Group 1 and 2 elements — trends, anomalies, compounds, diagonal relationship

Question

What are the key trends in Group 1 (alkali metals) and Group 2 (alkaline earth metals), what anomalies does lithium/beryllium show, and how does the diagonal relationship work?

Solution — Step by Step

Going down both groups:

Atomic radius increases (more electron shells)
Ionisation enthalpy decreases (outermost electron farther from nucleus)
Electropositive character increases (easier to lose electrons)
Metallic character increases
Melting point decreases for Group 1 (weaker metallic bonding as size grows)

For Group 2, melting points do not follow a smooth trend because crystal structure changes.

Li is the odd one out in Group 1. Why? Its tiny size and high charge density make it behave differently:

Li forms covalent compounds (LiCl is covalent, NaCl is ionic) — Fajans’ rule at work
$\text{Li}_2\text{CO}_3$ decomposes on heating; $\text{Na}_2\text{CO}_3$ does not
LiOH is a weak base; NaOH is a strong base
Li forms a stable nitride ( $\text{Li}_3\text{N}$ ); other alkali metals don’t form nitrides easily

Be is the odd member of Group 2 for the same reason — very small size, very high ionisation enthalpy:

BeCl $_2$ is covalent and exists as a polymer in solid state
Be does not react with water (other Group 2 metals do)
BeO is amphoteric; MgO, CaO etc. are basic
Be shows coordination number 4 (not 6) due to absence of d-orbitals

Li resembles Mg (diagonal neighbour) more than Na (vertical neighbour). Similarly, Be resembles Al.

\text{Li} \longleftrightarrow \text{Mg} \qquad \text{Be} \longleftrightarrow \text{Al}

The reason: moving one step right and one step down in the periodic table, the increase in nuclear charge (going right) and the increase in size (going down) roughly cancel out. So the charge-to-size ratio (ionic potential) stays similar.

Examples of Li-Mg similarity:

Both form nitrides ( $\text{Li}_3\text{N}$ , $\text{Mg}_3\text{N}_2$ )
Both carbonates decompose on heating
Both form covalent organometallic compounds

flowchart TD
    A["s-block Elements"] --> B["Group 1: Alkali Metals"]
    A --> C["Group 2: Alkaline Earth Metals"]
    B --> D["Li — anomalous"]
    B --> E["Na, K, Rb, Cs — typical"]
    C --> F["Be — anomalous"]
    C --> G["Mg, Ca, Sr, Ba — typical"]
    D --> H["Small size, high IE, covalent compounds"]
    F --> I["Small size, no d-orbitals, amphoteric oxide"]
    D --> J["Diagonal: Li resembles Mg"]
    F --> K["Diagonal: Be resembles Al"]
    J --> L["Similar charge-to-size ratio"]
    K --> L

Why This Works

The diagonal relationship boils down to polarizing power. An ion’s ability to distort the electron cloud of a neighbouring anion depends on its charge/size ratio. Li $^+$ (charge +1, radius 76 pm) has a similar polarizing power to Mg $^{2+}$ (charge +2, radius 72 pm) because doubling the charge while keeping size nearly the same roughly doubles the polarizing power — but Mg is compared to Na $^+$ (charge +1, radius 102 pm), which is much less polarizing.

Alternative Method

Use Fajans’ rules as a unifying framework. High cation charge, small cation size, large anion size, and pseudo-noble-gas configuration all favour covalent character. Check each “anomalous” property against Fajans’ rules — you will find every single one fits.

Common Mistake

Students often confuse the diagonal relationship with saying “Li and Mg have the same properties.” They do not. The diagonal relationship means they share similar trends — both form nitrides, both carbonates decompose, both are harder and higher melting than their group members. But Li is still a Group 1 metal with +1 oxidation state, and Mg is Group 2 with +2. The oxidation states remain different. JEE Main 2023 tested exactly this nuance.

Question

Solution — Step by Step

Why This Works

Alternative Method

Common Mistake

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