All matter is made up of elements which are fundamental substances which cannot be broken down by chemical means. There are 92 elements that occur naturally. The elements hydrogen, carbon, nitrogen and oxygen are the elements that make up most living organisms. Some other elements found in living organisms are: magnesium, calcium, phosphorus, sodium, potassium.

By the late 1800's many elements had already been discovered. The scientist Dmitri Mendeleev, a Russian chemist, proposed an arrangement of know elements based on their atomic mass. The modern arrangement of the elements is known as the Periodic Table of Elements and is arranged according to the atomic number of elements.

Here is an Interactive Table of Elements where you can learn more about each of the elements.

Every atom would like to have an electron configuration like a noble gases. In noble gases the outer electron shell is complete. This makes the element chemically inert. Helium is an example of a noble (inert) gas. It is not present in organisms because it is not chemically reactive.

All atoms would like to attain electron configurations like noble gases. That is, have completed outer shells. Atoms can form stable electron configurations like noble gases

by:

1. losing electrons
2. sharing electrons
3. gaining electrons.

For a stable configuration each atom must fill its outer energy level. In the case of noble gases that means eight electrons in the last shell (with the exception of He which has two electrons).

Atoms that have 1, 2 or 3 electrons in their outer levels will tend to lose them in interactions with atoms that have 5, 6 or 7 electrons in their outer levels. Atoms that have 5, 6 or 7 electrons in their outer levels will tend to gain electrons from atoms with 1, 2 or 3 electrons in their outer levels. Atoms that have 4 electrons in the outer most energy level will tend neither to totally lose nor totally gain electrons during interactions.

The Periodic Table of Elements will show you the electron configuration for any element you click on.

The atomic orbitals of the hydrogen atom can be visualized as a cloud around the nucleus. The orbital represents a probability of finding the electron at a particular location. Darker regions signify a greater probability. Shown below are the 1s (lowest orbital and the 2s orbital.

1s2s

Atomic orbitals do not always have the shape of a sphere. Higher orbitals have very unusual shapes.

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