Periodic Table

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Periodic Table — Quick Facts

The modern periodic law (Mendeleev, 1869; modified by Moseley with atomic numbers in 1913) states: “Properties of elements are periodic functions of their atomic numbers.”

Structure:

• 18 groups (vertical columns): Elements in the same group have similar chemical properties (same valence shell electronic configuration)
• 7 periods (horizontal rows): Each period represents filling of a new electron shell

Block Classification:

Block	Configuration	Groups	Character
s-block	ns¹⁻²	1 (alkali metals), 2 (alkaline earth metals)	Metals (except H)
p-block	ns² np¹⁻⁶	13–18	Non-metals, metalloids, metals
d-block	(n−1)d¹⁻¹⁰ ns⁰⁻²	3–12 (transition metals)	Metals, coloured compounds
f-block	(n−2)f¹⁻¹⁴ (n−1)d⁰⁻¹ ns²	Lanthanoids (4f), Actinoids (5f)	Inner transition metals

Ionic Radii Trend: Across a period (left to right): Cations shrink (more protons, same electrons). Anions also shrink (but less than cations). Down a group: Increasing (new electron shell added each period). For isoelectronic species (same number of electrons): More protons → smaller radius. Example: O²⁻ (1s²2s²2p⁶, Z=8) > F⁻ (Z=9) > Na⁺ (Z=11) > Mg²⁺ (Z=12) in size

⚡ Exam tip: Ionisation enthalpy is the energy required to remove an electron. It generally increases across a period and decreases down a group. Noble gases have the highest ionisation enthalpy in each period.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Periodic Table — NEET/JEE Study Guide

Periodic Trends in Properties:

1. Atomic Radii: Decreases across a period (Z effective increases → electrons pulled closer). Increases down a group (new shells added outweigh increasing nuclear charge).

2. Ionisation Enthalpy (IE): Energy needed to remove the most loosely held electron. First IE < Second IE < Third IE (successive ionisations get harder — removing positive ions is harder).

Anomalies:

• Group 13 (Boron family): IE₁ lower than Group 2 because p-electron is higher in energy and easier to remove than s-electron
• Group 16 (Oxygen family): IE₁ lower than Group 15 because paired electrons in p-orbital cause electron-electron repulsion, making removal easier

3. Electron Gain Enthalpy (Electron Affinity): Energy released when an electron is added to a neutral gaseous atom. Generally becomes more negative across a period (easier to add electrons as Z effective increases). Anomaly: Noble gases have positive electron gain enthalpy (energy must be supplied).

4. Electronegativity (Pauling Scale): Dimensionless quantity. Fluorine = 4.0 (most electronegative element). Caesium = 0.79 (least among stable elements).

Trend: Increases across a period, decreases down a group. Metallic character: Increases down a group (more electropositive), decreases across a period.

5. Diagonal Relationship: Some properties of elements show a diagonal pattern — Li resembles Mg, Be resembles Al, B resembles Si. Reason: Charge/radius ratio similar across the diagonal; the change in properties down a group approximately balances the change across a period.

Diagonal Pair	Similar Properties
Li (1) and Mg (2)	Both form nitrides (Li₃N, Mg₃N₂); both form oxides (Li₂O, MgO); both carbonates decompose on heating
Be (2) and Al (13)	Both form covalent oxides; both form complex fluorides (BeF₄²⁻, AlF₆³⁻); amphoteric oxides
B (13) and Si (14)	Both form acidic oxides (B₂O₃, SiO₂); both form volatile hydrides (BH₃, SiH₄ — unstable)

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Periodic Table — Comprehensive Notes

Effective Nuclear Charge (Z_eff): Z_eff = Z − S (Slater’s rules give approximate values) Across a period: Z_eff increases → electrons pulled closer → smaller atomic radius Down a group: Z_eff increases but new electron shells shield very effectively → radius increases

Screening/Shielding Constants (Slater’s Rules):

1. Write electron configuration as: (1s)(2s,2p)(3s,3p)(3d)(4s,4p)(4d)(4f)(5s,5p)…
2. Electrons to the right of the electron of interest contribute 0 to shielding
3. For ns or np electron: same group electrons = 0.35 each (1s electrons = 0.30); (n−1) shell = 0.85; (n−2) and lower = 1.00
4. For nd or nf electron: all electrons to the left = 1.00

Example — Z_eff for 3p electron in Cl (Z=17): Configuration: (1s²)(2s²2p⁶)(3s²3p⁵) Shields: 7 electrons in same (3s,3p) group × 0.35 = 2.45

• 2 electrons in 2s²2p⁶ × 0.85 = 1.70
• 2 electrons in 1s² × 1.00 = 2.00 Total S = 6.15 → Z_eff = 17 − 6.15 = 10.85

Variation of Z_eff Across Period 2:

Element	Z	Z_eff (approx)
Li	3	1.28
Be	4	1.91
B	5	2.58
C	6	3.22
N	7	3.85
O	8	4.49
F	9	5.13
Ne	10	5.75

This steady increase in Z_eff explains the decrease in atomic radius, increase in IE, and increase in electronegativity.

Metallic vs Non-Metallic Character: Metallic character decreases across a period (increasing Z_eff pulls electrons, more like non-metals). Non-metallic character increases across a period. Oxide character: Basic oxides on left, amphoteric in middle, acidic on right of periodic table. Example: Na₂O (strongly basic) → MgO (basic) → Al₂O₃ (amphoteric) → SiO₂ (weakly acidic) → P₄O₁₀ (acidic) → SO₃ (strongly acidic) → Cl₂O₇ (very strongly acidic)

Hydrogen — Unique Position:

• Resembles alkali metals (forms H⁺, like Li⁺, Na⁺) — especially in ionic hydrides (NaH, CaH₂)
• Resembles halogens (forms H⁻, like Cl⁻, F⁻) — especially in covalent hydrides
• Not truly group 1 or group 17 — placed separately at the top of the table

Alkali Metals (Group 1):

• ns¹ configuration
• Low IE (lowest in each period)
• Form M⁺ ions; ionic hydrides; ionic carbonates (except Li — Li₂CO₃ decomposes on heating)
• Li forms covalent compounds; smallest cation, most hydration energy
• Na, K most abundant on Earth; NaHCO₃ (baking soda), Na₂CO₃·10H₂O (washing soda), KCl (potash)

Halogens (Group 17):

• ns² np⁵ configuration
• High electronegativity, high electron affinity, high IE
• Form X⁻ ions; diatomic molecules (Cl₂, Br₂, I₂); interhalogen compounds (ICl, BrF₃)
• Oxidising power decreases down group: F₂ > Cl₂ > Br₂ > I₂
• Astatine (Z=85): Radioactive, only 25 known isotopes

Noble Gases (Group 18):

• Complete octet (ns² np⁶), except He (1s²)
• Very low reactivity — earlier called “inert gases”
• Xenon forms compounds (XeF₂, XeF₄, XeF₆, XeO₃)
• Uses: He in balloons and cryogenics; Ne in advertisement signs; Ar in incandescent bulbs; Rn in radiotherapy (historically)

NEET Pattern Analysis: Periodic properties contributes 1–2 questions per year. Key areas: predicting Z_eff, ranking atomic radii/IE/electronegativity across periods and down groups, diagonal relationships, and anomalous properties of first element in each group.

⚡ NEET 2021 Qn: Which has larger atomic radius: Na (Z=11) or Mg (Z=12)? Answer: Na. Both have 3 shells, but Na has 11 protons vs Mg’s 12 — more protons in Mg pull electrons closer, making radius smaller.