Lecture 1Current LetureAtoms and ElementsIf you cut a piece of graphite- If you cut a piece of graphite from the tip of the pencil into smaller and smaller pieces, you would eventually end up with individual carbon atoms. The word atom comes from the Greek word, atomos, meaning “indivisible”Imaging and moving individual atoms- On March 16, 1981, Gerd Binnig and Heinrich Rohrer worked late into the night in their laboratory. Their work led to the development of scanning tunneling microscopy (STM). STM is a technique that can image and even move, individual atoms and moleculesScanning Tunneling Microscopy- Binnig and Rohrer developed a type of microspore that could “see” atomsImaging and moving individual atoms- In spite of their small size, atoms are the key to connection the macroscopic and microscopic worlds. An atom is the smallest identifiable unit of an element. There are about 91 different naturally occurring elements and over 20 synthetic elements. Early ideas about the building blocks of matter- Leucippus (fifth century B.C.) and his student Democritus (460-370 B.C.) were first to propose that matter was composed of small, indestructible particles. “Nothing exists except atoms and empty space;everything else is opinion” –Democritus. They proposed that many different kinds of atoms existed, each different in shape and size, and that they moved randomly through empty space. - Plato and Aristotle did not embrace the atomic ideas. They held that matter had no smallest parts and that different substances were composed of various proportions of fire, air, earth, and water- Later scientific approach became the established way to learn about the physical world. and English chemist, John Dalton (1766-1844) offered convincing evidence that supported the early atomic ideas ofLeucippus and DemocritusModern atomic theory and the laws that led to it- The theory that all matter is composed of atoms grew out of observations and laws. The three most important laws that led to the development and acceptance of the atomic theory are as follows – the law of conservation of mass, the law of definite proportions, and the law of multiple proportionsThe law of conservation of mass- Antoine Lavoisier formulated the law of conservation of mass, which states that in a chemical reaction, matter is neither created nor destroyed. When a chemical reaction occurs, the total mass of the substances involved does not change. This law is consistent with the idea that matter is composed of small, indestructible particlesThe law of definite proportions- In 1797, a French chemist, Joseph Proust made observations on the composition of compounds. He summarized that all samples of a given compound, regardless of their source or how they were CHEM 130prepared, have the same proportions of their constituent elements. It is sometimes called the law of constant proportion. For example, the decomposition of 18.0 g of water results in 16.0 g of oxygen and 2.0 g of hydrogen, or an oxygen-to-hydrogen mass ratio of 8.0 or 8:1The law of multiple proportions- In 1804, John Dalton published this law which said that when two elements form two different compounds, the masses of element B that combine with 1 g of element A can be expressed as a ration of small whole numbers. An atom of A combines with either one, two, three, or more atoms of B (AB1, AB2, AB3, etc.).- Carbon monoxide and carbon dioxide are two compounds of the same two elements, carbon and oxygen. The mass ration of oxygen to carbon in carbon dioxide is 2.67:1, therefor, 2.67 g of oxygen reacts with 1 g of carbon. In carbon monoxide, however, the mass ration of oxygen to carbon is 1.33:1. The ratio of these two masses itself a small whole number.John Dalton and the atomic theory1. Each element is composed of tiny,indestructible particles called atoms2. All atoms of a given element have the same mass and other properties that distinguish them from the atoms of other elements3. Atoms combine in simple, whole-number ratios to form compounds4. Atoms of one elements cannot change into atoms of another elements. In a chemical reaction, atoms only change the way that they are bound together with other atoms. The discovery of the electron- J.J. Thompson (1856-1940) cathode rays experiments. Thomson constructed a partially evacuated glass tube called a cathode ray tube. He found that a beam of particles, called cathode rays, traveled from the negatively charged electrode (called the cathode) to the positively charged one (called the anode).- Thomson found that the particles that compose the cathode ray have the following properties: they travel in straight lines, they are independent of the compositions of the material from which they originate (the cathode), and they carry a negative electrical charge- J.J. Thomson measured the charge-to-mass rationof the cathode ray particles by deflecting themusing electric and magnetic fields. The value hemeasured was -1.76x10^3 coulombs (C) per gram.- Thomson had discovered the electron, a negativelycharged, low mass particle present within allatoms Millikan’s Oil Drop Experiment: The Charge of the electron- American physicist Robert Millikan (1868-1953),performed his now famous oil drop experiment in which he deduced the charge of a single electron. By measuring the strength of the electric field required to halt the free fall of the drops, and by figuring out the masses of the drops themselves (determined from their radii and density), Millikan calculated the charge of each drop. The measured charge on any drop was always a whole-number multiple of –1.96 × 10–19, the fundamental charge of a single electron. With this number in hand, and knowing Thomson’s mass-to-charge ration for electrons, we can deduce the mass of an electron. The structure of the atom- J.J. Thomson proposed that the negatively charged electronswere small particles held within a positively charged sphere.This model became known as the plum-pudding model. Rutherford’s gold foil experiment- In 1909, Ernest Rutherford (1871–1937), who worked under Thomson andsubscribed to his plum-pudding model, performed an experiment toconfirm Thomson’s model. Rutherford directed the positively charged particles at an ultra-thin sheet of gold foil. The Rutherford experiment gave an unexpected result. A majority of the particles did pass directly through the foil, but some were deflected, and some (approximately 1/20,000) even bounced back.