Dalton’s atomic theory and limitations - WAEC TUTORIALS
Major
postulates of Dalton’s atomic theory are:
(i) All forms
of matter are made up of very small particles called atoms ( from the Greek
word “Atom” meaning “indivisible”).
(ii) Atoms
cannot be created, divided or destroyed as a result of a chemical change.
(iii) All
atoms of an element are identical, and different from those of other elements.
(iv) Atoms of
elements combine in the ratio of whole numbers to produce a large number of
compound-atoms of a new substance.
The compound-atoms of a particular substance
are identical in all properties, and differ from those of other substances.
Thus,
according to Dalton’s atomic theory, hydrogen was considered to be composed of
only the atoms of hydrogen, while oxygen contained only the atoms of oxygen.
Limitations
of Dalton’s atomic theory
According to
Dalton’s atomic theory, an atom is the ultimate, discrete and indivisible
particle of matter. Later researches proved that Dalton’s atomic theory was not
wholly correct.
Dalton’s
atomic theory suffered from the following drawbacks:
i) Atoms of
the same or different types have a strong tendency to combine together to form
a new ‘group of atoms’. For example, hydrogen, nitrogen, oxygen gases exist in
nature as ‘group of two atoms’. This indicates that the smallest unit capable
of independent existence is not an atom, but a ‘group of atoms’.
ii) With the
discovery of sub-atomic particles, e.g., electrons, neutrons and protons, the
atom can no longer be considered indivisible.
iii) Discovery
of isotopes indicated that all atoms of the same element are not perfectly
identical. At least, they differ in their masses. Atoms of the same element
having different masses are called isotopes.
Dalton’s
atomic theory could not explain why certain substances, all containing atoms of
the same element, should differ in their properties. For example, charcoal,
graphite and diamond all are made up of only Carbon-atoms, but still their
properties are quite different.
No comments