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Atoms and Molecules




Bonds and Octet Rules
Chemical Equations
The Mole




Functional Groups
Periodic Table


Practice Quiz I
Practice Quiz II
Practice Quiz III

Introduction to Atomic Stucture

Atoms are the building blocks of all matter in the universe. Atoms are divided into two parts, the nucleus and the electron cloud. These two parts contain sub-atomic particles called electrons, protons, and neutrons.

Electrons orbiting within one shell around a nucleus
This is an imaginary construct being used for illustrative purposes.

The nucleus, in the center of the atom, consists of protons and neutrons. Protons and neutrons are packed so tightly in the nucleus that the average density of a nucleus is roughly 100 trillion (100,000,000,000,000) kilograms per cubic centimeter, which is the same as having the entire mass of the sun squeezed into the city of Chicago.

Surrounding the nucleus is the electron cloud, in which electrons orbit the nucleus at super-high speeds. Scientists know that electrons move around the nucleus in specific orbits, but since electrons move so fast, it is impossible to ever know exactly where an electron is. The space between the electron cloud and the nucleus is huge; if a nucleus were the size of a golf ball, the electron cloud would be located about 1.5 miles away.

Illustration of distance between nucleus and electron cloud

The electron cloud is divided into layers called energy levels, or shells. The maximum number of electrons that a given energy level or shell can contain is given by the formula 2n2, where n defines the energy level under consideration. Thus for the fifth shell, n = 5, the maximum number of electrons that this energy level can contain is 2 x 52, or 50.

Not all atoms have the maximum number of electrons in their valence shells. The diagram below illustrates a magnesium (Mg) atom. Mg contains a total of 12 electrons -- 2 in the first energy level (n = 1), 8 in the second energy level (n = 2), and 2 in the third energy level (n = 3).  Note that the third energy level does not contain the maximum number of electrons.

Electrons in a magnesium atom

This is an imaginary construct being used for illustrative purposes.

When n = 1, the maximum number of electrons is 2 x 12, or 2.
When n = 2, the maximum number of electrons is 2 x 22, or 8.
When n = 3, the maximum number of electrons is 2 x 32, or 18.

There have been several different theories over the centuries to describe the structure of an atom. The Greeks thought atoms were indestructible solid spheres, while scientists in the 1800's had the idea that electrons floated around randomly in a positive gel, resembling the English dessert plum pudding. This was called the plum pudding model.

The New Zealander-English physicist Ernest Rutherford performed an experiment in 1911 that enabled scientists to create the model of the atom that we have today. Rutherford fired a beam of positive alpha particles at a thin sheet of gold foil. He found that most of the alpha particles went straight through the foil, while a few were sharply deflected. Based on these observations, he came to two conclusions: an atom must contain a great deal of empty space, given that most of the alpha particles in the gold-foil experiment passed through the foil undeflected; there also has to be a large concentrated positive charge in the atom capable of deflecting positively charged alpha particles.

Perspective drawing of Rutherford's experiment

Outside Links

Rutherford's experiment (Interactive Java Tutorial)

Visionlearning Atomic Theory