5.4 Atomic Theory: Explaining
Chemical Facts and Laws
In earlier studies…..
Particle theory of matter was used to explain observations
of matter.
Can it explain what happens in the electrolysis of water?
The properties of the compound, water, are different from
the properties of the elements that make it up, hydrogen
and oxygen. As well as the particles that make up the
compound water, hydrogen and oxygen.
Since the particle theory of matter does not distinguish one
particle from another , a more powerful theory was needed.
Dalton’s Atomic Theory
• All matter is made up of small particles called
atoms.
• Atoms cannot be created, destroyed , or divided
into smaller particles.
• All atoms of the same element are identical in
mass and size, but they are different in mass and
size from the atoms of other elements.
• Compounds are created when atoms of different
elements link together in definite proportions.
How was it different?
Dalton suggested:
1. The smallest particle of matter is an atom.
2. Substances have different properties
because the atoms of which they are
composed are different.
3. Atoms can combine to form a compound.
Dalton’s theory in a modern
context
Although Dalton’s theory is closely related to
modern views of the structure of matter, some
aspects of it are no longer accepted.
• Certain kinds of “exotic” matter exist that are not
made up of atoms (e.g., particles such as positrons
and neutrinos, neutron stars).
• Atoms are themselves made up of smaller
particles.
• Atoms of the same element are not always
identical.
The Difference Between Laws
and Theories
Scientists use many terms to describe their work and
their results.
Terms that are often used include:
Fact, observation, law, hypothesis, conclusion and
theory.
In science, laws do not explain anything. They
describe and summarize what happens.
Theories are imaginative ways to explain why
something happens.
• Electrolysis of water
• Experimental technology to measure the mass of
hydrogen and oxygen