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Physical Structure
Main Concepts

Protons and Electrons

For every proton in an atomic center, somewhere, in an orbital, there will be an electron. A proton carries a positive charge (+) and an electron carries a negative charge (-), so the atoms of elements are neutral, all the positive charges canceling out all the negative charges.

Atoms differ from one another in the number of protons, neutrons and electrons they contain. However their arrangement always follows the same set of principles.

A Matter of Stability

Hydrogen and helium illustrate the fact that not all atoms have the same properties. Both are gasses, but hydrogen gas is violently explosive, whereas helium gas is completely inert and cannot be made to undergo any kind of chemical reactions under normal circumstances.

Atoms of hydrogen have a single proton in their center and a single electron in the lowest energy level. Helium atoms, on the other hand, have two protons and two electrons in the lowest energy level. The lowest energy level is filled with its maximum number of electrons. This is a very stable arrangement, and helium in consequence is an inert gas with few chemical properties.

Hydrogen only has one electron in its lowest energy level. This is a very unstable arrangement, and hydrogen gas undergoes a variety of reactions so as to reach a stable electron configuration where its energy level is either empty of electrons, or filled with electrons.

Atoms are at their most stable when their outermost energy level is either empty of electrons or filled with electrons.


Sodium atoms have 11 electrons. Two of these are in the lowest energy level, eight are in the second energy level and then one electron is in the third energy level. This is a very unstable arrangement, and the element sodium is a highly reactive, deadly white semi-solid that will burst into flames on exposure to the air or will burn through human flesh on contact. A reactive substance.

Chlorine atoms have 17 electrons. Two in the lowest, eight in the second and 7 in the third energy level. This too is a very unstable arrangement. This element is a gas at room temperature and was used in World War One as a poisonous attack weapon because of its high reactivity with human lungs. These two atoms were made for one another.

Sodium atoms readily give up the single electron in the outermost orbital. This electron is immediately picked up by a chlorine atom and fitted into the last empty space in its outermost orbital. Now both atomic arrangements are much more stable. They both have outermost orbitals which are filled with electrons.

However, there is a price to be paid for this stability. In giving up an electron, the sodium atom has lost a negative electrical charge. It still has all its positively charged protons, so the remaining structure is no longer electrically neutral. It has a net positive charge (+). Similarly, the chorine atom has picked up this extra negative charge and no extra protons, so it is now carrying a net negative charge (-).

These new atomic arrangements are called ions, and the process of electron exchange is called ionization.

Ionic Bonds

Ionic bonds are a type of linkage formed from the attraction between oppositely charged ions. Such bonds are created when the outermost electrons of one atom (such as sodium) are transferred permanently to another atom (such as chlorine). The atom that has lost an electron becomes a positively charged ion (called a cation), while the atom that picks up the extra electron becomes a negatively charged ion (called an anion).

Opposite charges attract one another while similar charges repel one another. So, the ions orient themselves in such a way that every positive ion becomes surrounded by negative ions and every negative ion becomes surrounded by positive ions. The ions so arrange themselves that the positive and negative charges alternate and balance one another out.

Sodium chloride is a solid food additive that is more stable than either of its constituent parts, all because the outermost energy levels of its atoms are filled with electrons.

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© 2003, Professor John Blamire