Here we briefly explore the man behind the theory and uncover its claims and serious flaws.
What are the five main points of Dalton's atomic theory?
In 1803 John Dalton proposed his atomic theory. He was a tutor at the time and based his findings on experimental evidence and the laws of chemical combinations.
Within it he made several interesting claims: -
1. That all matter is made of tiny, indivisible and indestructible particles called atoms. The atom is the smallest unit of matter that can take part in a chemical reaction.
2. All atoms of a given element are identical in mass and properties but differ from atoms of other elements.
3. Matter cannot be created or destroyed.
4. Atoms of different elements may combine with each other in fixed, simple, whole-number ratios to form compound atoms.
5. Atoms of the same element can combine in more than one ratio to form two or more compounds.
This makes his theory one of the most comprehensive attempts to define what a chemical reaction actually is - i.e. a rearrangement of atoms.
Who was John Dalton?
John Dalton was a British Chemist and meteorologist who was born on 6 September 1766 in Eaglesfield, Cumberland. He is best known for introducing atomic theory into chemistry.
Dalton was born into a Quaker family and was the son of a weaver.
In the 1790s, Dalton was appointed as a teacher of mathematics and natural philosophy at New College Academy in Manchester. At the age of 21, he had began keeping an extensive meteorological diary and later wrote a collection of essays on various meteorological topics.
"This work, Meteorological Observations, and Essays, was published in 1793. It created little stir at first but contained original ideas that, together with Dalton’s more developed articles, marked the transition of meteorology from a topic of general folklore to a serious scientific pursuit." - Encyclopedia Britannica.
Part of his work helped him to speculate that the Earth's atmosphere was a physical mixture of elements of roughly 80% nitrogen and 20% oxygen.
Through his experiments, he was also able to measure the capacity of air to absorb water vapor and observe that the variation of vapor pressure for all liquids is equivalent, for the same variation of temperature.
He was the first person to define partial pressure (in terms of physical law) and was later called "the father of meteorology" by another distinguished British scientist, John Frederic Daniell.
Building on his ideas of partial pressure, John later developed his famous Atomic Theory.
What are the failures of Dalton's atomic theory?
As interesting as Dalton's theory was, it had some very serious drawbacks.
1. His postulation that atom is indivisible has since been proven incorrect. As we all know today, atoms can indeed be further subdivided into a serious of subatomic particles.
Atoms are actually made up of protons, neutrons, electrons, and other subatomic particles. However, Dalton was right in the sense that chemical reactions only really occur at the atomic level.
2. Dalton claimed that all atoms of the same element are the same in all respects. However, as we have since discovered, an element can exist in different forms, called isotopes, which contain equal numbers of protons but different numbers of neutrons in their nuclei. Isoptopes differ from each other in relative atomic mass but not in chemical properties. For example, Carbon has 15 known isotopes (although not all of these are stable.
3. Similar to point 2 above, Dalton also claimed that all atoms of different elements are different in all respects. This has also since been proven incorrect in some circumstances.
For example, isotopes of different elements can differ in atomic number (number of protons) but have the same mass number. These are termed isobars. For example, Argon 40, Potassium 40 and Calcium 40 each have an atomic mass of 40 amu, but differ in their atomic number.
4. Dalton also claimed that atoms of different elements combine in a simple whole-number ratio when forming compounds. This is not observed in complex organic compounds like sugar - C12H22O11.
5. Dalton's theory does not address allotropes, the property of some chemical elements to exist in two or more different forms in the same chemical state. For example, carbon exists in the solids graphene, graphite, and diamond.
However, Dalton's atomic theory did have some merits.
- Dalton was one of the first people to recognize a workable distinction between fundamental particles of an element and compounds that they can form.
How did Dalton create his theory?
At the time that Dalton formulated his interesting theory, the field of chemistry was still emerging. Chemistry was becoming more rigorous and based more on prior experience and failures.
Two of the laws developed during this time which Dalton used in his theories were the Law of Constant Composition and the Law of Multiple Proportion as well as the Law of Constant Composition.
- Law of Constant Composition - This states that samples of a pure compound always contain the same elements in the same mass proportion. In basic terms, water is always composed of 1 gram of hydrogen and 8 grams of oxygen.
In other words, no matter how a compound is obtained or prepared, a pure version will always contain the same elements in the same mass proportion. For example, carbon dioxide (CO2) always contains carbon and oxygen in a 3:8 mass ratio. Water (H2O) always consists of hydrogen and oxygen in a 1:9 mass ratio.
This law, together with Dalton's law of multiple proportions, is the basis for stoichiometry in chemistry.
- The Law of Multiple Proportions - This states that when two elements combine with each other to form more than one compound, the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers.
That might sound a little strange so we'll include an example to help make it clearer.
"Two compounds are made of carbon and oxygen. The first compound contains 1.333 g of oxygen for each gram of C. The second compound contains 2.666 g of oxygen for each gram of carbon. The numbers 1.333 and 2.666 form a simple 1:2 ratio." - abetterchemtext.com.
The Law of Conservation of Mass - "The law of conservation of mass states that mass in an isolated system is neither created nor destroyed by chemical reactions or physical transformations." - courses.lumenlearning.com. In this sense, the products of any reaction must always have the same mass as the reactants.
Dalton's interesting theory was thus a product of attempting to understand these principles.
"Compounds are combinations of atoms and the combinations are always the same (Definite Proportions). Different compounds have different numbers of atoms, but since two atoms must weigh exactly twice one atom, there will be a simple ratio between those masses (Multiple Proportions)." -abetterchemtext.com.