A-Level Chemistry Revision: Bonding, Periodicity, Organic Mechanisms, and Transition Elements

Vocabulary practice flashcards covering chemical bonding, periodic trends, reaction mechanisms, transition metal complexes, and stereochemistry.

Instantaneous Dipole–Induced Dipole (id-id) Forces

Intermolecular forces of attraction that increase in strength with larger, more polarisable electron clouds resulting from higher molecular mass ($M_r$).

Intramolecular Hydrogen Bonding (Boiling Point Effect)

Hydrogen bonding occurring within a single molecule due to the close proximity of groups, which results in a lower boiling point compared to isomers forming intermolecular hydrogen bonds.

Intramolecular Hydrogen Bonding (Acid Strength Effect)

Internal hydrogen bonding that disperses the negative charge on the carboxylate ion ($COO^-$), stabilising the conjugate base monoanion and resulting in a stronger acid with a larger $K_a$ value.

Atomic Radius Trend across Period 3

Atomic radius decreases across Period 3 because valence electrons enter the same principal quantum shell ($n=3$) with relatively constant shielding, while the nuclear charge increases.

Electronegativity

The ability of an atom to attract electron density in a covalent bond.

Period 3 First Ionisation Energy Trend

Ionisation energy generally increases across Period 3 due to decreasing atomic radius and increasing nuclear charge (greater nuclear attraction for valence electrons), with exceptions at Aluminium ($Al$) and Sulfur ($S$).

Down-Group Ionisation Energy Trend

Ionisation energy decreases down a group because atomic radius increases, positioning valence electrons further from the nucleus and weakening nuclear attraction, thereby requiring less energy to remove.

Period 3 Metallic Melting Point Trend

The melting point of metals increases across Period 3 ($Na < Mg < Al$) because cationic charge increases and ionic radius decreases, yielding higher charge density and stronger electrostatic attraction to delocalised electrons.

Silicon ($Si$) Melting Point

The highest melting point across Period 3, resulting from a giant covalent structure with a continuous network of four strong covalent bonds per silicon atom requiring extensive energy to break.

Period 3 Simple Molecular Melting Point Trend

Melting points are determined by weak Van der Waals forces that increase with the number of electrons; sulfur ($S_8$, $128\,e^-$) has the highest melting point, followed by phosphorus ($P_4$) and chlorine ($Cl_2$).

Magnesium Oxide ($MgO$)

A basic oxide with a giant ionic lattice structure (GILS) that reacts readily with acids to form salt and water.

Aluminium Oxide ($Al_2O_3$)

An amphoteric oxide exhibiting partial covalent character due to the high charge density of $Al^{3+}$ polarising the $O^{2-}$ electron cloud; it reacts with both acids and bases.

Silicon Dioxide ($SiO_2$)

An acidic oxide with a giant molecular structure that reacts readily with bases to produce salt and water.

Group 2 Carbonate Thermal Stability

Thermal stability increases down Group 2 because increasing cation radius decreases charge density and polarising power, polarising the $CO_3^{2-}$ carbonate ion less and weakening the $C-O$ bond to a smaller extent.

$S_N1$ Mechanism Preferences

A nucleophilic substitution pathway preferred when electron-donating alkyl groups disperse positive charge to stabilise a carbocation, while bulky groups create steric hindrance that inhibits backside nucleophilic attack.

$S_N2$ Mechanism Preferences

A substitution pathway preferred when steric hindrance is minimal (allowing backside attack) and when electron-withdrawing groups would destabilise a carbocation intermediate.

Chlorobenzene ($C_6H_5Cl$) Inertness to Hydrolysis

Resistance to hydrolysis caused by sideways overlap of the chlorine p-orbital with the benzene $\pi$ system, delocalising a lone pair to impart partial double bond character to the $C-Cl$ bond.

Buffer Action upon Base Addition

Resistance to pH change achieved when added $OH^-$ reacts completely with a large reservoir of weak acid ($HA + OH^- \rightarrow H_2O + A^-$), keeping $[H^+]$ relatively constant.

Phenol Unreactivity in Esterification

The failure of phenol to form esters with carboxylic acids because the oxygen lone pair delocalises over the benzene ring, dispersing negative charge and rendering it a weaker nucleophile than aliphatic alcohols.

d–d Orbital Splitting

The lifting of degeneracy of 3d orbitals in the presence of ligands, where $d_{x^2-y^2}$ and $d_{z^2}$ orbitals experience greater repulsion and rise to a higher energy level than $d_{xy}, d_{xz}, d_{yz}$, separated by an energy gap $\Delta E$.

d–d Transition

The promotion of an electron from a lower-energy d-orbital to a higher-energy d-orbital upon absorbing a specific wavelength of visible light, resulting in the transmission or reflection of the complementary colour.

Colourless Complex Ions

Complexes that appear white or colourless due to having an empty d-subshell, a fully filled d-subshell (precluding d–d transitions), or absorption wavelengths lying outside the visible spectrum.

Carboxylate Resonance Stabilisation

The delocalisation of negative charge across the $O-C=O$ unit that disfavours nucleophilic attack at the carbonyl carbon (which would destroy resonance by forming an $sp^3$ carbon) in favour of substitution at the $C-Cl$ bond.

Biological Specificity of Enantiomers

The principle that stereoisomeric enantiomers exhibit different biological activities and potencies because chiral biological receptors selectively bind only one specific spatial enantiomer.