron is added to a neutral atom. When an atom readily accepts an electron, this process tends to release energy, meaning the electron affinity value is negative, indicating an exothermic reaction.
This trend is especially pronounced in nonmetals, as these elements strive to complete their outer electron shells. For instance, carbon ( C ) has a more favorable electron affinity compared to nitrogen ( N ). The reason why carbon is more favorable is related to electron repulsion. In nitrogen, the added electron must pair up in an already half-filled orbital, causing more electron-electron repulsion. Hence, this is why carbon often exhibits more favorable (or exothermic) electron affinity.

Ionization Energy

Ionization energy refers to the amount of energy required to remove the outermost electron from a neutral atom. It serves as an indicator of how strongly an atom's nucleus holds onto its electrons. Generally, atoms with full or nearly full electron shells have higher ionization energies.
Elements situated towards the right of the periodic table, such as noble gases, have higher ionization energies due to their stable electron configurations. Interestingly, first ionization energy increases across a period and decreases down a group.
In terms of our original problem, it's noteworthy that electronegativity and ionization energy often exhibit similar trends. Yet, selenium ( Se ) doesn't fit this trend when compared to bromine ( Br ) for higher ionization energy, illustrating how specific periodic trends may vary between elements.

Atomic Size

Atomic size, or atomic radius, defines an atom's size in terms of the typical distance from its nucleus to the boundary of its surrounding cloud of electrons. On the periodic table, atomic size increases as you move down a group due to the addition of more electron shells.
Conversely, atomic size decreases from left to right across a period. This decrease occurs because additional protons in the nucleus result in a stronger attraction between the nucleus and electron cloud, pulling electrons closer to the nucleus.
In the given scenario, selenium ( Se ) is compared with bromine ( Br ). Selenium has a larger atomic size because it lies in a higher period, causing more electron shells around its nucleus, increasing the overall size.

Electronegativity

Electronegativity measures an atom's ability to attract and bond with electrons. This property is crucial for understanding how atoms interact in chemical reactions and is an essential factor in predicting molecular structures and properties.
Generally, electronegativity increases across a period from left to right and decreases down a group. Fluorine ( F ) is known as the most electronegative element in the periodic table.
In our context, fluorine ( F ) is compared to chlorine ( Cl ). While both are highly electronegative, fluorine's proximity in the period to the noble gases and its small atomic size means it holds onto and attracts electrons more strongly than chlorine, making it slightly more electronegative.