Deep-Dive Guides · Main Group — s-Block

Main Group Elements — the s-Block deep dive

The s-block (Groups 1 and 2, plus H and He) is the most "trend-transparent" part of the periodic table: one or two loosely held ns electrons, no partly filled d/f shell to complicate matters, and an ionic chemistry that reads almost entirely off size, charge and lattice-vs-hydration energetics.

Free extract — the opening section of the guide, in full.

0. How to use this guide — the s-block logic map

Almost every fact in s-block chemistry is a consequence of three numbers and one competition. The numbers are the ionic charge (fixed: +1 for Group 1, +2 for Group 2), the ionic radius (grows down each group), and the resulting charge density (charge/radius, i.e. how "polarising" the cation is). The competition — the single most useful idea in the block — is between lattice enthalpy (favours small ions and high charge, holds solids together) and hydration enthalpy (also favours small, highly charged ions, drives dissolution and pulls E° values). Whenever you meet a trend, ask which of these two is winning.

ObservableGoverned byDown-group trend & why
Reactivity of the metalIonisation energy (ease of losing e⁻)Increases down group (IE falls as the ns electron moves further from the nucleus)
Solubility of a saltLattice enthalpy vs hydration enthalpyNo universal rule — depends on which term falls faster with size (see §3.5)
Thermal stability of carbonate/nitrateCation polarising powerIncreases down group (bigger, less-polarising cation destabilises the large anion less)
Covalent character of a compoundFajans' rules (small, high-charge cation → covalent)Greatest for Li⁺ and Be²⁺ (top of each group) — the root of both anomalies
Basic strength of MOH / M(OH)₂M–O vs O–H bond polarityIncreases down group (larger cation → weaker M–O → OH⁻ released more readily)
The three questions to ask of any s-block observation: (1) How big and how charged is the cation? (charge density sets polarising power). (2) Is this a lattice-energy story or a hydration-energy story? (solubility, E°, hydration numbers). (3) Is the species ionic or, because the cation is tiny (Li⁺, Be²⁺), significantly covalent? Answering these reconstructs most of the block's descriptive chemistry without memorising it.

The s-block: Groups 1 & 2 (blue), the reactive-metal frontier of the periodic tableHLiNaKRbCsGroup 1 (ns¹)BeMgCaSrBaGroup 2 (ns²)d-block and p-block(covered in separate guides)Diagonal relationships:Li ↔ MgBe ↔ Al(top-left element resembles thelower-right neighbour becausecharge density is similar)He sits in Group 18 despite an s² config.
Fig 0-1. The s-block. Group 1 (alkali metals, ns¹) and Group 2 (alkaline earths, ns²) form the reactive left margin; H formally sits atop Group 1 but is a non-metal. The red dashed diagonals mark the two famous diagonal relationships developed in §4.

Read the rest of this guide

The remaining 10 sections — Periodic trends across the s-block, Group 1 — the alkali metals, Group 2 — the alkaline earth metals, Anomalies & diagonal relationships and more — are part of ChemVidya Full Access, along with the other guides, the books, the study notes and 6,000+ practice questions with worked solutions.

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