ATOM
Atom is the smallest particle of a substance, which possesses its properties. Atom is an electrically neutral micro-system (particle), which is subject to the quantum laws and consists of positively charged atomic nucleus and negatively charged electrons. Electrons in an atom form electron shells (sheaths) according the main quantum numbers n = 1, 2, 3, …. Electrons in incomplete outer shells (valence electrons) define chemical properties of elements.
The size of an atom is determined by the size of the electron shell, which does not have, according to the quantum-mechanical theory, strictly defines borders. The size of an electron shell can be characterized by maximum position of electron density of the outer electron sheath.