FYUGP B.Sc Chemistry Semester 1: Unit 2 - Periodicity and Chemical Behaviour

This unit explores the periodic trends in atomic and ionic properties, including effective nuclear charge, ionization energy, electronegativity, and electron affinity. These notes are designed to help students understand the underlying principles of chemical behavior.

1. Effective Nuclear Charge (Z_eff)

The effective nuclear charge is the net positive charge experienced by an electron in an atom. It is influenced by the shielding effect of inner electrons.

• Shielding Effect: Inner electrons shield outer electrons from the full charge of the nucleus.
• Calculation: Z_eff = Z - S, where Z is the atomic number and S is the shielding constant.

2. Slater’s Rule

Slater’s Rule provides a method to estimate the shielding constant (S) for an electron in an atom:

• Electrons in the same group contribute 0.35 (except 1s, which contributes 0.30).
• Electrons in the (n-1) group contribute 0.85.
• Electrons in the (n-2) or lower groups contribute 1.00.

Example: For a 3p electron in phosphorus (P):

S = (2 × 0.85) + (8 × 0.35) + (2 × 1.00) = 5.2

Z_eff = 15 - 5.2 = 9.8

3. Covalent and Ionic Radii

Covalent Radius: Half the distance between the nuclei of two identical atoms bonded by a single covalent bond.

Ionic Radius: The radius of an ion in an ionic compound. It depends on the ion's charge and coordination number.

• Cations are smaller than their parent atoms.
• Anions are larger than their parent atoms.

Fig 1: Comparison of Covalent and Ionic Radii

4. Ionization Energy

Ionization energy is the energy required to remove an electron from a gaseous atom or ion.

• First Ionization Energy: Energy required to remove the first electron.
• Second Ionization Energy: Energy required to remove the second electron.
• Trends: Ionization energy increases across a period and decreases down a group.

Element	First Ionization Energy (kJ/mol)
Li	520
Na	496
K	419

5. Electronegativity

Electronegativity is the ability of an atom to attract shared electrons in a chemical bond. Different scales include:

• Pauling Scale: Most commonly used, based on bond energies.
• Mulliken Scale: Based on ionization energy and electron affinity.
• Allred-Rochow Scale: Based on effective nuclear charge and covalent radius.

Trends: Electronegativity increases across a period and decreases down a group.

6. Electron Affinity

Electron affinity is the energy change when an electron is added to a gaseous atom.

• Trends: Electron affinity generally increases across a period and decreases down a group.
• Exceptions: Noble gases have zero or positive electron affinity due to their stable configurations.

Understanding these periodic trends is crucial for predicting chemical behavior and reactivity. Practice problems and periodic table analysis will help reinforce these concepts.