209 years ago today...Dalton's 21 elements

"Sept. 3, 1803: Dalton Introduces Atomic Symbols"

by

Randy Alfred
   
September 3rd, 2008

Wired

1803: English chemist-physicist John Dalton starts using symbols to represent the atoms of different elements.

Dalton, considered the father of modern atomic theory, made a logbook entry that day titled, “Observations on the Ultimate Particles of Bodies and their Combinations.” It was the first use of symbols to represent the elements of modern chemistry.

He soon had a table of 21 elements arranged by atomic mass, which he presented in a scientific paper the following month. Eventually, he had 36 different symbols.

In his 1805 work, “A New System of Chemical Philosophy,” Dalton propounded the tenets of his atomic theory:

The chemical elements are made of atoms.

The atoms of an element are identical in mass.

Atoms of different elements have different masses.

Atoms combine only in small, whole-number ratios like 1:1, 1:2, 2:3, etc.

Atoms can not be created or destroyed.
Dalton’s symbols were not the ones we use today, but circles containing distinct symbols (a dot for hydrogen, a cross for sulfur), or circles containing letters (C for copper, L for lead). He used them singly to represent elements and in combination to show compounds.

A decade after Dalton formulated his symbols, Swedish chemist Jöns Jakob Berzelius simplified the system. Half of Dalton’s symbols used letters inside a circle to represent the element. Berzelius organized 47 elements with letters alone, and he based those letters not primarily on the English names, but on the Latin ones. In an era when all Europe’s learned men (and the few women who were allowed into schools and universities) knew Latin, the shared language was an international lingua franca.

All but a handful of Berzelius’ symbols are still used today. So it’s Au for gold and Ag for silver, not the circled G and S of Dalton’s original notation.

The simplified notation led the way for English analytical chemist John Newlands to formulate his Law of Octaves and a prototype periodic table of the elements in 1864, but it was Russian chemist Dmitri Mendeleev who really laid it all on the table with 63 elements in 1869. When he flipped his chart to a horizontal table two years later, he created a form much like what you see in chemistry textbooks and on the walls of chem labs today.

Alas, Mendeleev’s table was based on atomic mass rather than atomic number, so details like the placement of tellurium and iodine didn’t work out. He thought it was a question of inaccurate measurement or other experimental error. It was 1913 before English physicist Henry Moseley reorganized the periodic table by atomic number.

As for Dalton, his name lives on as alternate designation for the atomic mass unit or amu. Microbiologists and biochemists need a convenient measure for large organic molecules. Kilo-u or kilo-amu would be awkward, so a protein molecule might be said to have a mass of 35 kilodaltons, or kDA.

But it’s Berzelius’ symbols and what they mean that plague first-year chem students: You’ve got to “get it” before you can do anything else.