S2

S2.1 – Ionic Model

Atoms into Ions

  • Atoms can become ions by losing or gaining electrons.

  • Cations: positively charged ions formed by losing one or more electrons.

  • Anions: negatively charged ions formed by gaining one or more electrons.

Ionic Bonding

  • Ionic bonding occurs when cations and anions attract each other through electrostatic forces.

  • This typically happens between metals (which lose electrons and form cations) and nonmetals (which gain electrons and form anions).

  • Example: Sodium (Na) loses one electron to become Na⁺, while chlorine (Cl) gains one electron to become Cl⁻, resulting in the formation of NaCl (table salt).

Giant Ionic Structures

  • Ionic compounds form giant ionic lattices, where a regular arrangement of alternating cations and anions extends in three dimensions.

  • These structures are held together by strong electrostatic forces between the oppositely charged ions.

  • Properties of giant ionic structures include high melting and boiling points due to the strong attraction between the ions, as well as electrical conductivity when dissolved in water or molten condition.

S2.2 – Covalent Model

Covalent BondingCovalent bonding occurs when two atoms share pairs of electrons to achieve stability. This typically occurs between nonmetals.Example: In a water molecule (H₂O), each hydrogen atom shares an electron with the oxygen atom, resulting in a stable molecule.

Coordinate BondingCoordinate bonding, also known as dative bonding, occurs when one atom donates both electrons to form a covalent bond.Example: In the ammonium ion (NH₄⁺), the nitrogen atom donates a pair of electrons to bond with a hydrogen ion (H⁺).

Giant Covalent StructuresGiant covalent structures consist of a large network of atoms bonded together by covalent bonds. They have high melting and boiling points due to the strong bonds throughout the structure.Example: Diamond and graphite are both forms of carbon with giant covalent structures.

VSEPR (Valence Shell Electron Pair Repulsion)VSEPR theory explains the 3D arrangement of atoms in a molecule. It states that electron pairs around a central atom will arrange themselves to minimize repulsion.Example: In a methane molecule (CH₄), the four hydrogen atoms are arranged around the carbon atom in a tetrahedral shape.

Bond Molecule & PolarityBond polarity refers to the distribution of electrical charge over the atoms joined by the bond. Polar bonds occur when there is a significant difference in electronegativity between the bonded atoms, resulting in a dipole.Example: In HCl, chlorine attracts the shared electron pair more than hydrogen, making HCl a polar molecule.

ChromatographyChromatography is a technique used to separate mixtures based on the differences in their movement through a medium. It exploits differences in solubility and adsorption to separate components.Example: Paper chromatography can separate colored pigments in ink.

Forces Between MoleculesForces between molecules, also known as intermolecular forces, include dipole-dipole interactions, hydrogen bonds, and London dispersion forces. These forces affect properties such as boiling and melting points.Example: Water has strong hydrogen bonds, which contribute to its high boiling point compared to other similar-sized molecules.

The forces between molecules, also known as intermolecular forces, include various types such as dipole-dipole interactions, hydrogen bonds, and London dispersion forces.

  • Dipole-Dipole Interactions occur between polar molecules, which have a positive and a negative end due to differences in electronegativity between atoms. These positive and negative charges attract each other, holding the molecules together.

  • London Dispersion Forces are weak attractions that occur between all molecules, whether polar or nonpolar. These forces arise from temporary shifts in electron density, creating instantaneous dipoles that can induce dipoles in neighboring molecules, leading to attractions.

In summary, dipole-dipole interactions are stronger and occur specifically in polar molecules, while London dispersion forces are weaker and can happen in all types of molecules.

S2.3 – The Metallic Model

Metallic BondingMetallic bonding occurs when metal atoms collectively share their valence electrons in a delocalized manner. This creates a 'sea of electrons' around the metal cations, allowing for the free movement of electrons throughout the structure.

  • Characteristics of metallic bonding include:

    • Good electrical and thermal conductivity due to the mobility of the delocalized electrons.

    • Malleability and ductility, as the layers of atoms can slide over each other without breaking the metallic bond.

    • High melting and boiling points, attributed to the strong attraction between the positively charged metal ions and the delocalized electrons.

S2.4 – From Models to Materials

AlloysAlloys are mixtures of two or more elements, where at least one element is a metal. They are designed to have specific properties that make them desirable for various applications.

Bonding Models

  • The properties of alloys can be understood by considering the bonding models (ionic, covalent, and metallic) in the constituent materials.

  • Metallic bonds in alloys contribute to their strength, ductility, and conductivity. This is due to the delocalized electrons that are shared among metal atoms, allowing them to adapt and enhance each other's properties.

  • Variations in the size, type, and concentration of atoms in alloys can lead to changes in electronic structure and, consequently, the material’s overall properties.