Chapter 3 Chemistry

Chapter 3 Chemistry Atomic Theory The law of conservation of mass states that mass is neither created nor destroyed during ordinary chemical or physical reactions. The law of definite proportions holds that a chemical compound contains the same elements in exactly the same proportions by mass, regardless of the size of the sample or source of the compound. The law of multiple proportions states that if two or more different compounds are composed of the same two elements, 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. Atomic Structure An atom is the smallest unit of an element that maintains its properties. The nucleus is held together by nuclear forces, which are short-range forces that act between protons, neutrons, and protons. The atomic mass unit (amu) is exactly 1/12 the mass of a carbon-12 atom. The atomic number is the number of protons in the nucleus, and the average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element. Isotopes and Nuclides Isotopes are atoms of the same element that have different masses. The mass number is the total number of protons and neutrons in the nucleus. A nuclide is a general term for any isotope of any element. Mole and Molar Mass The mole is the SI unit for a substance, and it is defined as the amount of a substance that contains as many particles as there are atoms in exactly 12g of carbon-12. The molar mass is the mass of one mole of a substance, written in g/mol. Dalton's Atomic Theory Dalton's atomic theory states that all matter is composed of extremely small particles called atoms, atoms of a given element are identical in size, mass, and other properties, atoms cannot be subdivided, created, or destroyed, and atoms of different elements combine in simple whole-number ratios to form chemical compounds. Modern Atomic Theory Modern atomic theory has modified Dalton's theory to include that atoms are divisible into smaller particles and that a given element can have atoms with different masses. Scientists and Experiments Thomson discovered electrons and found that they have a very large charge for their tiny mass. Millikan found the exact mass of the electron and confirmed its negative charge. Rutherford discovered that the nucleus is a very small positively charged region inside an atom through his gold foil experiment. The gold foil experiment involved bombarding a thin piece of gold foil with alpha particles and observing how they were deflected. Rutherford found that about 1 in 8000 alpha particles were redirected back towards their source, leading him to conclude that it was due to the presence of a nucleus in the atom. anode ️the metal disk connected to the positive terminal of the voltage source cathode ray experiment ️investigators noticed that when a current was passed through a cathode-ray tube the surface of the tube directly opposite the cathode glowed and figured out that the particles composing the cathode rays were negatively charges Democritus ️Thought the atom was the basic particle and indivisible Aristotle ️Did not believe in atoms thought all matter was continuous chemical reaction ️the transformation of a substance of substances into one or more new substances Dalton ️turned Democritus's idea into a theory. He made his theory with explains the three laws covered in this chapter nucleus ️a very small region located near the center of the atom that contains at least on