The discovery of the atom

The atom is sometimes shown as a tiny "solar system," with electrons orbiting a nucleus

Science is detective work. Through ingenious experiments and some creative guessing, science discovers many things that are beyond the reach of our five senses . The atom is a good example. Oxygen and nitrogen atoms in the air we breathe are far too small to be seen, even with optical microscopes. How do we know that they exist? Democritus (460-370 BC) proposed that matter consists of indivisible particles, but this was philosophical speculation without solid experimental evidence. The detectives that first cracked this case were not physicists, such as Newton or Einstein. They were chemists, such as Lavoisier, Proust, and Dalton.

Making and breaking water

Until the 1700's, the periodic table of the elements was simple. Everything was thought to be made of air, earth, fire, and water. These were the "elements," that is, substances that are not divisible into chemically finer ingredients.

Matter was once thought to be composed of four elements: air, earth, fire, and water

By the end of that century, chemists had scales to accurately weigh the substances consumed or produced in chemical reactions. They had sealed containers to catch the gasses or liquids produced in reactions, so that nothing escaped. And they had a determination to rely on quantitative measurements rather than philosophical speculation.

Antoine Lavoisier (1743-1794) showed that water is not an element, but could be broken down into hydrogen and oxygen. Here is a drawing of one of his experiments, and a link to prettier drawings.

Lavoisier passed steam over heated iron and then collected the leftover water and the hydrogen gas produced

Lavoisier passed steam through a tube containing ribbons of heated iron. Some of the steam was consumed in a reaction that turned the iron into a blackish substance. The leftover steam was liquidfied and collected. The reaction also produced hydrogen gas, which was collected and trapped under a bell jar.. By weighing everything before and after the reaction, he found that mass was not being created or destroyed by the reaction. Some water disappeared in the reaction, but that mass reappeared in other forms. The iron became heavier as a result of its "blackening," and hydrogen gas was produced.

In hindsight we can draw a picture showing this reaction. Each water molecule (H2O) that reacts with the surface of the iron (Fe) gets split; its oxygen atom bonds to the iron and its two hydrogen atoms bond with each other to form a hydrogen molecule (H2), which escapes as gas.

An incoming water molecule (H2O, left) is split into an oxygen atom (O, right) that chemically bonds to the iron and a pair of hydrogen atoms that bond to form a molecule of hydrogen gas.

Lavoisier could also collect and mix oxygen and hydrogen gases and ignite them with a spark. Liquid water was produced in the reaction, because the hydrogen and oxygen were reuniting to form water molecules.

Work by Joseph Proust established that a compound such as water has definite proportions of the elements composing it. For example, splitting 100 grams of water into hydrogen and oxygen gasses always gives 11 grams of hydrogen and 89 grams of oxygen.

The first atomic theory

John Dalton (1766-1844) took the next step. He noticed a trend in the data, which is called the law of multiple proportions, best illustrated by an example. Iron oxide is made of iron (Fe) and oxygen (O). But there is more than one type of iron oxide, and some types are richer in oxygen content than others. One type, a black powder, has 28 grams of oxygen for every 100 grams of iron. Another type, a red powder, has 42 grams of oxygen for every 100 grams of iron. The numbers 42 and 28 have a ratio of 3 to 2. Dalton proposed that the black powder has the simplest composition, one oxygen atom for every iron atom, the iron atom being heavier. He explained the 3-to-2 ratio by proposing that the red powder has 3 oxygen atoms for every 2 iron atoms. These two compositions are shown below.

In samples of red and black power with the same amount of iron, Dalton explained the 3-to-2 ratio of the oxygen content using atoms

Dalton didn't know for sure that the black powder has an equal number of iron and oxygen atoms. This was a lucky guess. For water, guessing a one-to-one ratio gave him the wrong answer. Water has twice as many hydrogen atoms as oxygen atoms. Dalton also didn't know that the particles composing hydrogen or oxygen gasses are not individual atoms but molecules, each with two atoms bonded together.

Avogadro's hypothesis

Amedeo Avogadro noticed that to make water from hydrogen and oxygen gasses, the volume of hydrogen gas needed is double the volume of the oxygen gas. He made a lucky guess. He proposed that, for gasses, twice the volume means twice the number of particles, regardless of what kind of particles they are (as long as the temperature and pressure are the same). This leads to correct formula for water: H2O.

Avogadro guessed that the volume of a gas is determined by the number of particles. If hydrogen and oxygen consist of individual atoms, then it would take 1 liter of oxygen and 2 liters of hydrogen gas to make 1 liter of water vapor, since a water molecule is a single particle

But there was still a problem. If the particles of oxygen and hydrogen are individual atoms, as shown above, then 1 liter of hydrogen and 2 liters of oxygen would produce 1 liter of water vapor, because the water molecule is a single particle. In reality, 2 liters of water vapor are produced. Avogadro explained this by proposing that hydrogen and oxygen gasses are made of diatomic molecules. This leads to the correct prediction of relative volumes, as shown below.

Avogadro successfully explained the volumes consumed and produced in making water by assuming that the hydrogen and oxygen are composed of diatomic molecules

Building the Table

The 19th century saw much more chemical experimentation. More and more substances were identified as elements, that is, substances made of a particular type of atom. The absolute masses of the different types of atoms were not yet discovered, but hydrogen was found to be the lightest and the masses of atoms of other elements could be determined relative to hydrogen. When Dmitrii Mendeleev listed the elements in order of increasing atomic mass, he noticed repeating patterns in their chemical properties. For example, the 2nd element (helium), the 10th (neon), and the 18th (argon) are all chemically unreactive gasses. This lead ultimately to the modern periodic table, in which such similar elements are placed in the same column.

Dancing pollen grains

While air and water molecules are too small to be seen under a light microscope, we can see their effect on larger things such as microbes or grains of pollen. In 1827, Robert Brown saw that grains of pollen in water would dance around, and this came to be known as Brownian motion. The motion is caused by bombardment of the grain by water molecules. Einstein explored this is one his four 1905 papers.

The next chapter

At the beginning of the 20th century, the internal makeup of the atom and the physical reasons for its chemical behavior were not yet known. In another post we'll delve into how the physicists carried on the story.