Engineering Chemistry (BT-101) Unit 7 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

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

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.

$Z$ is the atomic number and $S$ is the screening (shielding) constant, found by Slater's rules.
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.
Example: for a 2p electron of N, $S = 4(0.35) + 2(0.85) = 3.10$, so $Z_{eff} = 7 - 3.10 = 3.90$.
$Z_{eff}$ 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

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

Key points.

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

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

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

Electronic Configuration

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

Key points.

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

<mark>Configurations follow the Aufbau principle, Pauli exclusion principle and Hund's rule.</mark>

Atomic and Ionic sizes

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.

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

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

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

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.

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.
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.
Down a group, EA generally decreases because the atom is larger and the added electron is farther from the nucleus and more shielded.
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.
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.
Down a group, EN decreases because added shells increase radius and shielding; F 4.0, Cl 3.0, Br 2.8, I 2.5.
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.
Ionic size (see above) decreases across a period and increases down a group, and these three properties all follow $Z_{eff}$ and atomic radius.
Pauling scale: $\chi_A - \chi_B = 0.208\sqrt{\Delta}$, where $\Delta$ is the excess bond energy in kcal/mol; Mulliken: $\chi = (IE + EA)/2$.

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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.

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

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

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.

Polarizability rises with size: large anions such as $I^-$ are more polarizable than $F^-$.
It rises with anionic charge ($S^{2-} > Cl^-$), and small highly charged cations polarize strongly.
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.
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).

Account

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

How unit 7 is examined

Effective Nuclear Charge

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

Electronic Configuration

Atomic and Ionic sizes

Electron affinity and electronegativity

Polarizability and Oxidation States

Last-minute revision

Memory hooks

Coverage checklist