The Atom

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Early Thought

In the 5th century B.C., Democritus held that matter was not infinitely divisible, and that if divided, it would become smaller and smaller etc. He called the smallest particle the atom. Later Aristotle held that matter was infinitely divisible and that there was no need for atoms. He introduced the idea of elements. Aristotle recognized four elements: fire, air, earth, and water; he said that you can go from one material to another by varying pro portions of four elements. Following this, the idea of the atom was dropped for 2000 years.

In the 1700's, science was engaged in the process of isolating and purifying materials particularly from minerals. It was known that:

John Dalton tried to explain observations by a new atomic theory. He said:

Picture of how atoms combined.

Evidence for atoms Brownian motion and have taken pictures of atoms. Atom is very small. 2 grams of hydrogen gas contains 6.022 x 10 atoms. This number is very large. If had this number of popcorn kernels would cover continental U.S. to depth of 9 miles-if divided Avagrodro's number of pennies among population of U.S., one person would have enough money to pay off national debt with 20 trillion dollars left over. Dalton also originated idea of symbols for elements. Give examples from the Periodic Table. Compound is combination of two or more elements. When atoms combine together, they form compounds. Show some examples and explain meaning of symbols such as NO, CuCl, etc.

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Parts of the Atom

At the turn of century, it was known that there were these three types of particles: electrons, neutrons, and protons. Define each and explain Thompson model of atom. Rutherford experiment-show setup the results of this experiment were: Results most particles went straight through, some particles deflected slightly, some particles bounced right back. Last result was entirely unexpected-"like firing a navy gun at piece of tissue paper and having it come straight back at you."

After this experiment, scientists were forced to devise picture of atom consisting of nucleus and electrons outside of nucleus. Nucleus has all of mass and is positively charged. Electrons are negatively charged.

The nucleus is composed of protons and neutrons. Protons are positively charged and neutrons are neutral. Here are some important terms when discussing atoms:

  1. atomic number = # of protons
  2. atomic number = # of electrons
  3. # of neutrons = atomic mass-atomic number
  4. atomic mass = # of protons + # of neutrons

Deviations from the "normal atom"

Sometimes an atom can have charge due to extra electron or missing electron. The name given to this type of atom is an ion. An ion is any atom that has an electrical charge. Examples include H or Cl¯. To be more specific, the term anion is given to an ion with a negative charge, and cation to an ion with a positive charge.

This image demonstrates how the sodium cation is formed. A neutral sodium atom has 11 protons and 11 neutrons. By "giving up" one electron, the atom now has a surplus of 1 proton, and therefore, a charge of "."

 This image demonstrates how the chloride anion is formed.  A neutral 
 chlorine atom has  protons and  neutrons.  By gaining one electron, the 
atom now has a surplus of 1 electron, and therefore, a charge of "¯."

Sometimes there are atoms with the same number of protons and electrons but different number of neutrons. This type of atom is called an isotope. Ex. Co C. Different isotopes of a given element have the same chemical properties.

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A Picture of the Atom

The recent picture of atom was presented by Neils Bohr and is known as the Bohr atom. In this model, the nucleus is in the center with electrons in orbit-etc. The stipulation is made that an electron can orbit in only certain well-defined paths and can never be found anywhere else.

This image represents cross sections of the spherical electron density patterns. The dark areas represent areas in which there is a high probability of finding electrons, while the white areas, called nodes, indicate areas where the electron density is zero.

To see a more detailed picture of specific electron orbitals, click here.

Further, when atom absorbs energy, not just any energy can be absorbed-must be equal in energy to energy difference between upper and lower energy levels. When atom absorbs proper amount of energy, causes electron to move from one orbit to another-without going in between. When atom loses energy, gives off energy in form of light-light energy is precisely equal to energy difference between upper level and lower level-causes lines to appear in spectrum.

Today we have a similar idea of atoms energy levels but have different picture. We know energy of atom but we can't know position of electron. Electron has certain probability of being at any point in space.

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The Periodic Table

We have seen that most materials can be decomposed into elements. At end of 18th century, knew of 26 elements-today have 109. In 18skjhf, Mendeleev devised the periodic table. In this table, each element has a unique symbol, plus an interesting name. Might show them some elements and explain where the elements are found. Today the elements are arranged in order of increasing atomic number-not atomic mass- Co and Ni. Elements in same columns have same chemical properties-due to arrangement of electrons in atom. In the lowest level of the atom, there is only room for two electrons-therefore two elements in the next energy level have room for 8 electrons-therefore have 8 elements etc.

Periodic Table

Not yet complete.

Click on any high-lighted element to reference further detail. Those elements which are shown in orange squares are non-metals, whereas those in blue squares are metals.

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