He also envisioned the atom as a solid billiard ball, not having any knowledge of subatomic structure. For example, he believed water to be HO not H2O. While some were correct, some were wrong. He had no way of knowing how many atoms were in a molecule and only made guesses at the ratios. Although, there were those that still contested the accuracy of his theory and refused to accept it without direct observations of atoms.Īnd, of course, Dalton did make mistakes and his atomic theory was not entirely correct. Theories like alchemy and phlogiston lost popularity and Dalton’s cohesive atomic theory gained acceptance. This era, the late 1700s and early 1800s, marked an important turning point in chemistry. With this atomic theory, chemists had a systematic, universal language that allowed them to describe the results of experiments and the chemical makeup of the world. Atoms recombine in whole numbers during a chemical reaction to form new compounds, which have fixed fractions of each element. Atoms of the same element are all identical and have fixed, unique mass and properties.Ĥ. This was a rejection of the still-popular theory of alchemy.ģ. Atoms of an element cannot be converted to a different element. All matter is made of atoms, which are indivisible units of an element that cannot be created or destroyed.Ģ. These pictures provided scientists with the imagery to understand both the simplicity and diversity of the chemical world as it allowed for infinite combinations of a core set of elements.ĭalton laid out his new atomic theory in the following four postulates:ġ. The atoms can combine in different ratios to create even more compounds. There are these fundamental particles of each element, the atoms, which then combine to form binary compounds, which themselves have unique properties. You can see how he began to conceive the atomic world. Here’s an image from his publication, “New System of Chemical Philosophy”. He began drawing images of molecules made of atoms, which could explain the whole number ratios observed in his atomic weight experiments. If carbon and oxygen can combine to form two different compounds at very specific ratios, there must be some indivisible particle from each element that combines in these very specific ratios. This discovery, along with the other laws mentioned, led Dalton to imagine the nature of matter on the atomic scale. This illustrates that there is exactly 2 times, not 1.5, not 2.3, but exactly 2 times the amount of oxygen in carbon oxide II than in carbon oxide I. In other words, the more H2O was added to the air, the more pressure was added to the air. He then observed that on humid days, the atmospheric pressure increased by the pressure of water vapor. He took note of the atmospheric pressure on normal, dry days. So, as we learn about Dalton’s experiments and theories, keep in mind that while they may seem like common sense to us now, what he was proposing was radical for the time.ĭalton’s extensive study of the atmosphere led him to develop the law of partial pressures, his first major contribution to chemistry and physics.

Scientists during this time were just beginning to understand the nature of the world and much of it was still a mystery. The phlogiston theory postulated that all combustible bodies contained phlogiston, a fire-like element, that would be released upon combustion.

Alchemy and the ill-conceived concept of phlogiston were still quite popular. The law of conservation of mass had just been proposed by Antoine Lavoisier in 1789, meaning that scientists had just realized that mass cannot be created or destroyed during a chemical reaction. At this point in history, scientists weren’t sure that atoms even existed (Dalton would come to play a significant role in this later). As he observed the world around him, he asked questions like, what’s in a cloud? How does water vapor become rain? While he contemplated these atmospheric questions, his curious mind wanted to probe further, so he wondered, “What makes up the water in the water vapor?”

So, he began his career simply by making meteorological observations, like wind speed and barometric pressure. He was a curious and thoughtful man but, given the time period, had no advanced equipment to probe the properties of matter. Today, we’ll delve into how people thought about chemicals back in Dalton’s day and note his many contributions to science along the way.ĭalton began his career as a humble, self-taught schoolmaster in the Lake District of England in the late 1700s. Hi, and welcome to this video on John Dalton, the scientist who proposed the first cohesive chemical doctrine.

When did dalton form his atomic theory