![]() Like atoms, molecules are incredibly small and light. Each glucose molecule is a unit that contains 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Each water molecule is a unit that contains two hydrogen atoms and one oxygen atom. ![]() A molecule may consist of two or more identical atoms, as in the molecules found in the elements hydrogen, oxygen, and sulfur, or it may consist of two or more different atoms, as in the molecules found in water. The atoms in a molecule move around as a unit, much like the cans of soda in a six-pack or a bunch of keys joined together on a single key ring. MoleculesĪ molecule consists of two or more atoms joined by strong forces called chemical bonds. It would require over 300,000,000,000,000 lead atoms (300 trillion ) to be weighed, and they would weigh only 0.0000001 gram (one ten millionth, or 1/10,000,000 of a gram). A billion lead atoms (1,000,000,000 atoms) weigh about 0.0000000000003 grams, a mass that is far too light to be weighed on even the world’s most sensitive balances. (credit c: modification of work by “Featheredtar”/Wikimedia Commons)Īn atom is so light that its mass is also difficult to imagine. These images provide an increasingly closer view: (a) a cotton boll, (b) a single cotton fiber viewed under an optical microscope (magnified 40 times), (c) an image of a cotton fiber obtained with an electron microscope (much higher magnification than with the optical microscope) and (d and e) atomic-level models of the fiber (spheres of different colors represent atoms of different elements). To put this in perspective, if a carbon atom were the size of a dime, the diameter of one strand would be larger than a football field. A single carbon atom in the web has a diameter of about 0.000000015 centimeter, and it would take about 7000 carbon atoms to span the diameter of the strand. Although the cross-section of one strand is almost impossible to see without a microscope, it is huge on an atomic scale. ![]() One of the smallest things we can see with our unaided eye is a single thread of a spider web: These strands are about 1/10,000 of a centimeter (one ten thousandth or 0.0001 cm) in diameter. Many aspects of Dalton’s atomic theory are still used today with only minor revisions.Īn atom is so small that its size is difficult to imagine. Since that time, repeated experiments and advanced imaging have confirmed many aspects of Dalton’s atomic theory, and it has become one of the central theories of chemistry. So when Dalton hypothesized that matter is composed of atoms, he was not just participating in philosophical discussion he was using the scientific method to test the hypothesis and develop a fundamental theory to describe many previous observations of the natural world. In the 150 years or so before Dalton, natural philosophy had been maturing into modern science, and the scientific method was being used to study nature. Dalton had something that the ancient Greek philosophers didn’t have he had experimental evidence. However, this hypothesis was not actually tested until the early nineteenth century by John Dalton (1766–1844), a British schoolteacher with a keen interest in science. Greek philosophers, Leucippus and Democritus, first hypothesized that matter is composed of atoms in the 5th century BCE. (credit a: modification of work by United States Geological Survey credit b: modification of work by “Erwinrossen”/Wikimedia Commons) (b) A scanning-tunneling microscope (STM) can generate views of the surfaces of solids, such as this image of a gold crystal. ![]() A special lab instrument called a scanning-tunneling microscope can take “pictures” of extremely small particles, like atoms. We even have visual evidence of atoms, as depicted in the figure on the right below. This atom would no longer be gold if it were divided any further. ![]() We now know that there is a limit, and the absolute smallest piece of gold is an atom (from the Greek atomos, meaning “indivisible” or “uncuttable”). But how far can you take this exercise, at least in theory? Can you continue cutting the gold into halves forever, making smaller and smaller pieces? Or is there some limit, some absolute smallest piece of gold? As the pieces become smaller and smaller, they are still gold. Continue cutting, making smaller and smaller pieces of gold. Cut one of those smaller pieces in half again. To demonstrate this, consider this thought experiment: Take some gold and cut it in half. The smallest particle of an element that maintains the identity and properties of that element is called an atom. ![]()
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