Strictly certain place is allocated to each chemical element in Mendeleyev's Table. Horizontal lines of the Table are called the Periods, and vertical – Groups. Number of the period corresponds to number of a valent cover of atoms of all elements which are in this Period. And the valent cover gradually is filled, from the beginning by the end of the Period. Change of properties of the elements which are in limits of one Period is explained by it.
Review an example of change of properties of elements of the third Period. It consists (as transfer, from left to right) of sodium, magnesium, aluminum, silicon, phosphorus, sulfur, chlorine, argon. The first element – Na (sodium). Extremely active alkaline metal. What are its pronounced metal properties and, especially, extreme activity explained by? That on its outer (valent) sheath only one electron. Reacting with other elements, sodium easily gives it, becoming a positively charged ion with a steady outer sheath. The second element – Mg (magnesium). Also very active metal, though significantly inferior on this indicator to sodium. On its outer sheath – two electrons. It also rather easily gives them, getting a steady electronic configuration. The third element – Al (aluminum). Has three electrons on an outer sheath. It is quite active metal too though in usual conditions its surface quickly becomes covered by an oxide film which interferes with the introduction of aluminum in reaction. However in a number of connections, aluminum shows not only metal, and acid properties, that is actually is an amphoteric element. The fourth element – Si (silicon). Has four electrons on an outer sheath. It is already the nonmetal low-active under usual conditions (because of formation of an oxide film on a surface). The fifth element – phosphorus. Pronounced nonmetal. It is easily possible to understand that, having five electrons on an outer sheath, it is much easier for it "to accept" others electrons, than to give the. The sixth element – sulfur. Having six electrons at the external level, she shows even more pronounced nonmetallic properties, than phosphorus. The seventh element – chlorine. One of the most active nonmetals. Extremely strong oxidizer. Accepting the one and only others electron, it completes the outer sheath to a steady state. And, at last, inert gas argon closes the Period. It has completely filled external electronic level. Therefore as it is easy to understand, it does not have need neither to give electrons, nor to accept them.