Chemical Molecules 
it can directly unite is strictly limited. All atoms 
of the same element have the same valency. 
Monovalent atoms are those which can, under no 
circumstances, unite with more than one other 
atom. The Hydrogen atom is an example of 
such an atom. Divalent atoms, as, for example, 
that of Oxygen, can unite with one other atom of 
similar valency or with two monovalent atoms. 
Similarly, a trivalent atom, such as that of 
Nitrogen, can unite with three monovalent 
atoms. A tetravalent atom, such as that of 
Carbon, can combine with four monovalent 
atoms. There are also pentavalent and hexa- 
valent atoms. Now, by indicating the valency 
of any given atom by a stroke for each mono- 
valent atom with which it is able to combine, 
chemists have been able to represent the mole- 
cule of every compound, or, at any rate, of every 
inorganic compound, by what is known as a 
graphic or structural formula. Thus, ethylic 
alcohol is represented by the formula :— 
H H 
| | 
H—C—C—O—H = C,H,O, 
i 
H H 
and methylic ether by the structural formula :— 
H H 
| | 
H—C—O—C—H =C,H,O. 
| | 
H H 
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