Properties and Activities of Living Protoplasm 83 



tively charged. Such a charged atom is called an ion. Atoms which 

 have gained electrons have a negative electrical charge and are called 

 anions; atoms which have lost electrons have a positive electrical charge 

 and are called cations. Since like charges repel and unlike charges 

 attract each other, we find that anions and cations combine. The amount 

 of combining which atoms can undergo is determined by the number of 

 electrons in the outer orbit at the beginning and the number that can 

 be gained or lost. Hence, hydrogen, having only one electron, tends to 

 lose it, becoming a (positive) cation. Oxygen, with six electrons in its 

 outer orbit, tends to gain two, becoming a (negative) anion. Thus two 

 atoms of hydrogen and one atom of oxygen combine to form a molecule 

 of water, since hydrogen loses only one electron and oxygen gains two 

 electrons (Fig. 27). 



> 



HYDROGEN 

 ATOM 



OXYGEN 

 ATOM 



HYDROGEN 

 ATOM 



WATER 

 MOLECULE 



Fig. 27. — Diagrams representing the formation of a molecule of water from two 

 atoms of hydrogen and one atom of oxygen. The union is represented by the shar- 

 ing of electrons in the outer orbits of the atoms concerned. P, proton (+ electrical 

 charge; A^^ neutron (electrically neutral) ; black circle with dash, an electron 

 (- electrical charge). The inner circle represents the nucleus of the atom; the 

 outer rings represent one or more orbits ("shells") of the electrons. 



Molecules may vary in complexity from the simple water molecule to 

 the extremely complex long carbon "chains" and "rings" present in 

 many living, organic materials. Molecules may contain only one kind of 

 atom (such as O2) or they may contain atoms of two or more different 

 kinds (such as CO2) ; in the latter case they form a compound. Atoms 

 are held together either by the attraction of opposite ions or by the shar- 

 ing of an electron by two different atoms. 



Many substances in living organisms are soluble in the large quantity of 

 water present in the protoplasm, and in solution many of them dissociate 

 to form ions. Hence, a molecule of common salt, sodium chloride 

 (NaCl), dissociates into (1) a positive sodium ion (Na^) while losing 

 its single outer electron and (2) a negative chlorine ion (CI'), gaining 

 one extra outer electron. A substance which dissociates to form ions is 



