Periodic Trends
Atomic and ionic radii, ionisation enthalpy, electron gain enthalpy and electronegativity. (Chemistry › Classification of Elements and Periodicity in Properties, NEET UG syllabus.)
What is Periodic Trends?
The distance from the center of the nucleus to the outermost shell of an atom.
Key points
- Define and explain atomic radius, ionic radius, ionization enthalpy, electron gain enthalpy, and electronegativity.
- Predict the trends of these properties across periods and down groups in the periodic table.
- Explain the underlying reasons for these trends based on effective nuclear charge and shielding effect.
- Identify and explain common exceptions to the general periodic trends.
Common exam trap
Confusing Ionisation Enthalpy with Electron Gain Enthalpy.
Definitions
- Term
Atomic Radius
- Meaning
The distance from the center of the nucleus to the outermost shell of an atom.
- Term
Ionic Radius
- Meaning
The effective distance from the center of the nucleus of an ion to its outermost shell.
- Term
Ionisation Enthalpy (IE)
- Meaning
The minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom in its ground state.
- Term
Electron Gain Enthalpy (EGE)
- Meaning
The energy change that occurs when an electron is added to an isolated gaseous atom in its ground state to form an anion.
- Term
Electronegativity
- Meaning
The tendency of an atom in a chemical compound to attract shared electrons towards itself.
Learning objectives
Define and explain atomic radius, ionic radius, ionization enthalpy, electron gain enthalpy, and electronegativity.
Predict the trends of these properties across periods and down groups in the periodic table.
Explain the underlying reasons for these trends based on effective nuclear charge and shielding effect.
Identify and explain common exceptions to the general periodic trends.
Compare the properties of isoelectronic species based on periodic trends.
Prerequisites
Basic understanding of atomic structure (protons, neutrons, electrons).
Knowledge of electron configuration (Aufbau principle, Hund's rule, Pauli exclusion principle).
Concept of effective nuclear charge (Zeff).
Understanding of the shielding/screening effect.
Common mistakes
Confusing Ionisation Enthalpy with Electron Gain Enthalpy.
Forgetting or misapplying exceptions to trends (e.g., IE of B vs Be, O vs N; EGE of F vs Cl).
Not considering the effect of nuclear charge for isoelectronic species when comparing ionic radii.
Assuming electronegativity has units.
Incorrectly applying the concepts of effective nuclear charge or shielding effect.
Keywords
Periodic table
atomic radius
ionic radius
ionization enthalpy
electron gain enthalpy
electronegativity
effective nuclear charge
shielding effect
period
group
cation
anion
isoelectronic
Practice preview
Which of the following correctly represents the general trend of atomic radius across a period in the modern periodic table?…
easy
What is the general trend of ionisation enthalpy down a group in the modern periodic table?…
easy
Arrange the following isoelectronic species in order of increasing ionic radius: N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺.…
medium