Atoms

by Gen:

It has been said that during the 20th century, man harnessed the power of the atom. We made atomic bombs and generated electricity by nuclear power. We even split the atom into smaller pieces called subatomic particles.

But what exactly is an atom? What is it made of? What does it look like? From the ancient Greeks to today... we have wondered what ordinary matter is made of.

The idea of the atom was first devised by Democritus in 530 BC. In 1808, an English school teacher and scientist named John Dalton proposed the modern atomic theory. Modern atomic theory states the following.

1. Every element is made of atoms
2. All atoms of any element are the same
3. Atoms of different elements are different
4. Atoms of different elements can combine to form compounds
5. In chemical reactions, atoms are not made, destroyed, or changed
6. In any compound, the numbers and kinds of atoms remain the same

The ability to weight atoms came from an observation by an Italian chemist named Amadeo Avogadro. Avogadro was working with gases (nitrogen, hydrogen, oxygen, chlorine) and noticed that when temperature and pressure was the same, these gases combined in definite volume ratios.

Avogadro said that at the same temperature and pressure, equal volumes of the gases had the same number of molecules. So, by weighing the volumes of the gases, he could determine the ratios of atomic masses. For example, a liter of oxygen weighed 16 times more than a liter of hydrogen.

To know the structure of an atom, we must know the following:

What are the parts of an atom?
How are these parts arranged?

Near the end of the 18th century, the atom was thought as nothing more than a indivisable sphere. However, a series of discoveries in the fields of chemistry, electricity, magnetism, radioactivity, and quantum mechanics in the late 19th and early 20th centuries changed all that.

In the late 19th century, chemists and physicists were studying the relationship between electricity and matter. They were placing high voltage electric currents through glass tubes filled with low-pressure gas (mercury, neon, xenon) much like neon lights. Electric current was carried from one electrode (cathode) through the gas to the other electrode (anode) by a beam called cathode rays. In 1897, a British physicist, J. J. Thomson did a series of experiments with the following results:

He found that if the tube was placed within an electric or magnetic field, then the cathode rays could be deflected or moved (this is how the the cathode ray tube (CRT) on your television works.

By applying an electric field alone, a magnetic field alone, or both in combination, Thomson could measure the ratio of the electric charge to the mass of the cathode rays.

He found the same charge to mass ratio of cathode rays was seen regardless of what material was inside the tube or what the cathode was made of.

Thomson concluded the following:

Cathode rays were made of tiny, negatively charged particles, which he called electrons.

The electrons had to come from inside the atoms of the gas or metal electrode.
Because the charge to mass ratio was the same for any substance, the electrons were a basic part of all atoms.

Because the charge to mass ratio of the electron was very high, the electron must be very small.

About the same time as Thomson's experiments with cathode rays, physicists such as by Henri Becquerel, Marie Curie, Pierre Curie, and Ernest Rutherford were studying radioactivity. Radioactivity was characterized by three types of emitted rays:

Alpha particles - positively charged and massive. Ernest Rutherford showed that these particles were the nucleus of a helium atom.
Beta particles - negatively charged and light (later shown to be electrons).
Gamma rays - neutrally charged and no mass (energy).

The experiment from radioactivity that contributed most to our knowledge of the structure of the atom was done by Rutherford and his colleagues. Rutherford bombarded a thin foil of gold with a beam of alpha particles and looked at the beams on a fluorescent screen, he noticed the following:

Most of the particles went straight through the foil and struck the screen.
Some (0.1 percent) were deflected or scattered in front (at various angles) of the foil, while others were scattered behind the foil.
Rutherford concluded that the gold atoms were mostly empty space, which allowed most of the alpha particles through. However, some small region of the atom must have been dense enough to deflect or scatter the alpha particle. He called this dense region the nucleus; the nucleus comprised most of the mass of the atom. Later, when Rutherford bombarded nitrogen with alpha particles, a positively charged particle that was lighter than the alpha particle was emitted. He called these particles protons and realized that they were a fundamental particle in the nucleus.

However, protons could not be the only particle in the nucleus because the number of protons in any given element (determined by the electrical charge) was less than the weight of the nucleus. Therefore, a third, neutrally charged particle must exist! It was James Chadwick, a British physicist and co-worker of Rutherford, who discovered the third subatomic particle, the neutron. Chadwick bombarded beryllium foil with alpha particles and noticed a neutral radiation coming out. This neutral radiation could in turn knock protons out of the nuclei of other substances.

In summary, science in the 20th century has revealed the structure of the atom. Scientists are now conducting experiments to reveal details of the structure of the nucleus and the forces that hold it together.