Lecture 2: Atoms and Molecules

Lecture 2 : Atoms and Molecules An atom is the tiniest speck of an element which possesses the characteristics of that element and is able to engage in a chemical bond. Take the element gold,Picture halving a gold nugget, then cutting one of the halves in two, and continuing this procedure until a piece of gold stayed that was so minuscule that it was not able to be divided (no matter how small your knife may be). This smallest piece of gold is an atom (from the Greek atomos, denoting “indivisible”) . This atom would not still be gold if it were separated anymore. The initial proposition that matter is comprised of atoms is credited to the Greek philosophers Leucippus and Democritus, who formulated their concepts in the 5th century BCE. Yet, it wasn't until the start of the nineteenth century that John Dalton (1766–1844), a British educator with a deep interest in science, supported this theory with quantitative measurements. From then on, repeated experiments have verified numerous components of this theory, and it has evolved into one of the principal theories of chemistry. Other aspects of Dalton’s atomic theory are still applied but with slight alterations (particulars of Dalton’s theory . An atom is so tiny that its scale is hard to envision. One of the smallest things we can observe with our naked eye is a single thread of a spider web: These filaments are approximately 1/10,000 of a centimeter (0.00001 cm) in diameter. Even though the cross-section of one strand is nearly impossible to discern without a microscope, it is immense on an atomic scale. A single carbon atom in the web has a diameter of about 0.000000015 centimeter, and it would require roughly 7000 carbon atoms to traverse the diameter of the strand. To place this in context, if a carbon atom was the size of a dime, the cross-section of one strand would be greater than a football field, which would demand about 150 million carbon atom “dimes” to encompass it. An atom is so light that its mass is also tough to picture. A billion lead atoms (1,000,000,000 atoms) weigh around 3 × 10−13 grams, a mass that is far too light to be weighed on even the world’s most delicate balances. It would demand over 300,000,000,000,000 lead atoms (300 trillion, or 3 × 1014) to be weighed, and they would weigh just 0.0000001 gram. It is uncommon to discover collections of individual atoms. Only a few elements, like the gases helium, neon, and argon, comprise a collection of individual atoms that move about independently of each other. Other elements, such as the gases hydrogen, nitrogen, oxygen, and chlorine, are composed of units that comprise pairs of atoms. One form of the element phosphorus consists of units composed of four phosphorus atoms. The element sulfur exists in diverse forms, one of which consists of units composed of eight sulfur atoms. These units are called molecules. A molecule consists of two or more atoms joined by strong forces called chemical bonds. 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. 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. Each water molecule is a unit that contains two hydrogen atoms and one oxygen atom. Each glucose molecule is a unit that contains 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Like atoms, molecules are incredibly small and light. If an ordinary glass of water were enlarged to the size of the earth, the water molecules inside it would be about the size of golf balls