SOME FUNDAMENTAL PHYSICS AND CHEMISTRY 



35 



Na+ 



o 



ci- 



ion may be defined as part of a molecule, consisting of one or more atoms 

 with an electric charge. Ions are either positive or negative, depending 

 on whether the atom has lost or gained electrons in producing them. 



When sodium and chlorine combine to form sodium chloride, which 

 is common table salt, a crystal of the salt is supposed to have the lattice 

 structure shown in Fig. 25. There is no sodium in the crystal, no chlo- 

 rine, but only sodium ions and chloride ions. There are not really any 

 sodium chloride molecules, since each chloride ion (observe the central 

 white one in the figure) is surrounded by six sodium ions at equal dis- 

 tances, and each sodium ion is surrounded by six chloride ions at eqi'.al 

 distances. One cannot say which 

 negative ion neutralizes a given 

 positive one, so that no specific 

 pair of ions can be said to form a 

 molecule. A molecule can hardly 

 be said to exist in a sodium chlo- 

 ride crystal, but only positive and 

 negative ions. 



Radicals. — In all the above 

 examples, the units of chemical 

 reactions have been atoms of ele- 

 ments or ions derived from them 

 by transfer of electrons. Very 

 often such reaction units are 

 formed of two or more different 

 elements. Sulfur (S) and oxygen, for example, may unite in the 

 proportion of one of the former to four of the latter. In this propor- 

 tion, however, their electric charges are not balanced, and the group bears 

 two units of negative electric charge — that is, two extra electrons. They 

 constitute a negative ion. In this form they act as a unit in combining 

 with atoms which have lost electrons (positive ions). Potassium (K) 

 may unite with them, but it takes two potassium ions to balance them, 

 and K2SO4 (potassium sulfate) is formed. A group of atoms acting as a 

 unit, as do the sulfur and oxygen (SO4") in this example, is called a radical. 

 Other groups of atoms (radicals) are positively charged (as NH4+), 

 forming positive ions. 



Acids, Bases, and Salts. — When a hydrogen atom (see Fig. 21) gives 

 up its electron, only its nucleus remains. This nucleus is a proton and is 

 positively charged: it may also be called a hydrogen ion (H+). Certain 

 substances in water readily yield up these protons to other substances, 

 and they possess certain properties as a consequence. They have a sour 

 taste, color litmus paper red, and do a number of other characteristic 

 things. SuTJStances which readily donate hydrogen ions (protons) are 



Fig. 25. — Crystal of sodium chloride, 

 showing lattice arrangement of sodium ions 

 and chloride ions. 



