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There are several concepts that go into our modern view of the atom. I have listed five and they are all "modern" in the sense that they have been developed within the past century.

[Concept 1] - Subatomic particles

Proton: positive charge, mass is about 2000 times that of the electron

Neutron: neutral or zero charge, mass is nearly the same as the proton

Electron: negative charge

Neutrino: neutral, massless (?)

The neutrino is a relatively unknown particle, but plays an important role in the reactions in the sun.

[Concept 2] - Forces that keep atoms together

Strong force: short range, attracts protons to neutrons, and attracts protons to other protons

Electromagnetic force: long range, attracts electrons to protons, repels electrons from electrons, and repels protons from protons.

[Concept 3] - Neutrons decay

n → p + e + neutrino

Or, conversely, a proton and an electron can fuse to create a neutron:

p + e → n + neutrino

[Concept 4] - Discrete Electron Orbits

Atomic energies are quantized.

[Concept 5] - Pauli's Exclusion Principle

No two electrons can occupy the same space.

Let's begin by thinking about how atoms actually formed in the early universe. Suppose in the dawn of time there was a mixture - a soup or a blob - of protons and electrons floating in space [Concept 1]. Most of space is empty, because these particles are very small. The only force felt by the particles is the electromagnetic force, because it is long-range. The strong force is like glue on the surface of the protons [Concept 2]. If a proton hits another proton, it will immediately stick to it. But protons repel each other due to their like charges, so they never do. Instead, protons attract electrons, which quickly begin orbiting around them like planets, forming hydrogen, an atom with one proton and one electron. This is the basic material of the universe, and the primary element in stars.

Once protons and electrons are paired, their overall charge is zero, so they have minimal interactions with neighboring particles. In stars, the temperature is so high (15 million degrees or more) that protons are moving fast enough to overcome the repulsive electromagnetic force and occasionally bump into other protons and stick to them. When this happens, one of the protons turns into a neutron [Concept 3] and forms deuterium (one electron orbiting a proton-neutron pair). Two deuterium atoms can also bump into each other and produce a helium atom with two protons and two neutrons stuck together via the glue of the strong force. This foursome is the nucleus of the helium atom with +2 units of positive charge. Two electrons orbit this nucleus to form a neutral atom.

The proton, the electron, and the neutron are the building blocks of all atoms. Light elements are formed in the center of stars as described above. Heavy elements are produced during supernova explosions when nuclei of light elements are brought into close proximity and stick together by the strong interaction glue. Atoms quickly arrange their components so they become neutral. Therefore, all atoms must have the same number of protons and electrons. Neutrons help keep the protons in the nucleus together, but as the number of positive protons increases, the repulsive electromagnetic charge eventually overcomes the strong force. When the number of protons in the nucleus reaches about a hundred, the nuclei fall apart on their own no matter how many neutrons are added to the mix. The atomic orbits are similar to our solar system, with the nucleus playing the role of the sun, and the electrons filling the role of planets. The sun is much heavier than the planets, just as the protons and neutrons are much heavier than the electrons. But there are a few important differences between the solar system and atoms. Planets can be at any distance from the sun, but electrons can only fall into special orbits [Concept 4] that have prescribed distances to the nucleus. The radii of these orbits are in the ratio of 1:4:9:16:25. In addition, in the solar system each orbit can hold any number of planets (or moons). For example the asteroid belt has thousands of planets (or "asteroids") rotating about the sun in the essentially same orbit. But atomic orbits are like rooms with only a few beds, and once they are filled, the door is closed and other electrons must find space in other (unfilled) orbits [Concept 5].

The way an atom behaves depends mainly on the number of electrons in the last orbit (the one farthest from the nucleus). In other words, complete or "filled" orbits do not participate in chemical reactions. For example, atoms with a single electron in their last orbit all have the same basic properties (e.g. Lithium, Sodium, Potassium, an so on). This leads to the regularities in the Periodic Table.


Elton Smith, Staff Scientist

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