The Greek word atomos means “uncuttable,” or “indivisible.”
As early as the fifth century BC, Greek philosophers Leucippus and Democritus argued that matter is comprised of unchanging, indivisible particles, which they called “atoms.” Their arguments were purely philosophical, not based on observations or experiments, but their atomic hypothesis was remarkably similar to modern atomic theory. They envisioned atoms as tiny particles constantly in motion, bouncing around with nothing but empty space between them. They also hypothesized that there are different types of atoms, and that atoms can bond with each other to form larger objects.See here for more details. However, the influential philosopher Aristotle rejected these ideas, and the atomic hypothesis fell out of favor until new evidence was discovered many centuries later, during the scientific revolution.
Some of the earliest experimental evidence for the existence of atoms involved the relation between pressure and volume of a gas. If you compress a gas into half of its initial volume, its pressure will be doubled. Conversely, if you let a gas expand to fill a volume twice as large, its pressure will be half of what it was initially. The pressure and volume of a gas are inversely proportional. This relation between volume and pressure is known as Boyle’s law, after philosopher and scientist Robert Boyle (1627 – 1691). The relation only holds so long as the temperature of the gas is unchanged, since temperature also affects pressure. Boyle’s law does not apply to all gases, but it is at least approximately true for many gases, especially in conditions of high temperature and low pressure.
Though Boyle’s law was discovered in the 17th century, no one understood the reason for this relation between pressure and volume until the 18th century, when Daniel Bernoulli (1700-1782) proposed an explanation that supported the existence of atoms. Bernoulli suggested that Boyle’s law could be explained by assuming that gases are comprised of microscopic particles that obey Newton’s laws of motion. Recall that pressure is the amount of force exerted against a surface, divided by the surface area. Bernoulli suggested that the force exerted by a compressed gas is due to its particles colliding with the surfaces that contain it. Doubling the amount of gas in a container will double the number of collisions, thus doubling the pressure. Compressing a gas to half its original volume will have the same effect, for the same reason: twice as many collisions will occur within any given area of the surrounding surface. Thus, the atomic hypothesis provided a simple explanation for Boyle’s law.
Further evidence for the existence of atoms was discovered by John Dalton, whose experiments in chemistry revealed a surprising fact about chemical combinations. Dalton’s discovery involved chemical elements that can be combined in different ways to form different chemical compounds. For example, carbon can be combined with oxygen to form either a poisonous gas that is lighter than air, or a harmless gas that is denser than air. (Those two gases are known today as carbon monoxide and carbon dioxide, with chemical formulas CO and CO2, respectively.) Similarly, nitrogen can be combined with oxygen to form either a colorless gas with a sweet smell, or a reddish-brown gas with a sharp odor. (These are known today as nitrous oxide and nitrogen dioxide, with formulas N2O and NO2, respectively.)
When a given amount of one element is chemically combined with a second element in different ways, differing amounts of the second element are needed. Dalton found that these amounts always differ (by mass) in ratios of small whole numbers—e.g. 1/2, 1/3, 2/3, 1/4, 3/4, etc. For example, exactly half as much oxygen is needed to make the lighter gas (carbon monoxide) from a given amount of carbon, compared to the amount of oxygen needed to produce the heavier gas (carbon dioxide) from that same amount of carbon. And exactly one fourth as much nitrogen is needed to make the reddish-brown gas (nitrogen dioxide) from a given amount of oxygen, compared to the amount of nitrogen needed to make the colorless gas (nitrous oxide) from that same amount of oxygen. Dalton discovered that this pattern of whole-number ratios holds for any two elements that can form more than one chemical compound. This fact has been called the law of multiple proportions.
To explain these observations, Dalton suggested that each chemical element consists of a different type of atom, and that chemical compounds consist of atoms linked together to form larger particles (now known as molecules). For instance, carbon monoxide (CO) is formed by linking one oxygen atom with each carbon atom; carbon dioxide (CO2) is formed by linking two oxygen atoms with each carbon atom. Nitrous oxide (N2O) is formed by linking two nitrogen atoms with each oxygen atom; nitrogen dioxide (NO2) is formed by linking two oxygen atoms with each nitrogen atom. Thus, the atomic hypothesis could explain the law of multiple proportions. For proposing this insightful explanation, Dalton is widely regarded as the “father of modern atomic theory.”
Like the ancient Greeks, Dalton thought that atoms are indivisible. Later discoveries revealed that atoms in fact can be divided, as we’ll see on the next page. Yet Dalton’s atomic theory wasn’t entirely off the mark. In his time, chemical elements were understood simply as substances that cannot be produced by combining other substances. And that is true of atoms too, as far as chemical combinations are concerned. Atoms can be chemically combined, but individual atoms cannot be chemically divided. (Splitting an atom is a nuclear division, not a chemical one.) Moreover, when an atom is divided into smaller pieces, it is no longer the same chemical element. In other words, the pieces of a broken atom do not have the same chemical properties as the original atom. So Dalton was correct that atoms are the smallest units of matter that comprise chemical elements, and that each chemical element corresponds to a different type of atom.