Consider three Period 3 elements: sodium (Na), magnesium (Mg), and aluminium (Al). As one … — JAMB UTME

Consider three Period 3 elements: sodium ( $Na$ ), magnesium ( $Mg$ ), and aluminium ( $Al$ ). As one moves from $Na$ to $Al$ across the period, there is a notable increase in both melting point and electrical conductivity per mole of metal. Which of the following best accounts for this observed trend in metallic bond strength and physical properties?

A
The ionic radius increases from $Na^+$ to $Al^{3+}$ , enabling larger cations to form a tighter lattice that conducts heat and electricity more effectively.
The number of delocalized valence electrons contributed per atom increases while cationic radius decreases, increasing electrostatic attraction and mobile charge density.Answer
C
Aluminium transitions from metallic bonding to directional covalent giant lattice bonding, requiring significantly higher heat to break.
D
Sodium contributes more delocalized electrons per atom into the lattice than aluminium because sodium possesses a lower first ionization energy.

Answer

The trend is best explained by the increase in the number of delocalized valence electrons contributed per atom combined with a smaller cationic radius, which increases electrostatic attraction and mobile charge carrier density.

Metallic bonding consists of electrostatic attractions between fixed positive metal cations and a surrounding delocalized sea of valence electrons. Moving from sodium to aluminium, each atom donates more valence electrons (

Na = 1e^-

Mg = 2e^-

Al = 3e^-

) into the electron sea while the ionic radius decreases (

Na^+ > Mg^{2+} > Al^{3+}

). The combination of higher cationic charge, smaller ionic radius, and greater electron density increases the electrostatic attraction, raising both melting points and electrical conductivity.

Step-by-Step Solution

Analyze the structural factors determining metallic bond strength.

Metallic bond strength depends directly on two main factors: (1) the charge on the metal cation (number of delocalized electrons donated per atom) and (2) the cationic radius (distance between cations and delocalized electrons).

Strength of electrostatic attraction follows Coulomb's law:

F \propto \frac{q_1 q_2}{r^2}

Compare valence electron contributions across Period 3 metals.

Sodium (

[Ne]3s^1

) donates 1 electron per atom (

Na^+

), Magnesium (

[Ne]3s^2

) donates 2 electrons per atom (

Mg^{2+}

), and Aluminium (

[Ne]3s^3

) donates 3 electrons per atom (

Al^{3+}

Higher delocalized electron count yields greater mobile charge density for electrical conductivity.

Compare cationic radii across the period.

Ionic radii decrease across the period:

Na^+ (102\text{ pm}) > Mg^{2+} (72\text{ pm}) > Al^{3+} (54\text{ pm})

Smaller cations allow delocalized electrons to approach closer to positively charged nuclei, dramatically strengthening electrostatic attraction.

Key Concept

Factors affecting metallic bond strength and properties (charge density and delocalized electron count)

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