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Electrons and Chemical Bonding

lithium atom, valence shells, lithium fluoride, francium, periodic law

The electrons in the valence, or outermost, shell of atoms determine the chemical behavior of most elements. The atoms of noble gases (helium, neon, argon, krypton, xenon, and radon) have complete, or full, valence shells. The configuration of a complete outer shell is very stable, so the noble gases usually exist as single atoms and rarely react with other elements. Atoms of the other elements attempt to imitate the stable configuration of the noble gases. They do this by donating, accepting, or sharing electrons in chemical reactions with atoms of the same element or atoms of other elements.

When atoms donate, accept, or share electrons with other atoms to complete their valence shells, they form chemical bonds. The resulting substance is called a compound. The type of bond depends on whether the electrons are transferred or shared.

An atom with few electrons in its valence shell will tend to donate these electrons to fill an almost complete shell in another atom. For example, an atom of lithium has two electrons filling its inner shell and a lone electron in an outer shell that could accommodate eight electrons. An atom of fluorine, on the other hand, has seven electrons in the outer shell (as well as two in the inner shell). The lithium atom transfers its outer electron to the fluorine atom. Both atoms now have filled outer shells. Fluorine has ten electrons, with eight electrons completing its outer shell. Lithium no longer has a second shell, but has two electrons completing the first shell. Because the lithium atom lost an electron, it now has a positive charge, while the fluorine atom gains a negative charge. Atoms that have an electrical charge are called ions. These oppositely charged ions attract each other, and an ionic bond forms between them. The compound created by lithium and fluorine is called lithium fluoride.

A covalent bond forms between atoms when the valence electrons of one atom are shared with another atom with no discrete transfer of electrons. For example, two atoms of hydrogen, each with a single electron (and just one shell), can share their electrons. Each hydrogen atomís shell is now complete with two electrons. This covalent bond yields a molecule of hydrogen. In molecules, each valence electron belongs to the molecule, not to the individual atoms.

When metal atoms combine with each other, the outermost electrons lose contact with their parent atoms. The remaining positively charged atomic centers form an ordered structure while the outer electrons move freely around the whole sample. These freely moving electrons, called conduction electrons, can carry heat energy and electric charge easily throughout the metal, making metals good conductors of heat (see Heat Transfer) and electricity.

Elements with atoms that have similar valence shell structures react in the same way to complete their outer shells. This predictable behavior led scientists to form the periodic law, which states that the physical and chemical properties of the elements tend to repeat at certain intervals as the atomic number (and number of electrons in the atom) increases. Elements that behave similarly are grouped in columns in the periodic table. For example, the valence shells of hydrogen and the alkali metals (lithium, sodium, potassium, rubidium, cesium, and francium) found in column 1 (or Ia) of the periodic table all contain a single electron, which makes them all highly reactive.


Wagne, Doris Jeanne, B.S., M.S., Ph.D.

Clinical Assistant Professor of Physics, Rensselaer Polythechnic Institute.

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