Scientists found that the stoichiometry of compounds changed periodically. If one ordered the elements by their atomic mass, you could group them by their chemical properties row after row. This grouping of elements by the compounds that they make became the periodic table. An example of a periodic chemical property is the reaction of metals with halogens (group VII elements) to make metal halides. Reacting a group I metal with Cl2 makes a binary ionic chloride with 1 metal to 1 chlorine (e.g. NaCl). Similarly, reacting group II metals give a stoichiometry of 1 metal to 2 chlorines and group I metals with Br2 give a 1:1 stoichiometry, for example MgCl2 and KBr, respectively. How can elements with such diffent masses such as Cl and Br react in the same stoichiometry with elements with such different masses as Na and K? It is because of they have the same number of valence electrons. Valence electrons are the higher energy, reactive electrons around an atom, which contrast with the lower energy, unreactive core electrons. …show more content…
These are the noble gas configurations. Group I elements have the general form of [NG] ns1, where [NG] is the noble gas configuration and n is the shell number for the one valence s-electron, for example Na: [Ne] 3s1. Elements gain, lose or share electrons to obtain a noble gas configuration. In our examples above, the metals lose all valence electrons to obtain a noble gas configurationn, one for group I and two for group II. The halogens gain an electron to obtain eight valence electrons and a noble gas configuration for the anion. Cl: [Ne] 3s23p5 + e- --> Cl-: [Ne] 3s23p6 equivalent to