Atomic weights

Following the formulation of his atomic theory, Dalton tried comparing the weights of atoms.

From his analysis of the combining weights of oxygen and hydrogen in water, he calculated that the weight of an oxygen atom compared to the weight of a hydrogen atom = 88.8 : 11.2 = 8:1.

But don't we believe that an oxygen atom is about 16 times heavier than a hydrogen atom?

Yes. Dalton assumed that the formula for water was HO. We know now that water is H2O, so oxygen is 8 times as heavy as 2 hydrogen atoms which makes it 16 times as heavy as 1 hydrogen atom.

What are the comparative or relative weights (or masses) of atoms?

• Dalton (1803) chose hydrogen = 1 as the standard for comparison.
• Berzelius chose oxygen = 100 as the standard.
• J.S.Stas, a Belgian,(1850...) chose oxygen = 16 as the standard.
• Currently, C-12 = 12 is the standard, and we speak of relative atomic mass rather than atomic weight.

Berzelius and Stas are both mentioned in this article, referred to on the previous page.

RELATIVE ATOMIC MASSES

In relation to the mass of the C-12 isotope of carbon which = 12.0000,

here are the weighted average masses of the atoms of some elements

in their isotope mixtures as they occur in nature:

argon = 39.948, carbon = 12.011, hydrogen = 1.0080, iron = 55.847

nitrogen = 14.0067, oxygen = 15.9994, sodium = 22.9898, zinc = 65.37

Read a summary of this section outlining the history of development of the understanding that all matter consists of atoms.

Here is an article which fills the gap between the Greeks and Dalton.

Another account of Dalton's life and work is interesting, (although the screen colors make it difficult to read).