Engineering Chemistry (BT-101) - Unit 7 Short Notes

How unit 7 is examined

This unit covers Zeff, orbital energies, electronic configuration, sizes, electron affinity, electronegativity, polarizability and oxidation states; electron affinity with electronegativity carries the marks, then polarizability.

Effective Nuclear Charge

Not asked since 2022

Definition. Effective nuclear charge $Z_{eff}$ is the net positive charge felt by an outer electron after the shielding of inner electrons is subtracted.

Formula. $$Z_{eff} = Z - S$$

Key points.

1. $Z$ is the atomic number and $S$ is the screening (shielding) constant, found by Slater's rules.
2. Slater's rules: an electron in the same group contributes 0.35 (0.30 for 1s), electrons in the shell one lower contribute 0.85, and all deeper electrons contribute 1.00.
3. Example: for a 2p electron of N, $S = 4(0.35) + 2(0.85) = 3.10$, so $Z_{eff} = 7 - 3.10 = 3.90$.
4. $Z_{eff}$ rises across a period and changes little down a group, which drives most periodic trends.

==$Z_{eff}$ is the atomic number minus the shielding constant, $Z_{eff} = Z - S$.==

Asked: [7 marks] (Jun 2025) Write brief note on (any two): Electron affinity; Effective Nuclear Charge; Aufbau rule; Atomic and Ionic size

Variations: S, P, d & f Orbital energies of atoms in periodic table

Not asked since 2022

Definition. The Aufbau principle says electrons fill orbitals in order of increasing energy, and the energy order follows the $(n+l)$ rule.

Key points.

1. A lower $(n+l)$ value means lower energy; if two orbitals have equal $(n+l)$, the one with lower $n$ fills first.
2. Filling order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d.
3. Within a shell the energy order is s < p < d < f, because s penetrates the nucleus best.
4. So the periodic table has s-block, p-block, d-block and f-block elements according to the last orbital filled.

Orbitals fill in increasing order of $(n+l)$, and s < p < d < f within a shell.

Asked: [7 marks] (Jun 2025) Write brief note on (any two): Electron affinity; Effective Nuclear Charge; Aufbau rule; Atomic and Ionic size

Electronic Configuration

Not asked since 2022

Definition. Electronic configuration is the arrangement of the electrons of an atom in its orbitals, written as $nl^x$.

Key points.

1. Aufbau principle: fill orbitals from lowest energy upward.
2. Pauli exclusion principle: an orbital holds at most two electrons, with opposite spins.
3. Hund's rule: degenerate orbitals get one electron each with parallel spins before pairing.
4. Half-filled and fully filled d subshells are extra stable, so Cr is $[Ar]3d^5 4s^1$ and Cu is $[Ar]3d^{10} 4s^1$.

Configurations follow the Aufbau principle, Pauli exclusion principle and Hund's rule.

Atomic and Ionic sizes

Not asked since 2022

Definition. Atomic radius is half the internuclear distance between two bonded like atoms; ionic radius is the radius of an ion in an ionic crystal.

Key points.

1. Across a period the size decreases, because $Z_{eff}$ rises while the electrons enter the same shell.
2. Down a group the size increases, because a new shell is added and shielding grows.
3. A cation is smaller than its atom (fewer electrons, same nuclear charge); an anion is larger (more electrons repel).
4. Order: cation < atom < anion; among isoelectronic ions, size falls as nuclear charge rises ($O^{2-} > F^- > Na^+ > Mg^{2+}$).

Cation < atom < anion; size decreases across a period and increases down a group.

Asked: [7 marks] (Jun 2025) Write brief note on (any two): Electron affinity; Effective Nuclear Charge; Aufbau rule; Atomic and Ionic size Asked: [14 marks] (Jun 2023, Dec 2023) Trends of ionic size in a group and in a period with reasons

Electron affinity and electronegativity

High weight

Definition. Electron affinity (EA) is the energy released when an electron is added to an isolated neutral gaseous atom: $X(g) + e^- \rightarrow X^-(g)$. Electronegativity (EN) is the tendency of an atom in a molecule to attract the shared pair of electrons towards itself; it has no unit on the Pauling scale.

Key points.

1. Modern periodic law (Moseley): the properties of elements are a periodic function of their atomic numbers; the modern table has 18 groups and 7 periods, with s, p, d and f blocks.
2. Across a period, EA generally increases (Li to F) because $Z_{eff}$ rises and the atom shrinks, so the nucleus holds the added electron more strongly.
3. Down a group, EA generally decreases because the atom is larger and the added electron is farther from the nucleus and more shielded.
4. Exceptions: Be, N and the noble gases have about zero or negative EA (filled s, half-filled p, filled p); Cl (349 kJ/mol) exceeds F (328) because F is so small that inter-electron repulsion is high.
5. Across a period, EN increases from left to right because of higher $Z_{eff}$ and smaller radius; Li 1.0, Be 1.5, B 2.0, C 2.5, N 3.0, O 3.5, F 4.0.
6. Down a group, EN decreases because added shells increase radius and shielding; F 4.0, Cl 3.0, Br 2.8, I 2.5.
7. In the s-block, EN and EA are low (metals, large size, low $Z_{eff}$) and both fall from Li to Cs; in the p-block both are high and rise to the halogens, so F is the most electronegative element.
8. Ionic size (see above) decreases across a period and increases down a group, and these three properties all follow $Z_{eff}$ and atomic radius.
9. Pauling scale: $\chi_A - \chi_B = 0.208\sqrt{\Delta}$, where $\Delta$ is the excess bond energy in kcal/mol; Mulliken: $\chi = (IE + EA)/2$.

