Chemistry IS The academic discipline concerned with the study of the composition, structure, properties, and speed of change of matter. The main building blocks of matter of interest to the chemist are elements, atoms, and molecules. A chemical element is a pure substance comprised only of one type of atom. The atom is the smallest unit of an element and is comprised of subatomic particles called protons (positively charged), neutrons (no charge), and electrons (negatively charged). The neutrons and protons reside in the central core of the atoms called the nucleus, whereas the electrons are smaller and free moving. The number of electrons in a given atom balances the protons, and this number is characteristic of any atom of a given element.

Dmitri Mendeleev, a Russian chemist, produced the first periodic table in 1869. In this table, elements are arranged in increasing order of the number of protons and are placed in columns based on common properties. The first element in the periodic table is hydrogen, which is comprised of only one proton and one electron. Other elements of interest include carbon, which has atoms containing 6 protons, oxygen with 8, and iron with 26. Although the number of protons and electrons is characteristic of an atom of a given element, the number of neutrons may vary. Two atoms of the same element, with different numbers of neutrons, are called isotopes. Carbon, for example, has three naturally occurring isotopes with 6, 7 or 8 neutrons; these are named carbon-12 (12C), carbon-13 (13C) and carbon-14 (14C), as both the number of neutrons and protons is taken into account in the nomenclature. Of the three isotopes, only 12C is stable.

Atoms are described as physically stable if the atom itself does not decay into its constituent parts or to smaller atoms. The vast majority of the elements toward the top of the periodic table are stable, whereas those with a greater number of protons (more than 85) toward the bottom of the table are not. The most stable element is iron. In contrast to their physical stability, the vast majority of elements are chemically reactive. The term chemical reactivity describes the reaction between atoms that combine to form molecules and compounds. An atom is unreactive to other atoms/ molecules (inert) if it has a closed outer shell of elec trons. The number of electrons required to do this varies as a function of the size of the atom (as predicted by quantum theory). The periodic table reflects this by placing the stable elements on the right-hand side, referred to as the inert or nobel gases. All other elements will undergo chemical reaction in which electrons from each atom are shared in order to achieve the same number of electrons as a nobel gas.

Chemistry is conventionally divided into three subdisciplines: organic, inorganic, and physical. Organic chemistry studies the reactions of molecules that have a carbon and hydrogen skeleton, with additional atoms of oxygen, nitrogen, phosphorous, and sulfur. Organic chemistry is so named, as these are the elements that make up life; most of its applications are the pharmaceutical and petrochemical industries. Inorganic chemistry deals with the reactions of molecules without a carbon and hydrogen-based skeleton, and has a wide range of applications, from catalysts to nuclear reactors. Physical chemistry is the study of the underlying structure of matter and the mechanisms and speed of chemical changes that underpin the other subdisciplines. Physical chemistry is also concerned with the interaction of radiation and matter. Other important subdivisions include biochemistry (study of organic molecules in cells), analytical chemistry (study of techniques to separate and quantify chemicals), atmospheric chemistry, ocean chemistry, and biogeochemistry.

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