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 * Chapter 3**

The first concept of the atom was created by Democritus. In 400 B.C, he called nature’s basic particle the “atom” based on the Greek word "Atomos" which means “indivisible”. After him, scientists began to gather evidence to support the atomic theory. The atomic theory was an explanation for the 3 following laws: The law of conservation of mass states that matter is neither created nor destroyed during chemical reactions or physical changes. Basically, it’s saying that the earth’s matter is constant. If the mass of a reaction is ten grams before the reaction, it will still be ten grams after the reaction. Next is the law of definite proportions which states that a chemical compound contains the same elements in exactly the same proportions regardless of size or source of the sample. In other words, a drop of water taken from Lake Erie has the same composition as a gallon of water from a kitchen sink. No matter how much of a sample you take, it will always have the same composition. Lastly is the law of multiple proportions which says that two different compounds may be composed of the same elements, but in different amounts and arranged in different ways as illustrated to the left.

The atomic theory was proposed in 1808 by a school teacher named John Dalton who wanted to explain the 3 basic laws stated above. His theory can be summed up by the following:
 * Dalton****’s Atomic Theory:**

Now Dalton’s theory was pretty good, but not all areas of it have proven to be correct. When Dalton stated that atoms of a given element are identical in size, mass, and other properties while atoms of different elements vary in the same areas, he was not correct. Isotopes, or atoms of the same element that differ in mass proved him wrong. For example, the element Carbon has three main isotopes. ¹²C, ¹³C, and ¹⁴C. Also, atoms can be subdivided into smaller particles, but they do not contain an elements properties. His entire theory was not discarded though, but modified which brings us to the modern atomic theory which includes 1) all matter is composed of atoms and that 2) atoms of any one element differ in properties of atoms of another element which were important concepts of Dalton’s that held true.
 * 1) All matter is composed of extremely small particles called atoms.
 * 2) Atoms of a given element are identical in size, mass, and other properties while atoms of different elements vary in the same areas.
 * 3) Atoms cannot be subdivided, created, or destroyed.
 * 4) Atoms of different elements combine in simple whole number ratios to form chemical compounds.
 * 5) In chemical reactions, atoms are combined, separated, or rearranged.

The first discovered subatomic particle was the electron and its discovery was a result of the relationship between electricity and matter. In the late 1800s investigators noticed that when a current of electricity was passed through a cathode ray tube, the surface opposite of the cathode glowed. From this, they hypothesized: Thus, they decided that the particles that composed the rays were negative. J.J Thomson carried out a series of experiments in 1897 that strongly supported this hypothesis. Through these experiments, Thomson concluded that all cathode rays are composed of identical negatively charged particles which were named electrons. He also came up with the plum pudding model of an atom which is not entirely accurate.
 * Discovery of the Electron and Other Parts of an Atom:[[image:07chemHSq25.gif width="152" height="156" align="right" caption="Plum Pudding Model by J.J. Thomson"]]**
 * 1) Cathode rays were deflected by a magnetic field in the same manner as a wire carrying electric current, which was known to have a negative charge.
 * 2) The rays were deflected away from the negatively charged object.

In 1909 Robert A. Milikan measured the charge of the electron. Scientists used this information to conclude that the mass of an electron is 9.109*10¨³¹. (nine point one zero nine times ten to the negative thirty-first) Ernest Rutherford is credited with the discovery of the other important part of the atom: the nucleus. In his Gold Foil Experiment, Rutherford shot alpha particles at a thin piece of gold. From watching the particles behavior, he concluded that there must a small, densely packed region at the center of an atom which was named the nucleus. This experiment also supported that the atom is mostly empty space. His discoveries led to the new planetary model of the atom. The final discovery about the atom was to prove Rutherford wrong. Niels Bohr said that the atom would collapse if it was what Rutherford said it was. He came up with the idea of energy levels.

An atom, which can be defined as the smallest particle of an element that retains the element’s physical properties, consists of two regions: the nucleus and the electrons. The nucleus is very small, extremely dense, and located at the center of the atom. Every nucleus contains at least one positively charged particle called proton (The mass of a proton is 1.673 * 10^-27) and at least one neutral particle called a neutron. (The mass of a neutron is 1.675 * 10^-27.) Surrounding the nucleus is a region that is very large compared to the nucleus and occupied by negatively charged particle called an electrons. Protons, neutrons, and electrons are known as subatomic particles.
 * Structure of the Atom:**

The atomic number of an element is how many protons are in the atom, and it is also how the elements are ordered in the Periodic Table. The electrons must balance with the protons to create the correct charge, which means that a neutral atom will have the same number of protons and electrons. As mentioned above, isotopes are atoms of the same element that differ in mass. A good example is of the carbon atom. 91.41% of carbons are Carbon 12. .07% are Carbon 13. .52% are Carbon 14. To find the atomic average, multiply the mass of the carbon by the percent and then add all three together. (12 * .9941) + (13*.0007) + (14 * .0052) which equals 12.4111.
 * Counting Atoms:**


 * Moles:**

A mole is the amount of a substance that contains as many particles as there are atoms in exactly 12 grams of carbon 12. It is a counting unit that replaces a number.One mole is equal to 6.022 * 10^23 atoms. This is called Avogadro's number. The mass of one mole of a substance is called molar mass. The molar mass is usually written in g/mol. The molar mass of oxygen is 16 grams per mole.

To convert the number of moles of an element to grams of that element, you multiply by the molar mass, and to convert grams to moles, you divide by the molar mass.

To convert 2 mol of carbon to grams, you multiply 2 by the molar mass of carbon, which is 12, and get 24 grams.


 * Vocabulary from this Chapter:**
 * Chemical reaction**-transformation of substance or substances into one or more new substances.
 * Atom****-**the smallest particle of an element that retains the chemical properties of that element. Composed of nucleus (center of atom made of neutrons and protons) and electrons (take up more space then nucleus)
 * Subatomic particles****-**protons neutrons and electrons
 * Nuclide**-a general term for a specific isotope of an element

To sum it all up, this focuses heavily on the atom from how it works to the great scientists who discovered it and unlocked many of its secrets.

Citations: [|www.doe.mass.edu] [] [] Harcourt Chemistry Book