All matter in the universe, whether it's very big or very small, is composed of atoms. Atoms are the building blocks of matter. All of the different kinds of atoms (also known as elements) are categorized and presented in the periodic table. This table groups the elements and shows their similarities and differences, their symbol, density, atomic weight, and things like that. Atoms are made up of a nucleus (composed of protons and neutrons) and electrons that orbit the nucleus. Protons have a positive charge, neutrons have a neutral charge (no charge) and electrons have a negative charge. For each proton in the nucleus there is an electron that orbits the nucleus. There are as many "plus" charges as there are "minus" charges, so overall the atom is electrically neutral. The atomic number of an atom tells you how many electrons orbit the nucleus (and also how many protons are in the nucleus; remember, there are an equal number of protons and electrons in a neutral atom). Hydrogen has 1 electron, so it's atomic number is 1. Helium has two electrons, so it's atomic number is 2. Lithium has three electrons, so it's atomic number is 3, and so on. For the most part, the first 18 elements in the periodic table represent most of the natural elements found in the universe. The rules that tell us what orbitals the electrons will be found in work very well for these 18 elements. For these elements, there are 2 electrons in the first (lowest) orbital, 8 electrons in the second orbital and 8 in the third.

In a very general sense, you can think of an atom like you would our solar system: the sun (nucleus) is at the center and the planets (electrons) orbit around it. Each electron's orbital level has an energy associated with it. This "model" of the atom was proposed by Danish physicist Niels Bohr about 1913 and is known as the Bohr model.

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Bohr model of the Boron atom

As modern atomic theory was progressing, in 1925 Austrian physicist Wolfgang Pauli came up with the Pauli Exclusion Principle. This states that no two electrons can be in the same quantum state at the same time (they can't have the exact same energy).

Atoms like to have their electron orbitals full of electrons. The elements in the far right column of the periodic table (Group Zero) have full outer electron shells. These are referred to as the "inert" or "noble" gases. They're referred to as inert because they don't usually react (combine) with other elements. Why? Because the outer orbitals are full. Remember, atoms like to have their outer electron orbitals full of electrons. Group Zero is a special class of elements. The group number that an element is located in on the periodic table tells us how many electrons are in the element's outer shell. Group One elements have 1 electron in their outer orbital. Group Two elements have 2 electrons in the outer orbital. And so on. On the right hand side of the table are higher group numbers (meaning more electrons in the outer orbitals). Regardless of which group they're in (with the exception of Group Zero), the outer electron orbitals are not full. Elements with few outer electrons have two options when combining with other elements: they can give up their few outer electrons or they can "steal" enough to finish filling up the orbital. For these elements, it's easier to give up their electrons. For elements needing only a few electrons to finish filling their orbitals, it's easier for them to "steal" a few than it is to give up what they already have.

The outermost electrons that are found in their normal orbitals are said to be in the valence band. However, it's possible for these electrons to receive energy and leave their normal energy band (orbital). If they receive enough energy they can break free from their nucleus and roam around to other atoms. In this condition they are said to be in the conduction band. The difference between the lowest energy level in the conduction band and the highest energy level in the valence band is called the bandgap energy. In some elements the bandgap energy is large. These elements are considered insulators. In other elements the valence and conduction bands are closer together and overlap. These are called conductors. There is also a separate category of elements where the bandgap energies fall in between insulators and conductors: semiconductors.





Now let's look at semiconductors.

Atoms have a nucleus made of protons (positively charged) and neutrons (no charge). Electrons (negatively charged) orbit the nucleus in different energy levels (orbitals). There are equal numbers of protons and electrons in electrically neutral atoms. Outer electrons can receive energy from outside the atom and rise to higher energy levels in the atom. If enough energy is received, the electron can leave the atom. Atoms like to have their outer electron orbitals full of electrons.