March 2005

Each green ball represents one
chlorine atom and the bar
between them represents the
bond between the atoms.
There are some chemicals that have so many roles in manufacturing essential products that, if they were to disappear tomorrow, life as we know it would change. Chlorine is one of those chemicals.

Chlorine and chlorinated disinfectants provide safe drinking water to billions of people. More than ninety-three percent of life-saving medicines are manufactured using chlorine. Durable chlorinated materials, such as vinyl ( PVC) and neoprene, provide families with housing materials, computer and auto parts, and sports and safety gear. Modern rescue and fire-fighting equipment used by first responders are made using chlorine chemistry. Police officers and soldiers rely on chlorine-based bullet-resistant gear. And patients requiring blood transfusions or other tube-delivered treatments benefit from flexible PVC medical equipment. With so many diverse roles in producing goods and services, chlorine truly is a molecular multi-tasker.

An atom can be thought of as structured somewhat like a solar system with the nucleus as a sun and the electrons surrounding the nucleus similar to the revolving planets.

Taking Its Place on The Periodic Table

As atomic number 17 on the Periodic Table of the Elements, chlorine takes its place between sulfur, atomic number 16, and argon, atomic number 18. Each chlorine atom consists of 17 protons (positively charged sub-atomic particles) in the atom's nucleus (central region), balanced by seventeen electrons (negatively charged sub-atomic particles) distributed around the nucleus.

The arrangement of an atom's electrons in "orbits" around its nucleus determines how reactive the element is because it is the electrons of atoms that interact, shuffling about, when chemical reactions occur. Electrons naturally arrange themselves around the nucleus of an atom in a particular sequence, starting from the orbit closest to the nucleus and advancing to the next one only when the previous orbit is filled to capacity.

As the illustration of the chlorine atom above demonstrates, the orbit closest to the chlorine nucleus holds no more than two electrons, and the next two orbits can hold a total of eight electrons each. Importantly, the most stable electron arrangement of any orbit is a full orbit. With seven electrons in its outer orbit, chlorine can be thought of as desperately seeking one more electron to achieve stability. As the drawing shows, the eighth space is vacant and available for immediate occupancy. Because it is so "anxious" to pull another electron into its orbit, chlorine is an extremely reactive element. In contrast, argon, sitting next to chlorine on the Periodic Table of the Elements, is very stable as a single atom because it has a full outer orbit of eight electrons.

Cl2-A Diatomic Molecule

A molecule is a chemically bonded unit of atoms with distinct chemical properties. Most molecules are combinations of two or more atoms of different elements (for example, carbon monoxide, CO, is made of two separate elements, while sodium hydroxide, NaOH contains three), but there are a few compounds that occur in nature as "twin molecules," or "diatomic" molecules--in which two atoms of the same element combine. Chlorine is counted among this group along with hydrogen (H2), iodine (I2), bromine (Br2), oxygen (O2), nitrogen (N2) and fluorine (Fl2).

With seven electrons in its outermost orbit--one electron short of a "stable eight"-- the element chlorine exists in nature as a diatomic molecule. This arrangement allows two chlorine atoms to share their outermost orbit electrons, achieving stability, compared to the single atom.

Released From the Salt of the Earth…

Chlorine, used to make so many important products, is obtained from the compound sodium chloride, NaCl, one of the many salts found in geologic deposits formed from the evaporation of ancient seas. Chlorine found in these salts originated in the minerals that make up the rocks of the continents. Over millions of years, the forces of weathering and erosion have broken down these minerals both chemically into simpler forms and physically into smaller particles, transporting them to the oceans.

A shock of electricity delivered to a brine made of water and sodium chloride generates chlorine gas (Cl2), sodium hydroxide (NaOH) and hydrogen gas (H2) according to the reaction:

Chemical Reactions and Conservation of Mass

All chemical reactions consist of reactants, to the left side of the arrow, and products, to the right side. The Law of Conservation of Mass states that matter can neither be created nor destroyed. Let's see if mass, in the form of numbers of atoms, is conserved in the equation that generates chlorine. Use the table below to tally and compare the number of atoms on each side of the chemical reaction. Keep in mind that the "2" in front of NaCl in the reactants above indicates two atoms of Na and two atoms of Cl.

If the numbers of atoms of each element are the same on the product side of the equation as on the reactant side of the equation, the chemical reaction is said to be balanced. Here's your tally sheet:

Follow-up Questions:

  1. List five problems that would arise if chlorine products were unavailable.

  2. Using the chlorine atom's electron orbits as a model, sketch atoms of sulfur and argon. Predict the reactivity of argon.

  3. Carl Wilhelm Scheele discovered chlorine in 1774 when he placed a few drops of hydrochloric acid (HCl) on manganese dioxide (MnO2):

MnO2 + XHCl -> MnCl2 + Cl2 + 2H20.

Balance Scheele's reaction equation by replacing the "X" in front of HCl with the appropriate number.

For a list of previous "Chlorine Compound of the Month" features, click here.


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