John Dalton

Democritus, a Greek philosopher, was the first to propose that matter is comprised of fundamental, indivisible particles, which he called atomos.

though this marks the first time matter was viewed in this way it was more of a philosophy than real science and no supporting evidence was gathered for a long time until John Dalton came along and provided the first empirical evidence that atoms exist he proposed that compounds consist of different types of atoms in whole number ratios and that chemical reactions involve the rearrangement of those atoms to give new combinations This idea explained certain observations.

For example, when a sample of water was split into hydrogen and oxygen, the result was always one ninth hydrogen and eight ninths oxygen by mass.

With atomic theory, this makes perfect sense if we understand that water is made up of tiny identical water molecules that all have the same exact number of hydrogen and oxygen atoms.

then no matter how much water there is it will always have the same ratio of hydrogen to oxygen by number of atoms and therefore also by mass it also explained the way certain elements combined to form different compounds If carbon reacts completely with either a certain mass of oxygen or precisely double that mass of oxygen, that probably means every single carbon atom is bonding to either exactly one or exactly two oxygen atoms.

This was the first solid evidence that atoms exist.

However, we soon discovered that Dalton's atoms were not the fundamental, indivisible units of matter that Democritus envisioned.

J.J. Thomson showed that the atom must consist of smaller particles when he discovered the electron.

That theory is indeed nothing more than the elaboration of Lavoisier's own principle.

John Dalton, 1766-1844, a Manchester Quaker, published in eighteen ten his new system of chemical philosophy where highly important conclusions are drawn both from lavoisier's facts and from experimental results of other chemists of these dalton gave an account and an explanation which has ever since been the soul of all chemical reasoning this explanation is known as his atomic theory the two facts of which dalton's theory is an explanation are as follows first lavoisier's fact that the total weight of substances remains always the same be they combined in ever so many different ways second that all substances be they in large or in small quantities combine with each other or separate from each other in definite and fixed proportions the theory of dalton was that these combinations take place between independent particles of matter which are indestructible and indivisible These atoms of the various elements have definite weights which are responsible for the proportion in which they are found to combine.

These facts of proportion in combination, or chemical affinity, could not be accounted for by the theory which regards matter as continuous, but only by the opposite theory, that it is discrete, i.e., divided up into particles.

Philosophical Corollaries These strictly scientific theories associated with the name of Laplace, Lavoisier, and Dalton tended to strengthen in the popular estimation the philosophical conclusions of writers like Holbach.

Aristotle, for instance, disagreed completely, stating instead that matter was made of four elements, earth, wind, water, and fire, and most later scientists followed suit.

Atoms would remain all but forgotten until 1808, when a Quaker teacher named John Dalton sought to challenge Aristotelian theory.

Whereas Democritus' atomism had been purely theoretical, Dalton showed that common substances always broke down into the same elements in the same proportions.

He concluded that the various compounds were combinations of atoms of different elements, each of a particular size and mass, that could neither be created nor destroyed.

Though he received many honors for his work, as a Quaker, Dalton lived modestly until the end of his days.

Atomic theory was now accepted by the scientific community, but the next major advancement would not come until nearly a century later with the physicist J.J. Thomson's 1897 discovery of the electron.

In what we might call the chocolate chip cookie model of the atom, he showed atoms as uniformly packed spheres of positive matter filled with negatively charged electrons.

It was not until 1808, however, that this atomic theory was really put on a firm foundation.

In that year, the English chemist John Dalton, 1766 to 1844, published a book in which he discussed atoms in detail.

Every element, he suggested, was made up of its own type of atoms.

The atoms of one element were different from the atoms of every other element.

The chief difference between the various atoms lay in their mass or weight.

Dalton was the first to try to determine what these masses might be.

He could not work out the actual masses in ounces or grams, for atoms were far too tiny to weigh with any of his instruments.

He could, however, determine their relative weights, that is, how much more massive one kind of atom might be than another.

This is a favorite topic for extended writing questions worth four to six marks.

The story begins with John Dalton in 1803.

Dalton proposed that atoms were tiny, indivisible, solid spheres.

He had no knowledge of subatomic particles.

His model was revolutionary for its time, but as we now know, incomplete.

Group number equals number of outer electrons.

Number seven, the development of atomic models.

Dalton's solid sphere, Thomson's plum pudding, Rutherford's nuclear model from alpha scattering, Bohr's energy levels, shows how scientific models evolve with new evidence.