To solve this question, let’s break down the concepts involved:
Nuclear Charge: This is the total positive charge of the nucleus, given by the number of protons. It increases as you move across a period (from left to right) or down a group (from top to bottom) in the periodic table.
Effective Nuclear Charge (Z_eff): This is the net positive charge experienced by an electron in an atom. It accounts for the shielding or screening effect of inner electrons. The effective nuclear charge increases across a period because the number of protons increases while the shielding effect does not increase as much. However, it decreases down a group because additional electron shells increase shielding, reducing the net positive charge felt by the outermost electrons.
Atomic Size: As effective nuclear charge increases across a period, the atomic size decreases because the electrons are pulled closer to the nucleus. Conversely, as you move down a group, the atomic size increases due to the addition of extra electron shells, which outweighs the increase in nuclear charge.
Metallic Character: Metallic character generally increases down a group and decreases across a period. This is because metals lose electrons more easily as you move down a group (due to the larger atomic size and weaker hold on valence electrons) and more difficultly as you move across a period (due to increasing effective nuclear charge making it harder to lose electrons).
Screening Effect: This refers to the reduction in the effective nuclear charge on the electron cloud due to the presence of other electrons between the nucleus and the electron in question. This effect increases down a group as more inner electron shells are added, which shields the outer electrons more effectively.
Inter-Electronic Repulsions: These are repulsive forces between electrons in an atom. They affect the atomic size and the distribution of electrons, but they are less directly related to the trends in effective nuclear charge and metallic character compared to the other concepts.
Identifying the Correct Reverse Trend:
Reverse Trend of Atomic Size: Atomic size increases down a group and decreases across a period.
Role of Screening Effect: The screening effect increases down a group but does not affect the periodic trend directly.
Reverse Trend of Metallic Character: Metallic character increases down a group and decreases across a period, which is the opposite trend to effective nuclear charge.
Role of Inter-Electronic Repulsions: This affects the size and energy levels of electrons but is not typically described as having a reverse trend relative to nuclear charge.
Given the explanation, the correct answer is:
(C) Reverse trend of metallic character
This is because metallic character decreases across a period (opposite to the increasing trend of effective nuclear charge) and increases down a group (opposite to the decreasing trend of effective nuclear charge).
