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Manchester Science Walk, Manchester, England

53° 28′ 40.63″ N, 2° 14′ 40.48″ W

The Atomic Theory

John Dalton was a late-18th-century chemist who lived and worked in Manchester. His most important work was his atomic theory, which laid the groundwork for chemistry as we know it. Dalton’s theory was that the elements are made from tiny particles called atoms, that the atoms that constitute an element are all the same, that atoms can be distinguished by their mass, that different elements have different atoms, and that chemical compounds are combinations of atoms from different elements. Only one part of his theory didn’t stand the test of time: he thought that atoms were indivisible.

In 1803, Dalton published a table of atomic masses for a variety of elements in his book A New System of Chemical Philosophy (see Figure 55-1). He gave hydrogen the nominal mass of 1, and all the other elements were weighed relative to hydrogen. Dalton was also the source of the Law of Multiple Proportions, which says that the ratio of one atom to another in a chemical compound is a whole number—since atoms could not be divided, they had to combine in fixed, whole-numbered ratios.

Dalton also favored the idea that atoms combined in binary relationships, and erroneously thought that water was HO and not H2O.

The best place to learn about John Dalton is at the Manchester Museum of Science and Industry, and a great way to get there is by following a free audio tour of scientific Manchester that ends at the museum.

The Manchester Science Walk is a 17-stop walk through central Manchester, narrated by an actor playing the part of John Dalton and available as a free download of MP3 files and a PDF map. The tour was created as part of the Manchester Science Festival, held each year in October.

The 2.5-kilometer walk starts at the central library, and it takes about 1.5 hours to reach the museum. The audio tour is largely historical, and explains Dalton’s life and the life in Manchester at the time.

Figure 55-1. Dalton’s elements and their weights

The museum’s Manchester Science Gallery highlights those Manchester scientists who have had a large impact: John Dalton, James Joule, Ernest Rutherford, and Sir Bernard Lovell (see Chapter 51). There’s also a working replica of the first stored-program computer (called the Baby), originally built in 1948.

Manchester was at the center of the textile industry and the early railway—in fact, the museum is housed inside the oldest surviving railway station in the world. It has an important collection of locomotives and demonstrations of machines used in cotton mills. Children will enjoy the tour though a Victorian sewer.

If you have time after the walk and museum, visit the Alan Turing statue in Sackville Street Gardens (see Chapter 66).

Practical Information

The website of the Manchester Museum of Science and Industry is at http://www.msim.org.uk/. Information about the Manchester Science Walk can be found at http://www.verm.co.uk/Sciencetour/, and about the Manchester Science Festival at http://www.manchestersciencefestival.com/.

The Changing Weight of Hydrogen

Although Dalton’s theory was correct when he assigned hydrogen the atomic weight of 1, today the atomic weight of hydrogen stands at 1.00794. The weight of a hydrogen atom seems like a simple thing to agree upon, but it has been the subject of controversy, learned commissions, and inquiries ever since Dalton introduced the idea. Even the term “atomic weight” is contested—some scientists, notably physicists, prefer “atomic mass” because weight is gravity dependent (see Chapter 69).

During the 19th century, atomic weights were calculated (with varying degrees of accuracy) by using Dalton’s assignment of 1 to hydrogen. Because there was disagreement about the actual weights of various elements, commissions were set up, culminating in the 1901 international agreement to calculate atomic weight by assigning oxygen the value 16. But following that there was international disagreement, and tables were published based on hydrogen with weight 1 and oxygen with weight 16.

A problem arose in 1929 with the discovery of two new isotopes of oxygen: 17O and 18O. These new isotopes created a problem because the atomic weight used by chemists is the weighted average mass of atoms of an element in their proportions as found in the Earth’s atmosphere and crust. When new isotopes are discovered or measurements made, the atomic weights have to be updated. For this reason, atomic weight tables are updated regularly.

But there was still disagreement between physicists (who were interested in the mass of specific isotopes) and chemists (who were weighing naturally occurring elements). In 1961, the atomic weights were changed based on assigning the carbon isotope 12C the weight 12. Because 12C is the most abundant form of carbon on Earth, this change was acceptable to chemists as it did little to change the weights they were already using, and it was acceptable to physicists because 12C is stable and has no free electrons: it has six protons, six neutrons, and six electrons.

Today, all atomic weights are based on 12C. The atomic weight of carbon (which is still a weighted average) is 12.0107.