The name of atom came from the Greek word "atomos" that means "indivisible". It happened still before it was revealed that it consists of a set of smaller particles: electrons, protons, neutrons. Did not begin to change the name, having accepted at the International congress of chemists in Karlsruhe in the 1860th year that atom is a smallest indivisible carrier of chemical properties of an element.
The kernel borrowing is a part of any atom it is insignificant small volume, but concentrated in himself practically all its weight, and the electrons rotating around a kernel on orbitals. Usually the kernel is neutral, that is, the total negative charge of electrons is counterbalanced with a total positive charge of the protons which are contained in a kernel. The neutrons which are in it as it is easy to guess from the name, do not bear any charge. If the quantity of electrons surpasses quantity of protons or concedes to it, atom becomes the ion loaded negatively or, respectively, positively. The structure of atom was a subject of hot discussions, since ancient times. Such outstanding people as the Ancient Greek scientist Democritus, the ancient Roman poet Tit Lucretius of Carrhae (the author of the well-known composition "About the nature of things"), considered that properties of the smallest particles are caused by their form and also existence (or absence) the sharp, acting elements. The famous physicist Thomson who opened an electron in 1897 offered the model of atom. According to it, it represents itself(himself) a certain spherical body in which, as if highlights in a pudding or cake, are electrons. So famous physicist Rutherford, Thomson's pupil, by practical consideration established impossibility of such model and offered the "planetary model" of atom. Further, efforts of many world famous scientists, such as Bohr, Planck, Schrödinger, etc., the planetary model gained the development. The quantum mechanics by means of which it was succeeded to explain "behavior" of atomic particles was created and to resolve the arisen paradoxes. Chemical properties of atom depend on a configuration of its electron shell. Its weight is measured in atomic units (one atomic unit is equal 1/12 mass of atom of isotope of carbon 12). The location of atom in Mendeleyev's Table depends on electric charge of a kernel. Atoms have so insignificant sizes that they cannot be made out even in the most powerful optical microscope. The image of an electronic cloud around an atomic nucleus can be received by means of an electronic microscope.