Elements, compounds and mixtures Revision Notes

Diamond: Structure and Properties

Diamond is an allotrope of carbon with a giant covalent structure. Each carbon atom is covalently bonded to four other carbon atoms, forming a rigid, three-dimensional tetrahedral lattice. This extensive network of very strong covalent bonds requires a large amount of energy to break. Consequently, diamond is extremely hard, has a very high melting point, and is insoluble in common solvents. It does not conduct electricity because all its valence electrons are used in strong covalent bonds, meaning there are no free (delocalised) electrons to carry charge.

Graphite: Structure and Properties

Graphite is another allotrope of carbon with a layered giant covalent structure. Each carbon atom forms three strong covalent bonds with other carbon atoms within the same layer, creating hexagonal rings. This leaves one valence electron per carbon atom delocalised (free to move) within the layer. The layers are held together by weak intermolecular forces. This structure explains graphite's properties: it is soft and slippery because layers can slide past each other, and it conducts electricity effectively due to the movement of delocalised electrons within the layers.

Fullerenes and Graphene: Structure and Properties

Fullerenes are molecular allotropes of carbon, typically forming hollow shapes like spheres (e.g., C60 buckminsterfullerene) or tubes. Each carbon atom is bonded to three others, featuring both five- and six-membered rings. Their unique structures give them properties like high tensile strength, good electrical conductivity, and lubricating properties, making them useful in nanotechnology. Graphene is a single layer of graphite, just one atom thick. It consists of carbon atoms arranged in a hexagonal lattice, with each carbon atom bonded to three others and possessing one delocalised electron. This makes graphene exceptionally strong, light, an excellent conductor of heat and electricity, and transparent, opening avenues for advanced electronic materials.

Understanding Mixture Components

To select an appropriate purification technique, it is crucial to understand the physical properties of the substances in the mixture. Key properties include solubility (whether a substance dissolves in a given solvent), boiling points (the temperature at which a substance changes from liquid to gas), and particle size or state (solid, liquid, gas). For example, a mixture of sand and salt requires different techniques than a mixture of alcohol and water because their properties differ significantly.

Techniques for Solid-Liquid Mixtures

For mixtures of solids and liquids: Filtration is used when an insoluble solid is mixed with a liquid, like sand in water. The solid (residue) is trapped by filter paper, and the liquid (filtrate) passes through. Evaporation or crystallisation is suitable for separating a soluble solid from a solvent. Evaporation removes all solvent by heating, leaving the solid. Crystallisation involves heating to concentrate the solution, then cooling to form pure crystals of the solid, which are then filtered and dried. The solvent is typically lost or not recovered during evaporation.

Techniques for Liquid-Liquid and Soluble Solid-Liquid Mixtures

Distillation is employed when you need to separate a desired liquid from a dissolved solid or from other liquids, particularly when the liquid solvent needs to be recovered or purified. Simple distillation separates a volatile liquid from a non-volatile solute or two liquids with significantly different boiling points. The liquid with the lower boiling point vaporises, condenses, and is collected. Fractional distillation is a more refined technique used to separate two or more liquids with similar, but distinct, boiling points, such as ethanol from water. It uses a fractionating column to provide a larger surface area for repeated vaporisation and condensation cycles, leading to a better separation.

Chromatography for Complex Mixtures

Chromatography is a powerful technique for separating components of a mixture that are dissolved in a solvent, often based on differences in their solubility and adsorption to a stationary phase. In paper chromatography, a solvent (mobile phase) moves up a piece of filter paper (stationary phase), carrying the dissolved components at different rates. Substances that are more soluble in the solvent and less attracted to the paper will travel further up the paper. This method is especially useful for separating coloured substances like inks or dyes, or identifying components in complex mixtures.

Atomic Number and Electron Count

The atomic number of an element is defined by the number of protons in the nucleus of its atoms. In a neutral atom, the number of electrons is equal to the number of protons, so the atomic number directly determines the total number of electrons an atom possesses. These electrons are arranged in shells or energy levels around the nucleus. The electron configuration, particularly the number of electrons in the outermost shell, is crucial for understanding chemical behaviour.

Valence Electrons and Reactivity

Valence electrons are the electrons located in the outermost occupied electron shell of an atom. These are the electrons involved in chemical bonding and reactions. Atoms react in ways that allow them to achieve a more stable electron configuration, typically a full outer shell (often eight electrons, known as the octet rule, or two for the first shell). Elements with few valence electrons (e.g., Group 1 metals with one) tend to lose them. Elements with nearly full outer shells (e.g., Group 17 halogens with seven) tend to gain electrons. Elements with full outer shells (e.g., Group 0 noble gases) are very unreactive due to their stable configuration.

Periodic Table Groups and Reactivity

Elements within the same group (vertical column) of the Periodic Table have the same number of valence electrons. This similarity in outer electron configuration leads to similar chemical properties and reactivity patterns. For instance, all Group 1 elements readily lose one electron to become positive ions, while all Group 17 elements readily gain one electron to become negative ions. This consistent behaviour within groups highlights the importance of valence electrons in determining an element's chemical nature.

Understanding Models in Chemistry

Chemical models are simplified representations used to explain and predict chemical phenomena. They help us visualise abstract concepts like atoms, molecules, and chemical bonds. Examples include dot-and-cross diagrams for showing electron transfer or sharing, and ball-and-stick models for representing molecular shapes. Each model focuses on particular features while often omitting others.

Limitations of Dot-and-Cross Diagrams

Dot-and-cross diagrams effectively show electron arrangement in the outermost shells of atoms involved in bonding. However, they are two-dimensional and do not convey the actual three-dimensional shape of a molecule. They also do not accurately represent the relative sizes of atoms or the continuous, dynamic movement of electrons. They are a static snapshot of electron configuration rather than a true depiction of electron clouds.

Limitations of Ball-and-Stick Models

Ball-and-stick models are useful for illustrating the three-dimensional arrangement of atoms within a molecule and the angles between bonds. The 'balls' represent atoms and the 'sticks' represent covalent bonds. A key limitation is that these models often exaggerate the empty space between atoms, implying distinct gaps where none truly exist. They also depict bonds as rigid sticks, while real chemical bonds are flexible and allow for vibration and rotation. Furthermore, they do not show the relative sizes of atoms accurately compared to space-filling models.

Why Multiple Models are Used

No single model can completely represent all properties of atoms and molecules. Different models highlight different aspects. For example, dot-and-cross diagrams are good for electron bookkeeping, while ball-and-stick models excel at showing molecular geometry. Space-filling models give a better sense of atomic size and close packing. Scientists use a combination of models, understanding the strengths and weaknesses of each, to build a comprehensive understanding.