Questions
3-5 questions per paper
Difficulty
Medium
Importance
Core foundation — never skip
Overview
The classification of elements provides the fundamental framework for predicting chemical behavior based on atomic structure. Mastering periodic trends is essential for board exams as it links electronic configuration to reactivity and physical properties. A solid grasp of how these properties vary across periods and down groups is a high-yield area for scoring.
Periodic Trends
Periodic trends describe how properties like atomic radius and metallic character shift across the table. Understanding these trends requires correlating the effective nuclear charge (Zeff) with the principle quantum number. It is critical to differentiate between screening effects and nuclear attraction when explaining these patterns.
- Atomic radius decreases across a period due to increasing Zeff
- Atomic radius increases down a group due to addition of shells
- Metallic character decreases across a period and increases down a group
- Non-metallic character increases across a period and decreases down a group
- Cation size is smaller than the parent atom while anion size is larger
Ionization Enthalpy
Ionization Enthalpy (IE) is the energy required to remove the most loosely bound electron from an isolated gaseous atom. The variation of IE is governed by factors like nuclear charge, atomic size, and electronic configuration stability. Fully filled and half-filled subshells possess extra stability, significantly impacting IE values.
- Successive IE values are always higher than the previous IE
- IE increases across a period as atomic size decreases
- IE decreases down a group as shielding effect increases
- Noble gases have the highest IE in their respective periods
- Nitrogen (2p3) has higher IE than Oxygen (2p4) due to stable half-filled configuration
Electronegativity
Electronegativity is the relative tendency of a bonded atom to attract shared electrons toward itself. Unlike IE, it is a property of an atom in a molecule rather than an isolated state. Pauling, Mulliken, and Allred-Rochow scales are common metrics used to quantify this ability.
- Fluorine is the most electronegative element on the Pauling scale
- Electronegativity increases left to right across a period
- Electronegativity decreases down a group as atomic size increases
- Pauling scale is the most widely used convention for electronegativity values
- Electronegativity difference determines the polarity of a chemical bond
Formula Sheet
Zeff = Z - sigma (Slater's Rule)
Mulliken Electronegativity = (IE + EA) / 2
Exam Tip
Always draw a quick sketch of the periodic table in the margin before solving trend questions to visualize the vector of increase or decrease for any given property.
Common Mistakes
- Confusing the periodic trends of ionic radius with atomic radius for isoelectronic species.
- Failing to account for the stability of half-filled and fully-filled orbitals when comparing IE values of adjacent elements.
- Assuming ionization enthalpy decreases uniformly without considering the shielding effect of d and f orbitals.
More Revision Notes
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