Section 3.1 Notes
Chemistry Class
Ms. Cox
Key Notes
The Atom: From Philosophical Idea to Scientific Theory
Early Greek thinkers, such as Democritus called nature’s basic particle an atom. Atom
comes from the work ATOMOS which means INDIVISIBLE or UNCUTTABLE.
The Greek philosopher’s opinions were not based on experimental evidence. Later
scientist, including John Dalton, proposed theories that were based on testable,
quantitative experimentation.
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The idea of atoms has been around since the time of the ancient Greeks. In the
nineteenth century, John Dalton proposed a scientific theory of atoms that can
still be used to explain properties of most chemicals today.
Law of Conservation of Matter: Matter and its mass cannot be created or
destroyed in chemical reactions.
Law of Definite Proportions: The mass ratios of the elements that make up a
given compound are always the same, regardless of how much of the compound
there is or how it was formed.
Law of Multiple Proportions: If two or more different compounds are composed
of the same two elements, then the ratio of the masses of the second element
combined with a certain mass of the first element can be expressed as a ratio of
small whole numbers.
In 1808, an English schoolteacher named John Dalton proposed an explanation for the
three newly discovered laws.
DALTON’s ATOMIC THEORY:
1. All matter consists of indivisible particles called atoms.
2. Atoms of the same element are IDENTICAL in shape and mass, but differ from
the atoms of other elements.
3. Atoms cannot be subdivided, created or destroyed.
4. Atoms of different elements may combine with each other in a fixed, simple,
whole number ratios to form compound atoms.
5. Atoms of same element can combine in more than one ratio to form two or
more compounds.
6. The atom is the smallest unit of matter that can take part in a chemical reaction.
BUT, we now know that atoms can be divisible into even smaller particles.