2.3-2.4 Modern Atomic Theory and the Laws That Led to it & The Discovery of the Electron

The Law of Conservation of Mass In a chemical reaction, matter is neither created nor destroyed. If 18.6 g K(s) reacted with I2(g) to produce 38.4 g KI(s), what was the mass of the I2(g) in this reaction? Mass of reactants = mass of products 18.6 g K(s) + ? g I2(g) = 38.4 g KI(s) Mass of I2(g) = 38.4 g – 18.6 g = 19.8 g

The Law of Definite Proportions All samples of a given compound, regardless of their source or how they were prepared, have the same proportions of their constituent elements. Mass ratio = mass of element 1 / mass of element 2 Two samples of carbon monoxide are decomposed into their constituent elements. One sample produces 17.2 g of oxygen and 12.9 g carbon, and the other sample produces 10.5 g oxygen and 7.88 g of carbon. Show that these results are consistent with the law of proportions.

Answer to Question on The Law of Definite Proportions The first sample: Mass of Oxygen 17.2 g ---------------------- = ---------- = 1.33 g :1.00 g Mass of Carbon 12.9 g The second sample: Mass of Oxygen 10.5 g ---------------------- = ---------- = 1.33 g :1.0 g Mass of Carbon 7.88 g

The Law of Multiple Proportions When two elements (call them A and B) form two different compounds, the masses of element B that combine with 1 g of element A can be expressed as a ratio of small whole numbers. Mass of element B to 1.00 g element A (larger mass) ---------------------------------------------------------------------- Mass of element B to 1.00 g element A (smaller mass)

Question on Law of Multiple Proportions Water and hydrogen peroxide, are both made up of the elements hydrogen and oxygen. In forming water, 8.0 g of oxygen combines with 1.0 g hydrogen. In hydrogen peroxide, there are 16.0 g of oxygen to 1.0 gram hydrogen. Show that these results are consistent with the law of multiple proportions. Mass of oxygen to 1.0 g hydrogen in H2O2= 16.0 g -------------------------------------------------------- --------- = 2.0 g Mass of oxygen to 1.0 g hydrogen in H2O = 8.0 g

John Dalton and Atomic Theory What was Dalton’s Atomic Theory? 1.) Each element is composed of tiny, indestructible particles called atoms. 2.) All atoms of a given element have the same mass and other properties that distinguish them from the atoms of other elements. 3.) Atoms combine in simple, whole number ratios to form compounds. 4.) Atoms of one element cannot change into atoms of another element. In a chemical reaction, atoms only change the way that they are bound together.

The Discovery of the Electron Who discovered the electron? How did this person discover them? J.J. Thomson discovered the electron using a cathode ray tube and deflected a stream of particles using electric and magnetic fields. Using the CRT, Thomson was also able to determine that an electrons mass-to-charge ratio was -1.76X108 C / g.  This number is not on your formula sheet and does not need to be memorized. However, if given this information, you may need to use it to solve future problems.

Millikans Oil Drop Experiment Robert Millikan, an American physicist, was able to determine the charge of a single electron to be -1.60X10-19 C.  This information is listed on your formula sheet.

Chapter 2 pg. 79-80 #’s 30, 32, 36, 39, 41, 46 Read Sections 2.5-2.7 pgs. 54-65