Topic 2 Chemistry: Halogens, Covalent Bonding, and Allotropes of Carbon

Vocabulary practice flashcards covering Group 7 halogens, displacement and redox reactions, covalent bonding, simple molecules, giant covalent lattices, and carbon allotropes.

Halogens

The elements found in Group 7 of the periodic table that have seven electrons in their outer shell and react by gaining one electron to form $1-$ halide ions.

Diatomic Molecule

A molecule consisting of exactly two atoms chemically bonded together, which is the natural form of all Group 7 halogens (such as $\text{F}_2$, $\text{Cl}_2$, $\text{Br}_2$, and $\text{I}_2$).

Monatomic

Existing naturally as single, unbonded atoms; characteristic of noble gases because they already possess a full outer shell of electrons.

Group 7 Physical State and Boiling Point Trend

Melting and boiling points increase down Group 7, shown by chlorine (gas, boiling point $-34\,^{\circ}\text{C}$), bromine (red-brown liquid, boiling point $59\,^{\circ}\text{C}$), and iodine (dark grey solid, boiling point $185\,^{\circ}\text{C}$).

Group 7 Reactivity Trend

Reactivity decreases going down Group 7 because the number of electron shells increases, placing the outer shell further from the positive nucleus with more shielding, which weakens the electrostatic attraction needed to attract and gain an incoming electron.

Displacement Reaction

A chemical reaction where a more reactive halogen displaces a less reactive halide ion from its aqueous salt solution.

Redox Reaction

A chemical reaction that involves simultaneous reduction (gain of electrons) and oxidation (loss of electrons).

Spectator Ion

An ion (such as the potassium or sodium metal ion in halogen displacement reactions) that does not take part in the transfer of electrons and remains unchanged in solution throughout the reaction.

Oxidation in Halogen Displacement

The loss of electrons by halide ions during a displacement reaction, such as iodide ions ($\text{I}^-$) losing electrons to form molecular iodine ($\text{I}_2$) when reacting with bromine water: $\text{Br}_{2\text{(aq)}} + 2\text{KI}_{\text{(aq)}} \rightarrow 2\text{KBr}_{\text{(aq)}} + \text{I}_{2\text{(aq)}}$.

Reduction in Halogen Displacement

The gain of electrons by a halogen molecule during a displacement reaction, such as bromine ($\text{Br}_2$) molecules gaining electrons to form bromide ions ($\text{Br}^-$).

Covalent Bond

The electrostatic attraction between the positively charged nuclei of two non-metal atoms and a shared pair of negatively charged electrons.

Molecule

A particle formed when two or more non-metal atoms are held together by covalent bonds.

Bonding in Phosphorus Trichloride ($\text{PCl}_3$)

A molecule containing 3 single shared pairs of electrons and 0 double bonds, where phosphorus shares electrons with three chlorine atoms so that all atoms achieve 8 outer-shell electrons.

Silicon Covalent Bonding Capacity

With an electronic configuration of 2.8.4, a silicon atom has four outer electrons and can form a maximum of 4 covalent bonds in a simple molecule to achieve a stable octet.

Intermolecular Forces

Weak forces of attraction between separate simple covalent molecules that require relatively little thermal energy to break, giving simple molecular substances low melting and boiling points.

Electrical Conductivity of Simple Covalent Substances

Simple covalent substances do not conduct electricity in solid, liquid, or molten states because they do not have free-moving ions or delocalised electrons.

Giant Covalent Structure

A continuous lattice held together by many strong covalent bonds extending throughout the structure, requiring vast amounts of energy to break and resulting in high melting points (e.g., silicon carbide, silicon dioxide, and diamond).

Diamond

A giant covalent allotrope of carbon in which every carbon atom forms 4 strong covalent bonds in a rigid tetrahedral network, making it exceptionally hard and an electrical insulator.

Graphite

An allotrope of carbon consisting of hexagonal layers held by weak intermolecular forces; each carbon forms 3 covalent bonds, leaving one delocalised electron per atom that allows electrical conduction and gives it soft, slippery properties.

Fullerenes

Allotropes of carbon that form hollow spheres or cages (such as $\text{C}_{60}$); they sublime at lower temperatures than graphite because they have a simple molecular structure with weak intermolecular forces between molecules.

Graphene

A single, two-dimensional layer of graphite that is one atom thick, highly transparent, extremely strong due to its covalent network, and an excellent conductor of electricity due to free delocalised electrons.

Delocalised Electrons

Electrons not fixed to a specific atom or covalent bond that are free to move throughout the structure, allowing substances like graphite and graphene to conduct electric current.