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ATOMS THE BUILDING BLOCKS OF MATTER DEMOCRITUS – 400 B.C. thought matter could not be divided indefinitely. First to use the term ATOM (indivisible). ARISTOTLE- 350 B.C -modified an earlier theory that matter was made of four “elements”: earth, fire, water, air. his theory persisted for 2000 years. LAW OF CONSERVATION OF MASS – Mass is neither created nor destroyed during ordinary chemical reactions or physical changes. • This implies that for any chemical process in a closed system, the mass of the reactants must equal the mass of the products. • Beginnings of the theory of conservation of mass were stated by Epicurus(341-270 BC), who wrote "the sum total of things was always such as it is now, and such it will ever remain,". Joseph Proust based on several experiments conducted between 1798 and 1804 suggested the Law of Definite Proportions- a chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or the source of the compound. For example: oxygen makes up 8/9 of the mass of any sample of pure water, while hydrogen makes up the remaining 1/ of the mass. 9 The Law of Multiple Proportions- (John Dalton) if two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers. HUH? When two elements can combine to form more than one compound and the same amount of the first element is used in each, then the ratio of the amounts of the other element will be a whole number. For example, carbon and oxygen combine in carbon dioxide (CO2) and carbon monoxide (CO). A sample of carbon dioxide containing 1 gram of carbon contains 2.66 grams of oxygen; a sample of carbon monoxide containing 1 gram of carbon contains 1.33 grams of oxygen. The ratio of the two weights of oxygen (2.66:1.33) is exactly 2:1 1. 2. 3. 4. 5. 6. John Dalton 1808 All matter is made of tiny particles, called atoms. Atoms are neither subdivided, created nor destroyed. (Dalton based this hypothesis on the law of conservation of mass) Atoms of different elements combine in simple whole number ratios, to form chemical compounds with more than one ratio being possible for a given combination of elements. (Dalton effectively explained the law of definite proportions and law of multiple proportions with this one) Each element is made of a different kind of atom, and the atoms of different elements have different masses. In chemical reactions atoms are combined, separated or rearranged. (Conservation of mass) Atoms of a given element are identical in size, mass and other properties. Atoms of different elements differ in size, mass and other properties. (Oops!) Atoms are divisible Isotopes exist Atomic theory has been modified but these haven’t: • All matter is composed of atoms. • Atoms of any one element differ in mass and properties from atoms of a different element. Read Ch 3-1 pg 69 q 1-3 pg 87 q 2 Chapter 3-2 The first evidence for sub-atomic particles came from experiments with the conduction of electricity through gases in sealed glass tubes at low pressures… The CATHODE RAY TUBE. If an object is placed in the path of the cathode ray then a shadow of the object is cast on the tube wall at the end. • Cathode rays travel in straight lines The cathode ray can push a small paddle wheel up an incline, against the force of gravity. • cathode rays carry energy and do work. The cathode ray is deflected from a straight line path by a magnetic field. • suggests that the two are related in some way. J.J. Thomson succeeds in deflecting the cathode ray with an electrical field. • The cathode rays bend toward the positive pole, confirming that cathode rays are negatively charged. Thompson was able to measure the charge/mass ratio of these electrons and found this ratio to be the same regardless of what gas was in the tube or what metal the electrodes were made from. (It was found that the mass of the electron is extremely small when compared to the charge) His research proved the existence of electrons and earned him the Nobel Prize for physics, 1906. Cathode_Ray_Tube 1909 Oil drop experiment – showed that the mass of an electron is about 1/2000 the mass of a hydrogen atom. Now known to be 1/1837. 9.109 X 10-28g Confirmed that electrons carry a charge. Millikan Oil Drop Nobel Prize in Physics, 1923 Two inferences about atoms could be made based on what was learned about the electron: 1. There must be a positive charge to balance the negative charge of the electron. 2. Atoms must have other particles that account for most of their mass 1911 Ernest Rutherford, Hans Geiger, Ernest Marsden GOLD FOIL EXPERIMENT- Involved the firing of radioactive particles through minutely thin metal foils (notably gold) and detecting them using screens coated with zinc sulfide (a scintillator). Rutherford found that although the vast majority of particles passed straight through the foil approximately 1 in 8000 were deflected, leading him to his theory that most of the atom was made up of 'empty space'. This model suggested that most of the mass of the atom was contained in a small bundle in the center of the atom and that it had a positive charge. He called it the nucleus. When compared with the overall size of the atom the nucleus is very small. Nobel Prize in Chemistry 1908….but not for the discovery of the nucleus. All atomic nuclei are made of protons and neutrons, except hydrogen. Protons- DETERMINE THE IDENTITY OF THE ATOM, positive charge. • Mass= 1.673 X 10-24g. Neutrons- no charge, slightly greater mass than a proton. • Mass = 1.675 X 10-24g. Strong Nuclear Force- a short-range force responsible for binding of protons and neutrons into atomic nuclei. • One of the four fundamental forces of nature: gravitational; electromagnetic; strong; weak forces. Protons and Neutrons are collectively referred to as nucleons. Imagine the nucleus to be the size of a golf ball. On this scale the first electron shell would be about one kilometer (0.62 miles) from the golf ball. The second shell- about four kilometers (2.49 miles) from the golf ball. The third -nine kilometers (5.59 miles). 200 000 000 hydrogen atoms side by side ≈ 1cm The nucleus contains most (99.9%) of the mass of the atom. Imagine a cube that is 1 mm on a side. If filled with nuclear matter, it would have a mass of about 200,000 tonnes. • 1 tonne=2200 lbs some experiments have shown the nuclear force has a "repulsive core," meaning that at very short distances, the force switches from attractive to repulsive, preventing the nucleus from collapsing in on itself. COUNTING ATOMS The number of protons in the nucleus of the atom. Shown above the symbol on the periodic table. Ex: Silicon, Si, Atomic # 14 Find the atomic number of oxygen, O. Silver, Ag. Atoms of the same element that have different masses. These different masses are due to different numbers of what atomic particle? Examples of Isotopes- Protium, deuterium, and tritium are all isotopes of hydrogen. PROTIUM Number Number of of protons neutrons 1 0 Number of electrons 1 DEUTERIUM 1 1 1 TRITIUM 1 2 1 The number of protons and neutrons in the nucleus of an atom. Relative and Average Atomic Masses Because atoms are so small an arbitrary value has been set as the standard. Relative – AMU A unit of mass equal to 1/12 the mass of the most abundant isotope of carbon, carbon- 12, which is assigned a mass of 12 amu. 1.660540 x 10-24 g Pg 85 q 2,3 Pg 87 q 19,20 A mole of paper clips (3 cm long), chained together, would wrap around the equator 450 trillion times. A mole of cantaloupes (6 inches in diameter) has the same volume as Earth (1.1 × 1012 km3). A mole of pennies distributed evenly to everyone in the world (7 billion people) would enable each person to spend 1.6 million dollars per minute for an entire year.