Evidence of Atoms & Elements

Instructor: Justin Wiens

Justin teaches college chemistry and has Bachelor and Doctorate degrees in chemistry.

In this lesson, we discuss evidence for the existence of atoms, the elements, and subatomic particles here on earth. We will also talk about how we know about the elements' broader existence in the cosmos.

Introduction To Atoms and Elements

Whether you realize it or not, atoms and molecules are all around you constantly, 24 hours a day, 7 days a week. All we really see are substances like water, dirt, or peanut butter, but what lies underneath had been the subject of intense scientific debate for centuries.

In this lesson, we discuss atomic theory and the elements, how we know what we know about atoms and molecules, and their existence both on the earth and in the cosmos.

Chemistry: It's Ancient History

Long before we had telescopes to see the planets and microscopes to see cells and atoms, the ancient Greeks had proposed the existence of the elements--sort of. They assumed that all substances were composed of either earth, water, wind, or fire in some proportions. A few Greek philosophers took this idea a step further and proposed that the elements were composed of indivisible particles that were too small to see.

For centuries, our knowledge of chemistry and physics continued to grow, but a fundamental understanding of the particles underlying all matter--atoms--was missing until just a few hundred years ago.

Atoms

It wasn't until John Dalton came along in the 17th century that the idea of atoms was explored in depth. John Dalton discovered that certain substances, when combined, did so in certain mass proportions or ratios. For example, when hydrogen gas and oxygen gas are combined to form water, they always do so in a mass ratio of 8 parts oxygen per 1 part hydrogen.

The only way to explain this is the idea that all samples of hydrogen are composed of the same ''stuff'', and similarly for all samples of oxygen. John Dalton extended this idea and said that the ''stuff'' is really the indivisible, microscopic particles called atoms.

The Electron

After a couple more centuries, additional experiments led scientists to believe that even atoms are composed of still smaller particles. Scientists had discovered that when a voltage is placed across a filament or plate in a vacuum where almost all gas has been removed, negatively charged rays are emitted. These rays were called as such because they exhibited some of the same properties as light.

However, the scientist J. J. Thomson discovered that they could be deflected with an electric or magnetic field, much like you could deflect a thrown paperclip's trajectory by placing a magnet near its path. This is not possible for light rays. Thomson concluded that, because particles can be deflected by a magnet, the cathode rays were really composed of small, negatively charged particles called electrons. Not only that, but these electrons came from either the cathode material or from residual gases in the vacuum chamber! It only makes sense, then, that atoms themselves must contain electrons.

The Proton

It's a rule in our universe that charged things tend not to stay that way: your batteries drain out, and the static shock you get from rubbing a balloon on your head is sure to be lost on the first unsuspecting passerby. Ouch!

So it serves to reason that atoms in nature try to be neutral if at all possible. But how can this be, if atoms only contain negatively charged electrons? The answer is that they must have a positive charge to neutralize the negative electrons. The scientist Ernest Rutherford gets credit for discovering that the center of the atom, called the nucleus, houses this positive charge in the form of particles called protons.

The Neutron

The explanation for atoms being neutral was satisfactory. However, scientists had discovered that some samples of elements had different masses than others that came from a different source, even though their chemical reactivity was identical. To be consistent with the atomic theory, the only explanation is that elements also have neutral particles called neutrons, which are also found in the atom's nucleus. Atoms of a given element can different numbers of neutrons. It took a few more decades for experiments to prove the existence of neutrons.

Do the particles get any smaller? Can we see them?

Protons and neutrons are composed of even smaller particles called quarks, which we know thanks to modern high-energy physics experiments. Electrons are called fundamental particles because they are not made of smaller particles.

Some modern microscopes can actually image atoms within a molecule, proving their existence, a few thousand years after they were proposed by the Greeks! We can't image protons, neutrons, and smaller particles, however, largely due to the quantum properties of these very small particles.

A network of carbon atoms is visible in this image of a carbon nanotube

Are There More Elements?

There are 118 elements currently on the Periodic Table, each with a certain number of protons and the same number of electrons.

Periodic Table of the Elements

The very heavy elements with atomic numbers near or above ~100 tend not to be stable for longer periods of time. Many of them can only be synthesized in the laboratory and have not been found in nature. Additional elements that we discover are likely to have only a fleeting existence before they decompose into smaller elements.

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