Core Idea 2: The Periodic Table - Trends, Period 3, Group 2, and Group 17

Vocabulary practice flashcards covering periodic trends, Period 3 physical and chemical properties, Group 2 alkaline earth metals, and Group 17 halogens.

Period 3 Atomic Radius Trend

Atomic radius decreases across Period 3 because nuclear charge increases while shielding effect remains approximately constant, leading to an increase in effective nuclear charge and stronger electrostatic attraction between the nucleus and valence electrons.

1st Ionisation Energy Dip at Aluminium

The first ionisation energy of $\text{Al}$ ($[\text{Ne}]\,3\text{s}^2\,3\text{p}^1$) is lower than that of $\text{Mg}$ ($[\text{Ne}]\,3\text{s}^2$) because the $3\text{p}$ electron removed from $\text{Al}$ occupies a higher energy level than the $3\text{s}$ electron removed from $\text{Mg}$, requiring less energy to remove.

1st Ionisation Energy Dip at Sulfur

The first ionisation energy of $\text{S}$ ($[\text{Ne}]\,3\text{s}^2\,3\text{p}^4$) is lower than that of $\text{P}$ ($[\text{Ne}]\,3\text{s}^2\,3\text{p}^3$) because of inter-electronic repulsion between the paired electrons sharing the same $3\text{p}$ orbital in sulfur.

Period 3 Melting Point Trend

Increases across the metals ($\text{Na} < \text{Mg} < \text{Al}$) due to greater delocalised electron count and smaller cationic radius, reaches a peak at $\text{Si}$ due to its giant covalent lattice, and drops sharply across simple molecular non-metals where melting points follow molecular size: $\text{S}_8 > \text{P}_4 > \text{Cl}_2 > \text{Ar}$.

White Phosphorus ($\text{P}_4$)

A highly reactive, waxy solid allotrope composed of tetrahedral $\text{P}_4$ molecules that spontaneously ignites in air and must be stored underwater.

Octet Expansion

The ability of Period 3 elements (such as $\text{P}$ and $\text{S}$) to accommodate more than eight valence electrons and exhibit high oxidation states ($+5$, $+6$) through the use of vacant, energetically accessible $3\text{d}$ orbitals for bonding.

Melting Point Difference Between $\text{MgO}$ and $\text{Na}_2\text{O}$

$\text{MgO}$ has a significantly higher melting point than $\text{Na}_2\text{O}$ because $\text{Mg}^{2+}$ has a higher ionic charge and smaller ionic radius than $\text{Na}^+$, giving $\text{MgO}$ a much larger lattice energy ($|\Delta H_{\text{latt}}| \propto \frac{|q^+ \times q^-|}{r_+ + r_-}$).

Covalent Character in Aluminium Oxide ($\text{Al}_2\text{O}_3$)

The partial covalent nature resulting from the high charge density and polarising power of the small $\text{Al}^{3+}$ cation, which distorts the electron cloud of the $\text{O}^{2-}$ anion and lowers its melting point relative to $\text{MgO}$.

Aluminium Chloride ($\text{AlCl}_3$) Dimerisation

At temperatures above $180\,^\circ\text{C}$, simple molecular $\text{AlCl}_3$ vaporises and exists as a molecular dimer, $\text{Al}_2\text{Cl}_6$, held together by coordinate/dative covalent bonds and intermolecular instantaneous dipole–induced dipole attractions.

Amphoteric Oxide

An oxide that exhibits both acidic and basic chemical behavior by reacting with both acids and bases; exemplified in Period 3 by $\text{Al}_2\text{O}_3$.

Reaction of $\text{P}_4\text{O}_{10}$ with Water

A vigorous reaction in which tetraphosphorus decaoxide dissolves in water to produce an acidic solution of phosphoric(V) acid ($\text{pH} \approx 2$) via $\text{P}_4\text{O}_{10}\,(s) + 6\text{H}_2\text{O}\,(l) \rightarrow 4\text{H}_3\text{PO}_4\,(aq)$.

Hydration of Chlorides

The physical attraction and ion-dipole interaction between water molecules and ions (e.g., $\text{Na}^+$, $\text{Cl}^-$) without the breaking of covalent $\text{O}-\text{H}$ bonds of water.

Hydrolysis of Chlorides

A chemical reaction in which water coordinates to a central atom or cation and an $\text{O}-\text{H}$ bond of water is cleaved, releasing $\text{H}^+$ (or $\text{H}_3\text{O}^+$) to generate an acidic solution.

Resistance of $\text{CCl}_4$ to Hydrolysis

Carbon tetrachloride does not hydrolyse because carbon lacks vacant, energetically accessible $d$ orbitals to accept lone pairs from water, and the small carbon atom is sterically shielded by four large chlorine atoms.

Diagonal Relationship

The similarity in physical and chemical properties observed between diagonally adjacent elements of the second and third periods (such as $\text{Li}$ and $\text{Mg}$, $\text{Be}$ and $\text{Al}$, $\text{B}$ and $\text{Si}$) due to comparable electronegativities and similar cation charge densities.

Group 2 Reducing Power Trend

Reducing power increases down Group 2 as atomic radii increase and valence electrons are lost more readily, which is corroborated by the standard electrode potential ($E^\ominus$) becoming increasingly negative from $\text{Mg}$ ($-2.37\,\text{V}$) to $\text{Ba}$ ($-2.91\,\text{V}$).

Thermal Stability of Group 2 Carbonates

Increases down the group from $\text{MgCO}_3$ to $\text{BaCO}_3$ because the increasing cationic radius reduces cation charge density, thereby polarising and distorting the large $\text{CO}_3^{2-}$ oxyanion less and weakening the $\text{C}-\text{O}$ bond to a lesser extent.

Group 17 Volatility Trend

Volatility decreases (boiling point increases) down Group 17 from $\text{Cl}_2$ to $\text{I}_2$ because the number of electrons per molecule increases, resulting in greater polarisability of the electron cloud and stronger instantaneous dipole–induced dipole (id-id) interactions.

Group 17 Oxidising Power Trend

Oxidising power decreases down Group 17 ($\text{Cl}_2 > \text{Br}_2 > \text{I}_2$) as atomic radius increases and the tendency to gain electrons to form halides decreases, evidenced by increasingly less positive $E^\ominus$ values.

Thermal Stability of Hydrogen Halides ($\text{HX}$)

Decreases down Group 17 from $\text{HCl}$ to $\text{HI}$ because the increasing size of the halogen atom leads to less effective orbital overlap with hydrogen, resulting in a lower $\text{H}-\text{X}$ bond energy.