Atomic Structure and the Periodic Table

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Atomic Structure and the Periodic Table — Key Facts Atomic number Z = number of protons; Mass number A = protons + neutrons; Isotopes = same Z, different A Electronic configuration: aufbau principle (1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d…); Hund’s rule (maximise unpaired electrons before pairing); Pauli exclusion (max 2 electrons per orbital, opposite spins) Ionisation energy: energy to remove outermost electron; generally increases across period, decreases down group ⚡ Exam tip: Ionisation energy of Mg is higher than Na because Mg has higher nuclear charge with same shell

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Atomic Structure and the Periodic Table — JAMB Chemistry Study Guide

Quantum numbers:

• Principal $n$: 1, 2, 3… (shell, determines energy)
• Azimuthal $l$: 0 to $n-1$ (subshell: s=0, p=1, d=2, f=3)
• Magnetic $m_l$: $-l$ to $+l$ (orbital orientation)
• Spin $m_s$: $+\frac{1}{2}$ or $-\frac{1}{2}$ (electron spin)

Electron configurations: 1s² (He); 1s² 2s² 2p⁶ (Ne); [Ne] 3s² 3p⁶ (Ar) Cr: [Ar] 3d⁵ 4s¹ (not 3d⁴ 4s²) — half-filled d subshell is stable Cu: [Ar] 3d¹⁰ 4s¹ (not 3d⁹ 4s²) — completely filled d subshell is stable

Periodic trends:

• Atomic radius: decreases across period (L→R), increases down group (R→L) due to more shells
• Ionisation energy: generally increases across period, with dips at group 13 (extra penetration of p electron) and drops at group 18
• Electron affinity: tendency to gain electron; most negative at group 17 (halogens); group 18 has positive EA
• Electronegativity: F = 4.0 (most electronegative); increases across period, decreases down group
• Ionic radius: cations smaller than parent atom; anions larger; isoelectronic series (e.g., N³⁻ > O²⁻ > F⁻ > Na⁺ > Mg²⁺ > Al³⁺)

Common student mistakes: Confusing atomic radius trend (atomic radius of K is larger than Ca despite Ca having more protons — extra shell wins); thinking IE always increases across a period (it doesn’t at group 13/18 anomalies); not understanding why Cr and Cu have anomalous configurations.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Atomic Structure and the Periodic Table — Comprehensive Chemistry Notes

Bohr’s atomic model limitations: Bohr model assumed electrons orbit nucleus in fixed shells without radiating energy. However, according to Maxwell’s equations, accelerating charges radiate electromagnetic waves and should spiral into nucleus. Bohr imposed quantisation as a postulate without theoretical justification.

Quantum mechanical model (Schrödinger equation): The Schrödinger equation $H\psi = E\psi$ doesn’t give electron orbits but probability clouds (orbitals). The square of the wavefunction $|\psi|^2$ gives probability density of finding an electron at a point.

Pauli exclusion principle: No two electrons in an atom can have the same set of four quantum numbers. Since electrons in the same orbital have same $n, l, m_l$, they must differ in $m_s$ (spin). Thus maximum 2 electrons per orbital.

Effective nuclear charge ($Z_{eff}$): Inner electrons shield outer electrons from full nuclear charge. $Z_{eff} = Z - S$ where $S$ = shielding constant. Slater’s rules: for ns or np electron: same group (n-1) electrons shield 0.85 each; (n-2) and below shield 1.00 each. Increases across period (more protons, similar shielding) → atoms get smaller.

Ionisation energy trends — detailed: Across period 2: Li < Be > B < C < N > O < F < Ne Exceptions:

• Be to B: IE decreases because B’s outermost electron is in 2p (higher energy than 2s) and less tightly held
• N to O: IE decreases because O already has paired electrons in 2p orbitals (electron-electron repulsion)
• N (1s² 2s² 2p³): half-filled p subshell is relatively stable

Screening in d and f electrons: d and f electrons are poor at shielding because their shapes (dumbbells, clover) don’t shield well. This causes the lanthanide contraction (4f series) and leads to unexpected properties in heavier elements.

JAMB exam patterns:

• 2023 JAMB: Write the electron configuration of Fe (Z=26) in terms of s, p, d notation
• 2022 JAMB: Why is ionisation energy of nitrogen higher than oxygen?
• 2021 JAMB: State Hund’s rule and apply it to carbon’s 2p² configuration
• 2020 JAMB: Which has larger atomic radius — Na or Al? Explain

Periodic table groups:

Group	Name	Valency electrons	Key property
1	Alkali metals	ns¹	Very reactive, +1 ion
2	Alkaline earth	ns²	Reactive, +2 ion
13-17	Main group	Variable	Form ions or bonds
18	Noble gases	ns² np⁶	Unreactive

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📊 JAMB Exam Essentials

Detail	Value
Questions	180 MCQs (UTME)
Subjects	4 subjects (language + 3 for course)
Time	2 hours
Marking	+1 per correct answer
Score	400 max (used for university admission)
Registration	January – February each year

🎯 High-Yield Topics for JAMB

• Use of English (Grammar + Comprehension) — 60 marks
• Biology for Science students — 40 marks
• Chemistry (Organic + Physical) — 40 marks
• Physics (Mechanics + Optics) — 35 marks
• Mathematics (Algebra + Geometry) — 40 marks

📝 Previous Year Question Patterns

• Q: “The process of photosynthesis requires…” [2024 Biology]
• Q: “The electronic configuration of Fe is…” [2024 Chemistry]
• Q: “Find the value of x if 2x + 5 = 15…” [2024 Mathematics]

💡 Pro Tips

• Use of English carries the most weight — master grammar rules and comprehension strategies
• JAMB syllabus is your Bible — questions come directly from it. Download and use it.
• Past questions are highly predictive — repeat patterns appear every year
• For Science students, Biology and Chemistry are high-scoring if you study NCERT-level content

🔗 Official Resources

• JAMB Official
• JAMB Syllabus

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