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The Dalton atomic model—also known as the billiard ball model—was proposed by English chemist John Dalton in 1803. Built on the laws of conservation of mass, definite, multiple, and reciprocal proportions, it pictured atoms as tiny, indivisible spheres unique to each element. The model was later overturned by the discovery of the electron (1897), the nucleus (1911), and other subatomic particles.
For someone to think beyond the constructs and limits of their intellect is difficult, but for a person to conceive something that has never been imagined by any other human mind is a feat that belongs to the brain of a genius. That is precisely what John Dalton did. Much of the Dalton Atomic Model has been disproven, but he is still considered the man who laid the foundation for the atomic theory we celebrate today. Before we dive into the Dalton atomic model, let’s take a brief overview of the atomic theory that had been established before it.
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The Basic Laws Of Atomic Theory
The first thing we will be looking at is the Law of conservation of mass. It was a law founded by Antoine Lavoisier in 1789. The law of conservation of mass states that the net change in the mass of the reactants and the products after a chemical reaction is zero. This implies that mass can neither be destroyed nor created. This implies that mass always remains constant in a chemical reaction. A few inaccuracies were noted in this law later, due to the fact that during some reactions, the mass can interconvert with heat and bond energy.
The second law is the Law of constant proportions (also called the law of definite proportions). It states that any given chemical compound always contains its constituent elements in the same fixed proportion by mass. The significance of this law is that a sample of a compound has the same elements in the same proportions regardless of where the compound was made, how it was obtained or how much of it you have.
The next law is the law of multiple proportions, which states that when two elements form two or more compounds between them, the ratio of the masses of the second element in each compound can be expressed in the form of small whole numbers. The law was proposed by Dalton himself after studying the previous two laws.
The fourth and final law is the law of reciprocal proportions, which states that when two different elements combine separately with the same fixed mass of a third element, the masses in which they do so bear a simple ratio to the masses in which they combine with each other. Jeremias Benjamin Richter proposed this law in 1791.
John Dalton Atomic Model
Based on the above laws that were laid down, Dalton formulated his idea on the fundamental nature of matter, which is how he came up with the concept of atoms. After studying the laws above with great scrutiny, he came up with the following postulates:
- Matter is made of very tiny particles called atoms.
- Atoms are indivisible structures that can neither be created nor destroyed during a chemical reaction (based on the law of conservation of mass).
- All atoms of a particular element are similar in all respects, including their physical or chemical properties.
- Inversely, atoms of different elements show different properties, having different masses and different chemical properties.
- The relative number and kinds of atoms in a given compound are always in a fixed ratio (based on the law of definite proportions).
Based on the above postulates, Dalton was able to come up with one of the first models for the atom. Another name for his model is the billiard ball model. He defined atoms as tiny indivisible spherical objects that cannot be divided any further. He was unaware at the time (as was everyone, for that matter) of the concept of the nucleus, protons or electrons. If you had asked Dalton to draw an atom, he would have simply drawn a circle!
He did try to classify the atoms and was the first scientist to assign symbols to different elements. However, despite his best efforts, many discrepancies and fallacies crept into the very foundation of his theory. The second postulate could eventually no longer be accepted: J.J. Thomson’s discovery of the electron in 1897, followed by Rutherford identifying the nucleus in 1911 and the proton a few years later, and finally Chadwick uncovering the neutron in 1932, showed that atoms are themselves built from smaller subatomic particles—and so are not indivisible after all. The third postulate was also found to be false, as isotopes are atoms of the same element that share the same number of protons but have different numbers of neutrons. Finally, the fourth postulate was proven wrong by the existence of isobars—atoms of different elements that share the same mass number (the same total of protons and neutrons) but have different atomic numbers.
After J.J. Thomson announced his plum-pudding model of the atom, it reinforced what was still true in Dalton’s remaining postulates—that elements are built from atoms with characteristic properties. A few years later, Rutherford’s gold-foil experiment overturned the plum-pudding distribution of charge and led to his 1911 nuclear model, in which a tiny, dense, positively charged nucleus is surrounded by orbiting electrons. Niels Bohr then refined this picture in 1913 with quantized electron orbits, and the fully quantum-mechanical atom we know and admire today—with electrons described as probability clouds (orbitals) rather than fixed orbits—was developed shortly after by Heisenberg (1925) and Schrödinger (1926)!
