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UM CHEM 1110 - Atomic & Subatomic Theory

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Atomic (& Subatomic) Theory 1Preview The lectures in this unit cover an introduction to chemistry and matter, working with numbers and units, and an introduction to atoms and the periodic table. This lecture covers atomic and subatomic theory. Atomic & Subatomic Theory I. Atomic Theory Atomic theory is the theory that all matter can be broken down to individual “fundamental pieces” which cannot be divided further. Matter can be divided into two types of pure substances: elements and compounds. All the pure substances for which atoms are the smallest particle possible are called elements. Compounds have a molecule as the smallest particle; these molecules are made of chemical combinations of elements (not mixtures). The following are four fundamental assumptions laid out by John Dalton, the father of atomic theory: 1 – All matter is composed of atoms 2 – The atoms of a given element differ from the atoms of all other elements. 3 – Compounds are combinations of atoms; each molecule of the same compound has the same number and types of atoms. 4 – Atoms are not created or destroyed by chemical reactions. Assumption #3 is another way of expressing the Law of Constant Composition that we discussed in our lecture on matter – that is, all pure substances have a constant composition. Both compounds and elements are considered pure substances. Assumption #4 is another way of expressing the Law of Conservation of Mass, which you may be familiar with from physics. Check out the periodic table at the front of your book. It includes all the elements discovered so far, organized in what might appear to be a very peculiar fashion. Generally, the table is arranged so that elements with similar properties are listed together. For example, all the metals are on the left hand side and all the nonmetals are on the right hand side. There are ~110 known elements in existence. Every substance and speck of matter you deal with on a day to day basis is made from one or more of these elements. Some are combined chemically into new pure substances with unique properties and some are combined physically as mixtures. Elements each have a name and a symbol. The periodic table always includes the symbols of each element. Some tables also include the full name. Both are appropriate. Therefore when I make reference to carbon I can just use the symbol C. Cl is used for Chlorine, and so on. In an exam setting I provide you with a copy of the periodic table which contains the symbols AND names of the elements. See the main page of the website for a copy of it.Atomic (& Subatomic) Theory 2------------------------------------------------------------------------------------------------------------------------ Take a minute to complete your Atoms & the Periodic Table worksheet section I. This has you look up the atomic symbols for various elements. Not all symbols are intuitive and some of these might be new elements to you! This should help familiarize you with where some different elements are on the periodic table. I recommend using the table from the website that is used on the exam. The more familiar with it you are, the easier it will be to reference it quickly! ------------------------------------------------------------------------------------------------------------------------ Elemental symbols do a good job of representing elements in their pure state. But what about the other category of pure substances: compounds? How do we represent them? First we must understand that since atomic theory tells us compounds are combinations of elements and atoms are the smallest units of an element possible, then compounds must contain whole number ratios of atoms. A compound that contains both hydrogen and oxygen, for instance, could potentially have 1 hydrogen atom and 1 oxygen atom. It could also have 1 hydrogen atom and 2 oxygen atoms, or 2 hydrogen atoms and 1 oxygen atom. But it cannot have ½ of a hydrogen atom or ¾ of an oxygen atom. We represent compounds by listing the symbols of the elements included and indicate the respective abundance of each element as a subscript following the symbol. So a compound that contains 8 hydrogen atoms and 3 carbon atoms is represented by the formula C3H8, and a molecule that contains 2 hydrogen atoms and 1 oxygen atom is H2O – if no number is listed then it represents just 1 of that atom. Sometimes parentheses may be used to indicate more than one group of atoms. The formula Ba(NO3)2 indicates 1 barium atom, 2 nitrogen atoms, and 6 oxygen atoms. Just as with mathematical formulas, the parentheses means the 2 at the end must be distributed through to understand how many of each atom type there are. You may wonder why we do not just express this compound as BaN2O6. We will address that question later in the semester when we talk about bonding in molecules! ------------------------------------------------------------------------------------------------------------------------ Before you continue, take a minute to complete your Atoms & the Periodic Table Worksheet sections II and III. These have you put together compound formulas from information about the number of each type of atom, and vice versa. It is good practice to help you recognize these relationships! ------------------------------------------------------------------------------------------------------------------------Atomic (& Subatomic) Theory 3II. Subatomic Theory In chemistry we make a big deal out of the fact that atoms are the smallest pieces of matter from which all other matter is made; but that is not completely accurate. As scientists studied atoms and matter, they discovered that atoms themselves are made of even smaller particles! Atoms are composed of smaller particles with different charge and mass values, as follows: Protons are positively charged particles. They have a charge of +1 relative to other subatomic particles. They have a mass of approximately 1 relative to other subatomic particles. They reside in a dense mass in the middle of an atom known as the nucleus. Neutrons have no charge – they are neutral. They also have a mass of approximately 1 relative to other subatomic particles. They also reside in the nucleus. Electrons are negatively charged particles. They have a charge of –1 relative to the proton: the charge is exactly the same but opposite. Their mass is ~ 2000 times less than that of a proton or


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