Properties and Activities of Living Protoplasm 81 



to hold one of the whirling electrons in its orbit. Thus, the number of 

 electrons is determined by the number of positive protons in the nucleus. 

 The atom as a whole is electrically neutral (neither positive nor nega- 

 tive). The nucleus of the oxygen atom contains eight positively charged 

 protons which hold the eight negatively charged electrons in the two 

 orbits. The nuclear protons and neutrons are held together by intra- 

 atomic forces (Figs. 25 to 27). 



HYDROGEN MOLECULE 



OXYGEN MOLECULE 



Fig. 26. — Diagram representing the structures of the molecules of the gases 

 hydrogen and oxygen. In the case of hydrogen, the mutual utilization of one 

 electron from each atom (total, two electrons) is involved in the combination. In 

 oxygen, two electrons from each atom (total, four electrons) are involved in the 

 combination. P, proton (+ electrical charge) ; A''^ neutron (electrically neutral) ; 

 black circle with dash, an electron (- electrical charge). The inner circle repre- 

 sents the nucleus of the atom; the outer rings represent one or more orbits 

 ("shells") of the electrons. 



2. A series of negatively charged electrons ("planetary" electrons) 

 which revolve in one or more concentric orbits ("shells") about the nu- 

 cleus and whirling at inconceivable speed. Modern suggestions state 

 that electrons may be "whirlpools of energy." Depending on the kind 

 of atom, there may be from one to seven concentrically arranged orbits, 

 and each orbit has a maximum of electrons which it can accommodate 

 (although sometimes an orbit may not have its maximum number). In 

 general, the inner orbit must be filled to capacity before a second appears. 

 The maximum numbers of electrons in the various orbits are suggested 

 below : 



ORBIT 



First 



Second 



Third 



Fourth 



Fifth 



Sixth 



Seventh 



number of electrons 



(maximum) 



2 



8 

 18 

 32 

 18 

 12 



2 



