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Classification of Elements and Periodicity - Periodic Trends in Chemical Properties

Grade 11ICSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Periodicity in Valence: The valency of elements in a period increases from 11 to 44 and then decreases to 00 with respect to hydrogen. Down a group, the number of valence electrons remains the same, resulting in constant valency.

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Anomalous Properties of Second Period Elements: Elements like LiLi, BeBe, BB, CC, NN, OO, and FF show different behavior from the rest of their groups due to their exceptionally small size, high electronegativity, high ionization enthalpy, and non-availability of dd-orbitals.

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Diagonal Relationship: Certain elements of the second period show similarities with elements of the third period placed diagonally to them. For example, LiLi resembles MgMg, and BeBe resembles AlAl due to their similar ionic sizes and charge/radius ratios (polarizing power).

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Nature of Oxides: Across a period, the nature of oxides changes from strongly basic to amphoteric and finally to strongly acidic. For example, in Period 3: Na2ONa_2O (Basic) →\rightarrow Al2O3Al_2O_3 (Amphoteric) →\rightarrow Cl2O7Cl_2O_7 (Acidic).

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Chemical Reactivity: Reactivity is highest at the two extremes of a period and lowest in the center. High reactivity on the left is due to low ionization enthalpy (ease of losing electrons), while on the right, it is due to high electron gain enthalpy (ease of gaining electrons).

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Electronegativity and Metallic Character: Metallic character decreases across a period as electronegativity increases. Conversely, metallic character increases down a group as electronegativity and ionization enthalpy decrease.

📐Formulae

Zeff=Z−σZ_{eff} = Z - \sigma

Polarizing Power∝Ionic Charge(Ionic Radius)2\text{Polarizing Power} \propto \frac{\text{Ionic Charge}}{\text{(Ionic Radius)}^2}

Valency (for Groups 1, 2, 13, 14)=Group Number or Valence Electrons\text{Valency (for Groups 1, 2, 13, 14)} = \text{Group Number or Valence Electrons}

Valency (for Groups 15-17)=8−Valence Electrons\text{Valency (for Groups 15-17)} = 8 - \text{Valence Electrons}

💡Examples

Problem 1:

Show the trend in the nature of oxides for the elements of the third period: Na,Mg,Al,Si,P,S,ClNa, Mg, Al, Si, P, S, Cl.

Solution:

Na2ONa_2O (Strongly Basic), MgOMgO (Basic), Al2O3Al_2O_3 (Amphoteric), SiO2SiO_2 (Weakly Acidic), P4O10P_4O_{10} (Acidic), SO3SO_3 (Strongly Acidic), Cl2O7Cl_2O_7 (Very Strongly Acidic).

Explanation:

As we move from left to right across a period, the electropositive character decreases and the electronegative character increases, leading to a transition from basic to acidic oxides.

Problem 2:

Why does LiLi show a diagonal relationship with MgMg?

Solution:

Due to similar atomic and ionic radii: r(Li+)≈76 pmr(Li^+) \approx 76 \text{ pm} and r(Mg2+)≈72 pmr(Mg^{2+}) \approx 72 \text{ pm}.

Explanation:

The diagonal relationship is a result of the 'balancing' of trends. Moving right increases electronegativity/decreases size, while moving down decreases electronegativity/increases size. Diagonally, these effects cancel out, resulting in similar polarizing power.

Problem 3:

Explain why Nitrogen (NN) cannot form NCl5NCl_5, whereas Phosphorus (PP) can form PCl5PCl_5.

Solution:

NN has the electronic configuration 1s22s22p31s^2 2s^2 2p^3. It lacks vacant dd-orbitals in its valence shell (n=2n=2).

Explanation:

Because Nitrogen has no 3d3d orbitals, it cannot expand its octet to accommodate five chlorine atoms. Phosphorus (n=3n=3) has vacant 3d3d orbitals, allowing for sp3dsp^3d hybridization and the formation of PCl5PCl_5.

Periodic Trends in Chemical Properties Revision - Class 11 Chemistry ICSE