http://www.ck12.org Chapter 4. Atomic Structure
in the case of gases). Chemical compounds always contain the same proportion of elements by mass, regardless of
amount, which provided further support for Proust’s law of definite proportions. Dalton also observed that there was
more than one mass ratio in which two elements could combine.
TABLE4.1: Combining Ratios in Elements in Compounds
CO and CO 2 NO 3 and N 2 O 3 NO 3 and N 2 O 2
FeCl 2 and FeCl 3 N 2 O 3 and N 2 O 4Notice inTable4.1 that the elements combine in different but predictable ratios with other elements. Carbon can
combine in a 1:1 ratio with oxygen to form carbon monoxide –a poisonous gas. Change the ratio to 1:2 carbon to
oxygen, and you get carbon dioxide, which is a gas that we exhale. His work led to the development of thelaw of
multiple proportions. This law states that if two elements form more than one compound between them, then the
ratios of the masses of the second element that combine with a fixed mass of the first element will be ratios of small
whole numbers.
Dalton’s Atomic Theory (1804)
From his experiments and observations, as well as the work of contemporary scientists, Dalton proposed a new
theory of the atom. This later became known as Dalton’s atomic theory. The general tenets of this theory were as
follows:
- All matter is composed of extremely small particles called atoms.
- Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ
in size, mass, and other properties. - Atoms cannot be subdivided, created, or destroyed.
- Atoms of different elements can combine in simple whole number ratios to form chemical compounds.
- In chemical reactions, atoms are combined, separated, or rearranged.
Dalton’s atomic theory has been largely accepted by the scientific community, although a couple of modifications
have been made since its conception. We know now that (1) an atom can be further sub-divided, and (2) not all
atoms of an element have identical masses.
Lesson Summary
- Early Greek philosophers thought that matter could be divided and subdivided into smaller and smaller pieces
without limit. - In 400 BC, Democritus proposed that there was a point at which matter could no longer be divided any further.
He suggested that all matter was composed of tiny indivisible particles, which he called atoms. - Joseph Proust found that compounds always contain the same proportion of elements by mass, regardless of
amount. This was later called the law of definite proportions. - Antoine Lavoisier proposed the law of conservation of mass, which states that in a chemical reaction mass is
not created nor destroyed. - John Dalton discovered that certain combinations of elements could combine in multiple ratios. This was
called the law of multiple proportions. - In 1804, John Dalton proposed a modern atomic theory. This theory is still largely accepted by the scientific
community, with a couple modifications.