The atoms of elements possess a power of combination, or socalled valency, more or less peculiar in character. Hydrogen is a socalled univalent elementthat is, it has only one capability of chemical attachment to another element. Thus, it may be made to combine with chlorine, and the combination may be graphically represented as follows: Hydrogen, then, and the other elements which have a similarly restricted valency are called monads. Oxygen, on the other hand, generally behaves as a 4*valent element, and can combine, for example, with 2atoms of hydrogen or other monovalent element, as shown by the constitutional formula of water, HaO, which may be graphically expressed thus: In other combinations, oxygen behaves as a tetradas, for example, when it exists in combination with hydrogen as hydrogen dioxide. This represents the view that i atom of oxygen is attached, on the one hand, directly to the 2 atoms of hydrogen, and the other atom of oxygen is itself not in direct attachment or combination with the hydrogen atoms. Nitrogen is an example of a frcvalent element, although in some compounds it behaves as a pentad or pentavalent. For example, in ammonia it exists in combination with 3 atoms of hydrogen, and may be represented as follows whereas, combined as m chloride of ammonium, it would appear to exist in pentavalent combination, thus: Carbon is an example of the tetrad c lassthat is to say, it has the power of combining with 4 atoms of hydrogen or 2 atoms of oxygen, as expressed in the two follow >ng graphic formulae:

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Definition of  VALENCIES