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{{subpages}} '''Chemistry''' is the [[science]] of materials. Chemists consider that all of the materials in the world are matter, primarily made up of [[atom]]s. The combination of at least two atoms connected by a [[chemical bond]] leads to [[molecule]]s. [[Ion]]s are derived from atoms or molecules by loss or gain of one or more [[electron]]s leading to charged particles. [[Salt]]s are composed of [[ion|cations]] (positively charged ions) and [[ion|anions]] (negatively charged ions), so that the substance or material is neutrally charged (without net charge). Chemists use their view of [[matter]] at the [[atom]]ic to [[molecule|molecular]] level to explain how different materials interact, and how they change under varying conditions. Chemists can induce practical changes of substances, and make new compounds including [[drug]]s, [[explosive]]s, [[cosmetic]]s and [[food]]s. Chemical synthesis of materials involves bringing together substances in bulk under conditions that they can interact to give different substances. These interactions are called [[chemical reaction]]s, and always involve rearrangement of electrons around the reacting atoms of each molecule. The ''formation of bonds'' between atoms or molecules means ''sharing electrons'' between the composite atoms, and this can transform one substance into another; such as the synthesis of [[water (molecule)|water]] (H₂O) from two gases: [[hydrogen]] (H₂) and [[oxygen]] (O₂). Some chemical interactions require energy: the substances must be mixed and heated for a [[ chemical reaction]] to occur. These energy requiring reactions are called [[endothermic reaction]]s. Whereas other chemical reactions release heat, and those energy releasing reactions are called [[exothermic reaction]]s. On a molecular level, reactions can also be initiated by the addition or removal of electrons using [[electromagnetic radiation]] ([[light]]).

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