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Gallium is the chemical element with atomic number 31 and symbol $\mathrm{Ga}$. It is a metal. Its melting point lies just above ordinary room temperature. Compounds such as gallium nitride are important semiconductors; their electronic properties arise from the compound lattice, not from the low melting point of elemental gallium.^[ref]

Indium is the chemical element with atomic number 49 and symbol $\mathrm{In}$. It is a metal. It is a soft group-13 metal, commonly forming $+3$ compounds. Indium-containing transparent conducting oxides and indium gallium nitride illustrate how the element contributes to optical and electronic materials without behaving as a free metal.^[ref]

Germanium is the chemical element with atomic number 32 and symbol $\mathrm{Ge}$. It is a metalloid. Crystalline germanium is a semiconductor with a diamond-like crystal structure. Its electronic and infrared-optical properties connect chemical bonding in a solid with devices and spectroscopy.^[ref]

Niobium is the chemical element with atomic number 41 and symbol $\mathrm{Nb}$. It is a transition metal. Niobium and several niobium alloys are used as superconductors. Its common $+5$ chemistry also appears in oxides, including lithium niobate, whose optical and electrical behavior differs from that of the metal.^[ref]

Radium is the chemical element with atomic number 88 and symbol $\mathrm{Ra}$. It is a alkaline earth metal. All its isotopes are radioactive. Its chemistry resembles that of barium, often favoring the $+2$ state, while radioactive decay distinguishes its long-term behavior from an ordinary stable alkaline-earth salt.^[ref]

Chromium is the chemical element with atomic number 24 and symbol $\mathrm{Cr}$. It is a transition metal. A chromium-rich passive oxide layer helps stainless steel resist corrosion. Chromium(III) and chromium(VI) compounds have substantially different bonding, reactivity, and environmental behavior, so an oxidation state is essential when discussing a chromium compound.^[ref]

Plutonium is the chemical element with atomic number 94 and symbol $\mathrm{Pu}$. It is a actinide. It has no stable isotopes and exhibits several accessible oxidation states. Its solid has multiple structural phases, so both nuclear composition and crystal structure matter when specifying a plutonium sample.^[ref]

Curium is the chemical element with atomic number 96 and symbol $\mathrm{Cm}$. It is a actinide. It has no stable isotopes and is produced through nuclear transformations. Its common trivalent chemistry helps organize its separation from other actinides, while its radioactive decay gives it physical behavior absent from a stable chemical analogue.^[ref]

Erbium is the chemical element with atomic number 68 and symbol $\mathrm{Er}$. It is a lanthanide. Erbium ions in glass can absorb and emit infrared light near telecommunications wavelengths. Erbium-doped optical amplifiers exploit these electronic transitions, linking an element's energy levels to long-distance optical communication.^[ref]

Vanadium is the chemical element with atomic number 23 and symbol $\mathrm{V}$. It is a transition metal. It supports several oxidation states, producing differently colored ions in solution. Vanadium-containing alloys strengthen steel, while vanadium oxides participate in oxidation catalysis; the same element therefore plays distinct structural and chemical roles.^[ref]

Lanthanum is the chemical element with atomic number 57 and symbol $\mathrm{La}$. It is a metal. The $+3$ oxidation state dominates its chemistry. Lanthanum compounds occur in optical glasses and catalytic materials; the properties of those compounds should be distinguished from the reactive silvery elemental metal.^[ref]

Thorium is the chemical element with atomic number 90 and symbol $\mathrm{Th}$. It is a actinide. It has no stable isotopes, but thorium-232 is sufficiently long-lived to occur naturally. The $+4$ oxidation state is characteristic of its chemistry, distinguishing it from the more usual $+3$ chemistry of many neighboring f-block elements.^[ref]

Gadolinium is the chemical element with atomic number 64 and symbol $\mathrm{Gd}$. It is a lanthanide. The $\mathrm{G}{\mathrm{d}}^{3+}$ ion has seven unpaired 4f electrons, giving it strong paramagnetism. This connects atomic electron configuration with magnetic behavior and with the effect of coordinated gadolinium on nearby nuclear-spin relaxation.^[ref]

Neptunium is the chemical element with atomic number 93. Its isotope neptunium-239 occurs between uranium-239 and plutonium-239 in a neutron-capture and beta decay production chain.

Antimony is the chemical element with atomic number 51 and symbol $\mathrm{Sb}$. It is a metalloid. Its common oxidation states include $+3$ and $+5$. Elemental antimony is brittle, while its compounds and alloys have distinct materials applications; potassium antimony tartrate is an example of a coordination compound rather than the free element.^[ref]

Malonate is the dianion of malonic acid, with two carboxylate groups separated by one methylene group. Its three-carbon skeleton resembles part of the four-carbon skeleton of succinate.

Malonate is a classical inhibitor of succinate dehydrogenase: structural similarity allows binding without the normal substrate transformation. The example illustrates how an enzyme inhibitor can probe the contribution of a particular reaction to a metabolic pathway.

Ubiquinol is the reduced form of ubiquinone. Its quinone headgroup has accepted two electrons and two protons, while its hydrophobic side chain anchors it in a cell membrane.

In the mitochondrial respiratory chain, diffusion of ubiquinol within the membrane transfers reducing equivalents between protein complexes. Oxidation back to ubiquinone releases the carried electrons and protons through a controlled sequence of steps.

Flavin adenine dinucleotide is a redox coenzyme with a flavin ring system linked to an adenine-containing nucleotide unit. Reduction of the flavin gives FADH2.

In succinate dehydrogenase, an enzyme-bound flavin accepts reducing equivalents during conversion of succinate to fumarate. The flavin is reoxidized through the respiratory electron-transfer machinery, connecting a carbon-metabolism reaction with the mitochondrial respiratory chain.

Fumarate is the dianion of fumaric acid, a four-carbon dicarboxylic acid with a trans carbon-carbon double bond. The geometry distinguishes it from its cis isomer.

In the citric acid cycle, oxidation of succinate yields fumarate, and addition of water across the double bond yields malate. This sequence changes the functional groups while preserving the four-carbon framework.^[ref]

Succinyl-CoA is coenzyme A carrying a four-carbon succinyl group through a thioester linkage. It is formed during oxidative decarboxylation of 2-Oxoglutarate in the citric acid cycle.

Conversion of succinyl-CoA to succinate can be coupled to formation of a nucleoside triphosphate. This is a direct energy-conserving step in the cycle, distinct from the later use of respiratory electron carriers to produce ATP.^[ref]