Elements, compounds and mixtures Study Guide

Dmitri Mendeleev's periodic table, published in 1869, was a groundbreaking achievement. He arranged the then-known elements primarily by increasing atomic mass and grouped them based on their chemical properties. This allowed him to predict the existence and properties of undiscovered elements, such as 'eka-silicon' (germanium), 'eka-aluminium' (gallium), and 'eka-boron' (scandium), which were later found to match his predictions remarkably well. However, Mendeleev's arrangement had a few anomalies where elements had to be placed out of strict atomic mass order to fit their chemical properties, for example, tellurium (atomic mass 127.6) was placed before iodine (atomic mass 126.9). This was necessary because iodine clearly belonged with the halogens, and tellurium with the oxygen group. These 'inversions' highlighted a limitation of using atomic mass as the sole criterion for arrangement.

The refinement of the periodic table came with the discovery of the proton and the concept of atomic number (the number of protons in an atom's nucleus), largely due to the work of Henry Moseley in the early 20th century. Moseley used X-ray spectroscopy to determine the atomic number of different elements. He found that when elements were arranged by increasing atomic number, the aforementioned anomalies in Mendeleev's table disappeared, and the periodicity of chemical properties became perfectly consistent. For instance, according to atomic number, tellurium (atomic number 52) naturally precedes iodine (atomic number 53), aligning perfectly with their chemical properties. The atomic number is a more fundamental property than atomic mass because it defines the element and dictates its electronic configuration, which in turn determines its chemical behaviour. Therefore, the modern Periodic Table arranges elements strictly in order of increasing atomic number, resolving the ambiguities of Mendeleev's original mass-based arrangement and providing a more robust and chemically accurate organizational framework.

Carbon is a unique element due to its ability to form a vast array of natural and synthetic organic compounds. This remarkable versatility stems primarily from three key properties of carbon. Firstly, carbon atoms can readily form strong covalent bonds with other carbon atoms. This allows them to link together to create long chains of varying lengths, and also to form intricate ring structures. This self-bonding capability, known as catenation, is fundamental to the structural diversity of organic molecules.

Secondly, carbon atoms can form single, double, or triple covalent bonds with other carbon atoms, as well as with atoms of other elements like hydrogen, oxygen, nitrogen, sulfur, and halogens. This variation in bonding allows for different arrangements of atoms and different chemical properties, even with the same number of atoms. For example, ethene contains a carbon-carbon double bond, while ethane contains a carbon-carbon single bond; these differences lead to distinct chemical behaviours.

Thirdly, carbon typically forms four covalent bonds. This tetravalency means that each carbon atom can connect to up to four other atoms, allowing for branching in carbon chains and the formation of complex three-dimensional structures. This extensive bonding capacity, combined with catenation and variable bond types, enables carbon to be the backbone for millions of different molecules, from simple hydrocarbons to complex biological polymers like DNA and proteins, and synthetic materials such as plastics and pharmaceuticals. These molecules can be grouped into 'families' (homologous series) that share similar structures and chemical properties, further illustrating carbon's ability to create diverse yet systematically related compounds.