Learning Chemistry is like climbing a ladder because it is necessary to learn certain things before they can be applied to learn other things. Another example of this might be that an architect can not build a building without the foundation, and this is exact what this unit is for. This unit is a foundation. In this unit, the objective is to understand how and why certain principles came into existence. There are five important theories to be discussed.
The first theory of importance to Chemistry is the Law of Conservation of Mass. This theory was created by Antoine Lavoisier and its definition is that mass can not be created nor destroyed. For instance, if 25 grams react in a chemical reaction, there should be 25 grams of elements left after the reaction has taken place. For example if 25 grams of water are boiled in a beaker and all of it evaporates, nothing is left in the beaker, so where did all the water go? The answer to this is that water evaporated into steam and floats up into the air, even though there is nothing left in the beaker, in reality, 25 grams were produced.
The next law of importance was created by Joseph Proust and it is called Law of Definite Proportions. The Law of Definite Proportions states that a given compound will always contain exactly the same proportion of elements by weight. For example, water weighs 18 grams and 2 of the grams are due to the weight of hydrogen and the other 16 grams are due to the weight of oxygen. So every time there is a water molecule, there will be 16 grams of oxygen and 2 grams of hydrogen.
The third theory was created by John Dalton. Dalton postulated (stated) that when two elements form a series of compounds, the ratios of the masses of the first element can always be reduced to small whole numbers. In common terms, in compounds there can not be half of an atom. This theory is called the Law of Multiple Proportions. Here is an example that will hopefully clarify things.
Dalton wasn't done theorizing with the Law of Multiple Proportions. He continued to explain why substances act the way that they do with his atomic theory. Dalton's Atomic Theory is printed below.
Dalton's Atomic Theory
The final fundamental theory that is important to understand now is Avogadro's Hypothesis. Avogadro said that if two gases occupy the same volumes at the same temperature and pressure, then the number of particles in each gas will be the same. So basically, if I have 10 liters of oxygen gas and 10 liters of helium gas in the same room with me, each gas will have the exact same number of particles in it.
J. J. Thomson, while experimenting with a cathode and electricity, he discovered that atoms contain negatively charged particles called electrons. A cathode is a partially evacuated tube with electrodes located on it at opposite ends. Electricity was then applied to the electrodes and a stream of electrons, which he called a cathode ray, ran straight across the tube from electrode to electrode. He then applied an electric field around the center of the cathode and noticed that the electrons bent towards the positive side of the electric field which meant that the ray must be negatively charged. He then adjusted the amount of pull that the electric field had on the cathode ray and he was able to determine the mass to charge ratio of the electron. The mass to charge ratio of an electron is -1.76 * 108 C/g.
There was also another man doing similar experiments named Robert Millikan. Robert Millikan, using charged oil drops and the mass to charge ratio that Thomson determined, determined the mass of an electron. The mass of an electron is 9.11 * 10-28 grams.
At the time of Thomson's discovery of an electron, scientists had already determined that an atom was electrically neutral. Because an atom was electrically neutral, and he had just discovered a negatively charged particle, Thomson guessed that there must also be a positively charged particle in the atom. He went even further and made a model of what an atom looked like. It was called the plum pudding model because he said that there was a dense cloud which had a positive charge and randomly scattered inside this cloud were these negatively charged electrons. (His model looked much like a chocolate chip cookie would look because the dough part would be the positively charged part and the chocolate chips would be the negatively charged part.)
Late in the nineteenth century, Henri Becquerel, discovered that certain elements spontaneously emit high energy radiation which was called radioactivity. Three types of particles were given off when radiation occurred. The first particle was called the alpha particle and it was a +2 charged particle. The second particle was called a beta particle and it was a high speed electron. The third particle was called a gamma particle and it was high energy light.
Rutherford discovered the nucleus of an atom and proved Thomson's plum pudding model wrong. Rutherford shot alpha particles at a thin sheet of gold foil and observed some very interesting results. The first observation that he made was that most particles went straight through the gold foil. This led him to believe that atoms are very spacious and have lots of open space in them. The second observation that he made was that some of the particles got bent on their way through. This led him to the conclusion that some of the positively charged alpha particles came close to the positively charged particle in the atom. The third observation that he made was that some of the particles got reflected straight back and this led him to the conclusion that the alpha particles came in contact with the positively charged particle in the atom.
Through this whole experiment, Thomson's plum pudding model was proved incorrect, and Rutherford proposed a new model of the atom which is much like today's view of an atom. Rutherford concluded that there was a dense positively charged center of the atom, called the nucleus, and there were electrons floating around the center at large radii (relative to the size of the nucleus).
The atom has gone through many, many experiments throughout the years. Today, scientists know that the nucleus of the atom is composed of two different types of particles. The first is called the proton. The proton is a positively charged particle and has a charge equal to that of the electron, except it is positively charged, not negatively charged. (Simply if the proton has a positive charge of 1, then the electron has a negative charge of 1. This is why the atom has a neutral charge.) The second particle of the nucleus is called the neutron. The neutron has no charge, but has a mass equal to that of a proton. The electron is a particle that "orbits" the nucleus (much like the moon orbits the earth), but has relatively no mass,when compared to protons and neutrons.
Isotopes are atoms with the same number of protons but a different number of neutrons. For example, Carbon 12 (Carbon is the name of the elements and 12 refers to the mass of the atom) has 6 protons, 6 neutrons, and 6 electrons, whereas Carbon 13 is an isotope of Carbon 12 and has 6 protons, 7 neutrons, and 6 protons.
|Table of Contents:
Unit 1 - Section 1
Unit 1 - Section 3