Chlorine is one of the more noxious elements. A greenish gas at room temperature, elemental chlorine is diatomic - which is to say two chlorine atoms are bound together by a single bond. Like all halogens (elements from Group 7 on the periodic table), chlorine is an oxidizer. It is toxic to humans, causing damage first to any moist surfaces it contacts, such as the lungs, eyes, etc. and burning the skin with a slightly longer exposure. Like any good oxidizer, it also reacts with metals, sometimes violently. (Sodium and chlorine, when mixed, react explosively to produce our innocuous table salt. Less drastic, but still potent, it can eat through aluminum foil at a visible rate.) Most people are familiar with chlorine from swimming pools, where the chlorinated compounds added to the water to kill bacteria and algae release chlorine gas in low quantities. When there is much chlorine in the water, it causes irritation, burning lungs, and red eyes.
Physical Data - Elemental Chlorine:
The diatomic element chlorine has an average molecular mass of 70.905 amu.*
Chlorine will turn to a liquid (boiling point) at -34.04 degrees Celsius, and will become a solid (melting point) at -101.5 degrees Celsius.*
Physical Data - Atomic Chlorine:
Atomic chlorine (atomic number 17) has 17 protons, 17 electrons, and either 18 (75.78% of the time)* or 20 (24.22% of the time)* neutrons.
The average atomic mass of chlorine is 35.454 amu.
Its electron configuration is 1s2, 2s2, 2p6, 3s2, 3p5 - or in shorthand: [Ne] 3s2, 3p5
It has a high electronegativity (3.0 on a scale of 4)
As mentioned earlier, chlorine is a strong oxidizer. It reacts readily with most other elements and compounds to gain an electron. It will form ionic compounds with metals. The chloride ion carries a charge of minus one, and pairs with the metal ions to balance charges accordingly. Chlorine and hydrogen behave similarly, forming HCl - hydrogen chloride. When dissolved in water, it splits into the ions again, and is known as hydrochloric acid (or muriatic acid to alchemists and pool boys). Hydrochloric acid is also the acid your stomach uses to aid in digestion. Many drug compounds are mixed with hydrochloric acid to create a chloride salt of the drug that will dissolve in the body when the drug compound itself will not.
Aside from salts and stomach acid, chlorine does not play a significant role in the chemistry of the human body. It does, however, show up a good deal in organic chemistry. Chlorine is easy to add wherever double bonds are found. It can also replace the -OH in carboxylic acids, forming acid chlorides. Probably the most common occurrence of chlorinated carbon compounds in daily life comes in the form of PVC - polyvinylchloride. Chlorine tends to make polymers harder or more brittle. This is put to good use in PVC pipes, which are strong and reliable materials. The negative side is that it virtually never biodegrades, and burning it produces toxic compounds.
A recent debate arose concerning a chlorinated compound - sucralose, also known by the trade name "Splenda". Sucralose is a compound derived from sugar by replacing hydroxyl groups with chlorine atoms at three places. The result is that the human body can no longer metabolize the sweetener, making it effectively a zero-calorie sweetener. The debate centers on whether the compound is safe or toxic. Because many chlorine compounds (such as the infamous DDT) are known to be toxic, there is understandable concern by some groups. The toxicity studies presented by the manufacturers to the FDA were sufficient for them to allow the product to market. Long term use by humans will ultimately resolve the debate, but the relevant fact here is that the debate is driven, more than anything, by chlorine.
* indicates numerical data from the 82nd CRC Handbook of Chemistry and Physics
* other data, sadly, is simply burnt into my brain from teaching it