Diagram.

Answer frame. Open with the definitions of EA and EN and Moseley's law; draw the period-2 arrow diagram; develop points 2-4 for EA, then 5-6 for EN, then 7-8 for s and p blocks and ionic size; close with the sentence that all trends follow $Z_{eff}$ and atomic radius. For the EN-only note, give definition, points 5-6 and the Li to F and halogen examples.

Electronegativity increases across a period and decreases down a group, because of $Z_{eff}$ and atomic radius.

Pitfall: Do not say F has the highest EA; Cl does, though F has the highest EN.

Asked: [14 marks] (Jun 2023, Dec 2023) How do you explain electro negativity, ionic sizes and electron affinity of s, p-block elements? What is periodic table? Write trends of ionic size, electronegativity and electron affinity in a group and in a period, with reasons Asked: [7 marks] (Dec 2024) Write short notes on Electro negativity variation in groups and periods Asked: [7 marks] (Jun 2025) Write brief note on (any two): Electron affinity; Effective Nuclear Charge; Aufbau rule; Atomic and Ionic size Asked: [7 marks] (Jun 2025) Write brief note on (any two): Electro negativity; Polarizability; Oxidation states

Polarizability and Oxidation States

Low weight

Definition. Polarizability is the ease with which the electron cloud of an atom or anion is distorted by an electric field or a nearby cation: $\mu_{ind} = \alpha E$. Oxidation state is the apparent charge an atom carries in a compound, assigned by electronegativity.

Key points.

1. Polarizability rises with size: large anions such as $I^-$ are more polarizable than $F^-$.
2. It rises with anionic charge ($S^{2-} > Cl^-$), and small highly charged cations polarize strongly.
3. Fajans' rules: covalent character increases with small, highly charged cation, large, highly charged anion, and a cation with a non-noble-gas (18-electron) shell.
4. Oxidation states: s-block elements show fixed +1 or +2; p-block show variable states (inert pair effect down a group); d-block show many states, e.g. Mn +2 to +7.

==Polarizability is the ease of distortion of an electron cloud, $\mu_{ind} = \alpha E$, and it increases covalent character (Fajans' rules).==

Answer frame. Open with the definition and $\mu_{ind} = \alpha E$; list the factors; close with Fajans' rules and one example (AgI is more covalent than NaCl).

Asked: [7 marks] (Dec 2024) Explain about polarisability Asked: [7 marks] (Jun 2025) Write brief note on (any two): Electro negativity; Polarizability; Oxidation states

Last-minute revision

• $Z_{eff} = Z - S$; Slater: 0.35 same group, 0.85 one shell lower, 1.00 deeper.
• Aufbau: lower $(n+l)$ fills first; s < p < d < f in a shell.
• Cr is $[Ar]3d^5 4s^1$, Cu is $[Ar]3d^{10} 4s^1$.
• Size: cation < atom < anion; decreases across a period, increases down a group.
• EA is the energy released on adding an electron to a gaseous atom; Cl has the highest EA.
• EN is the tendency to attract shared electrons; F is highest at 4.0 (Pauling).
• EN and EA rise across a period and fall down a group.
• Mulliken: $\chi = (IE + EA)/2$.
• Polarizability: $\mu_{ind} = \alpha E$; large anions are most polarizable.
• Fajans' rules give covalent character in ionic compounds.
• Mn shows +2 to +7; s-block oxidation states are fixed.

Memory hooks

• Zeff: "Z minus S", the nucleus minus the shield.
• Sizes: cation Cuts, anion Adds.
• Pauling EN of period 2: "Little Bears Believe Can Nothing Offend Fish" is Li Be B C N O F (1.0 to 4.0).
• Fajans: small cation, big anion, more covalent.

Coverage checklist

• Effective Nuclear Charge: Jun 2025 (any two).
• Variations: S, P, d & f Orbital energies of atoms in periodic table: Jun 2025 (Aufbau rule).
• Electronics Configuration: no past question; Aufbau, Pauli, Hund covered.
• atomic & Ionic sizes: Jun 2025 (any two); Jun 2023, Dec 2023 (ionic size trends).
• electron affinity & electro negativity: Jun 2023, Dec 2023 (14 marks); Dec 2024 (EN note); Jun 2025 (both notes).
• Polarizability & Oxidation States: Dec 2024 (polarisability); Jun 2025 (note).