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Please, see if you are eligible to read or download our books by creating an account. Our site matches keywords, searched from 3rd-party sites, to affiliate-networks offering unlimited access to licensed entertainment content. We allows visitors, otherwise looking for free-content to enjoy more for less. In the scope of its subject, chemistry occupies an intermediate position between physics and biology.
The history of chemistry spans a period from very old times to the present. Since several millennia BC, civilizations were using technologies that would eventually form the basis of the various branches of chemistry. It was unsuccessful in explaining the nature of matter and its transformations, but, by performing experiments and recording the results, alchemists set the stage for modern chemistry. In origin, the term is borrowed from the Greek χημία or χημεία. The basis of such a chemical transformation is the rearrangement of electrons in the chemical bonds between atoms. The number of atoms on the left and the right in the equation for a chemical transformation is equal.
The type of chemical reactions a substance may undergo and the energy changes that may accompany it are constrained by certain basic rules, known as chemical laws. Most chemists specialize in one or more sub-disciplines. The atom is the basic unit of chemistry. In a neutral atom, the negatively charged electrons balance out the positive charge of the protons. 1,836 times that of an electron, yet the radius of an atom is about 10,000 times that of its nucleus. The properties of a compound bear little similarity to those of its elements.
When a compound has more than one component, then they are divided into two classes, the electropositive and the electronegative components. Thus, molecules exist as electrically neutral units, unlike ions. However, not all substances or chemical compounds consist of discrete molecules, and indeed most of the solid substances that make up the solid crust, mantle, and core of the Earth are chemical compounds without molecules. Examples of pure chemical substances. A collection of substances is called a mixture. The mole is defined as the number of atoms found in exactly 0. Diagram showing relationships among the phases and the terms used to describe phase changes.
In addition to the specific chemical properties that distinguish different chemical classifications, chemicals can exist in several phases. Many substances exhibit multiple solid phases. Atoms sticking together in molecules or crystals are said to be bonded with one another. More than simple attraction and repulsion, the energies and distributions characterize the availability of an electron to bond to another atom.