the latter, two. Evidently two atoms of H are needed, as H-O-H,
or

H
= O , or H2O. In the union of Zn and O, each has two bonds;
H

hence they unite atom with atom, Zn = O, or ZnO.

Write the grapbic and the common symbols for the union of H^I and
Cl^I; of K^I and Br^I; Ag^I and O^II; Na^I and S^II; H^I and
P^III. Study valences. It will be seen that some elements have a
variable quantivalence. Sn has either 2 or 4; P has 3 or 5. It
usually varies by two for a given element, as though a pair of
bonds sometimes saturated each other;. e.g. =Sn=, a quantivalence
of 4, and |Sn=, a quantivalence of 2. There are, therefore, two
oxides of tin, SnO and SnO2, or Sn=O and O=Sn=O. Write symbols
for the two chlorides of tin; two oxides of P; two oxides of
arsenic.

The chlorides of iron are FeCl2 and Fe2Cl6. In the latter, it
might be supposed that the quantivalence of Fe is 3, but the
graphic symbol shows it to be 4. It is called a pseudo-triad, or
false triad. Cr and Al are also pseudo-triads.

Cl Cl | | Cl--Fe--Fe--Cl | | Cl Cl

Write formulae for two oxides of iron; the oxide of Al.

57. A Radical is a Group of Elements which has no separate