Elements, compounds and mixtures Study Guide

Dot and cross diagrams are visual representations used in chemistry to show the electron arrangement in the outermost shells of atoms when they form bonds. These diagrams help to illustrate how atoms achieve a stable electron configuration, typically a full outer shell, like noble gases. For simple covalent substances, atoms share electrons. Each shared pair of electrons is represented by one dot and one cross, indicating that each electron in the pair comes from a different atom. Only the valence (outer shell) electrons are shown. For example, in a molecule of hydrogen chloride (HCl), hydrogen contributes one electron (shown as a dot) and chlorine contributes one electron (shown as a cross) to form a shared pair, while the remaining non-bonding electrons on chlorine are also shown. For binary ionic substances, one atom transfers electrons to another. The atom that loses electrons becomes a positively charged ion (cation), and the atom that gains electrons becomes a negatively charged ion (anion). In these diagrams, the transferred electrons are represented by dots or crosses to show their original atom. The ions are then enclosed in square brackets, with the charge written outside the brackets, and the full outer electron shells of both ions are drawn, clearly showing which electrons have been gained or lost. For instance, in sodium chloride (NaCl), sodium (Na) transfers its one outer electron to chlorine (Cl), resulting in Na+ and Cl- ions. The Na+ ion will be shown without its original outer electron, and the Cl- ion will be shown with an additional electron from Na in its outer shell.

Chromatography is a powerful laboratory technique used to separate mixtures. Both paper chromatography and thin layer chromatography (TLC) rely on the principle that different components in a mixture will travel at different speeds through a stationary phase when carried by a mobile phase. The stationary phase in paper chromatography is a specialized piece of chromatography paper, while in TLC it's a thin layer of adsorbent material (like silica gel or alumina) spread over a glass or plastic plate. The mobile phase is a solvent or mixture of solvents that moves up the stationary phase by capillary action, carrying the mixture's components with it.

To perform either technique, a small spot of the mixture is applied near one end of the stationary phase, known as the 'baseline' or 'origin'. The stationary phase is then placed vertically in a container with the mobile phase at the bottom, ensuring the solvent level is below the applied spot. As the solvent moves up, it dissolves the components of the mixture and carries them along. Components that are more soluble in the mobile phase and less adsorbed by the stationary phase will travel further and faster. Conversely, components that are less soluble in the mobile phase and more strongly adsorbed by the stationary phase will travel shorter distances or slower.

Eventually, the solvent front reaches near the top of the stationary phase, and the process is stopped. The separated components appear as distinct spots or bands. If the components are coloured, they are immediately visible. If they are colourless, a locating agent (such as ninhydrin for amino acids) or exposure to UV light might be needed to visualise them. The relative distance travelled by each component can be quantified by its retention factor (Rf value), which is the ratio of the distance travelled by the spot to the distance travelled by the solvent front.