The chemical elements ordered in the periodic table
Steam and liquid water are two different forms of the same chemical (pure) substance: water.
A thermite reaction using iron(III) oxide. The sparks flying outwards are globules of molten iron trailing smoke in their wake.
Estimated distribution of dark matter and dark energy in the universe. Only the fraction of the mass and energy in the universe labeled "atoms" is composed of chemical elements.
Colors of a single chemical (Nile red) in different solvents, under visible and UV light, showing how the chemical interacts dynamically with its solvent environment.
Antoine Lavoisier developed the theory of combustion as a chemical reaction with oxygen.
Periodic table showing the cosmogenic origin of each element in the Big Bang, or in large or small stars. Small stars can produce certain elements up to sulfur, by the alpha process. Supernovae are needed to produce "heavy" elements (those beyond iron and nickel) rapidly by neutron buildup, in the r-process. Certain large stars slowly produce other elements heavier than iron, in the s-process; these may then be blown into space in the off-gassing of planetary nebulae
Native sulfur crystals. Sulfur occurs naturally as elemental sulfur, in sulfide and sulfate minerals and in hydrogen sulfide.
As seen from the equation CH4 + 2O2 -> CO2 + 2 H2O, a coefficient of 2 must be placed before the oxygen gas on the reactants side and before the water on the products side in order for, as per the law of conservation of mass, the quantity of each element does not change during the reaction
Abundances of the chemical elements in the Solar System. Hydrogen and helium are most common, from the Big Bang. The next three elements (Li, Be, B) are rare because they are poorly synthesized in the Big Bang and also in stars. The two general trends in the remaining stellar-produced elements are: (1) an alternation of abundance in elements as they have even or odd atomic numbers (the Oddo-Harkins rule), and (2) a general decrease in abundance as elements become heavier. Iron is especially common because it represents the minimum energy nuclide that can be made by fusion of helium in supernovae.
Potassium ferricyanide is a compound of potassium, iron, carbon and nitrogen; although it contains cyanide anions, it does not release them and is nontoxic.
An example of organic reaction: oxidation of ketones to esters with a peroxycarboxylic acid
Mendeleev's 1869 periodic table: An experiment on a system of elements. Based on their atomic weights and chemical similarities.
Cranberry glass, while appearing homogeneous, is a mixture consisting of glass and gold colloidal particles of about 40nm in diameter, giving it a red color.
Isomerization of azobenzene, induced by light (hν) or heat (Δ)
Dmitri Mendeleev
Chemicals in graduated cylinders and beaker.
Representation of four basic chemical reactions types: synthesis, decomposition, single replacement and double replacement.
Henry Moseley
Illustration of a redox reaction
Sodium chloride is formed through the redox reaction of sodium metal and chlorine gas
Ferrocene – an iron atom sandwiched between two C5H5 ligands
Precipitation
In this Paterno–Büchi reaction, a photoexcited carbonyl group is added to an unexcited olefin, yielding an oxetane.
Schematic potential energy diagram showing the effect of a catalyst in an endothermic chemical reaction. The presence of a catalyst opens a different reaction pathway (in red) with a lower activation energy. The final result and the overall thermodynamics are the same.
Solid heterogeneous catalysts are plated on meshes in ceramic catalytic converters in order to maximize their surface area. This exhaust converter is from a Peugeot 106 S2 1100
Mechanism of electrophilic aromatic substitution
E2 elimination
Electrophilic addition of hydrogen bromide
Acid-catalyzed addition-elimination mechanism
The Cope rearrangement of 3-methyl-1,5-hexadiene
Illustration of the induced fit model of enzyme activity
Thermite reaction proceeding in railway welding. Shortly after this, the liquid iron flows into the mould around the rail gap.

A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species.

- Chemical element

A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another.

- Chemical reaction

Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., without breaking chemical bonds.

- Chemical substance

Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur.

- Chemical reaction

When different elements undergo chemical reactions, atoms are rearranged into new compounds held together by chemical bonds.

- Chemical element

Chemical substances may be combined or converted to others by means of chemical reactions.

- Chemical substance
The chemical elements ordered in the periodic table

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Chemical compound

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A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) composed of atoms from more than one element held together by chemical bonds.

A compound can be converted to a different chemical substance by interaction with a second substance via a chemical reaction.