In the previous two chapters, you learned how the modern atomic theory
is used to describe the structures of atoms of different elements. The electron
structures of different atoms successfully account for the position of elements
in the periodic table as well as for groups of elements with similar properties.
On a large scale, all metals were found to have a similarity in electron structure,
as were nonmetals. On a smaller scale, chemical families such as the alkali
metals were found to have the same outer electron configurations. Thus, the
modern atomic theory accounts for observed similarities between elements in
terms of atomic structure. So far, only individual, isolated atoms have been discussed; we have not
considered how atoms of elements join together to produce compounds. There is
a relationship between the electron structure of atoms and the reactions they
undergo to produce specific compounds. Understanding this relationship will
explain the changes that matter itself undergoes. For example, hydrogen is a
highly flammable, gaseous element that burns with an explosive reaction. Oxygen,
on the other hand, is a gaseous element that supports burning. As you know,
hydrogen and oxygen combine to form water. Water is a liquid that neither burns
nor supports burning. What happens when atoms of elements such as hydrogen and
oxygen join to form molecules such as water? Why do such atoms join and why
do they stay together? Why does water have different properties from the elements
that combine to produce it? And finally, why is water H2O and not H3O or H4O? Answers to questions about why and how atoms join together in certain numbers
are provided by considering the electronic structures of the atoms. Chemical
substances are formed from the interactions of electrons as their structures
merge, forming new patterns that result in molecules with new properties (Figure
11.1). It is the new electron pattern of the water molecule that gives water
different properties than the oxygen or hydrogen from which it formed. Understanding
how electron structures of atoms merge to form new patterns is understanding
the changes that matter itself undergoes, the topic of this chapter. |