Tuesday, December 1, 2009

Dalton's Atomic Theory

These are pretty straightforward and require no explanation. Although you don't use these on tests or anything, it's good stuff to know (not to mention our whole understanding of chemistry is based on this):

Each element is made up of tiny particles called atoms.
The atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways.
Chemical compounds are formed when atoms combine with each other. A given compound always has the same relative numbers and types of atoms.
Chemical reactions involve reorganization of the atoms--changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction.
These ideas form the basis of what a lot of AP Chem is all about; chemical reactions. It's nothing more than breaking some bonds and making some bonds, to put the atoms in a different order to create new things.

Intro to Atomic Structure

An atom consists of a nucleus and electrons orbiting around it. The nucleus is made up of positively charged particles called protons, and neutral particles called neutrons. The electrons are negatively charged particles.

Here is a simplified picture:

This atom has two protons (red), so it is a helium atom.

One thing that pictures like the above cannot express is the relative sizes of the nucleus and the atom. If the atom was really that big (diameter 108 pixels), then the nucleus would be 1/100th of a pixel big! Or on a bigger scale, if the nucleus was the size of a pea, then the atom would be as wide as a football field. Another thing is mass. Virtually all the mass is concentrated in the nucleus. The 'pea' would weigh 250 million tons!

There are two numbers to know about any atom. One is the atomic number, which is the number of protons (and electrons in a neutral atom), and the atomic mass number, which is how much the thing weighs. One proton's weight is close to one neutron's weight which is close to one. Electrons weigh virtually nothing when compared to the huge protons, so they don't affect the total mass. So the atomic mass number is simply the number of protons plus the number of electrons.

The notation of writing atoms is like this:

The top number represents the mass number.

The bottom number represents the atomic number.

Some things to remember while solving these types of problems:

(Note: Problems of this simplicity will not be on the AP test, I guarantee that. We are not so lucky. But you will be tested in class on this stuff.)

The bottom number is always the number of protons.
The bottom number is also usually the number of electrons, but only in a neutral element. Ions come later.
The number of neutrons is always the top number minus the bottom number. All you're doing is taking the total mass, taking away the portion that is due to the protons, and what you got left is the neutrons. Likewise, if you know number of neutrons and protons, to find this number all you got to do is add.

Sample Problem

Here is a sample atom. How many electrons, protons, and neutrons does it got?


The bottom number is always the number of protons you have. So there are 9 protons. It's a neutral atom, by the fact that there are no plus or minus signs anywhere, so you can assume there are 9 electrons as well. There are 19 total nucleons (protons and neutrons) in the nucleus, and 9 are protons. So there are 10 neutrons. Easy.

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