Animated Solution for Chemistry - Periodicity in Properties: In general, the property (magnitudes only) that shows an opposite trend in comparison to other properties across a period is

Welcome to a fascinating exploration of the periodic table! When we look at the elements arranged in a period (a horizontal row), we are essentially watching a story unfold. Imagine hopping from one element to the next, say from Lithium ($Li$) all the way across to Neon ($Ne$). As we make this journey, the fundamental architecture inside the atoms undergoes a systematic change, which in turn dictates their chemical and physical properties.

Effective Nuclear Charge

The secret to understanding almost all periodic trends lies in one master variable: the effective nuclear charge ($Z_{\text{eff}}$). As we move from left to right across a period, the atomic number ($Z$) increases. This means we are adding protons to the nucleus, making it more positively charged.

Simultaneously, we are adding electrons, but here is the catch: these new electrons are entering the exact same outermost principal shell. Because they are in the same shell, they do not shield each other very effectively from the pull of the nucleus. Mathematically, we express this as $Z_{\text{eff}}=Z-\sigma$, where the shielding constant ($\sigma$) increases at a slower rate than the actual nuclear charge ($Z$). Consequently, the effective nuclear charge steadily increases. The nucleus pulls harder and harder on the electron cloud!

EN, IE, and EGE

Now, think about what a stronger nuclear pull means for the atom's properties. If the nucleus is holding onto its electrons tightly, it becomes much harder to remove an outermost electron. Therefore, the Ionisation Enthalpy (IE) increases.

Similarly, the atom's tendency to attract shared electrons in a chemical bond, which we call Electronegativity (EN), also increases because the nucleus has a stronger attractive force.

Furthermore, when an atom gains an extra electron, the stronger nuclear pull means more energy is released. Thus, the magnitude of the Electron Gain Enthalpy ($|{\Delta }_{eg}H|$) goes up too. All three of these properties—Electronegativity, Ionisation Enthalpy, and the magnitude of Electron Gain Enthalpy—follow the exact same upward trend across a period.

Atomic Radius

But what about the physical size of the atom? Because that powerful effective nuclear charge is pulling the outermost electrons closer and closer to the center, the overall electron cloud shrinks.

So, the Atomic Radius actually decreases as we move from left to right. Notice how this is the exact opposite of the other properties we just discussed! While EN, IE, and $|{\Delta }_{eg}H|$ all increase due to the stronger nuclear grip, the atomic radius is the lone property that decreases. Therefore, atomic radius is the correct answer to our question.