Particles and Their Properties
The key properties of an atom (like oxygen) are described by the Atomic Number (which is 8 for Oxygen) and the total number of nucleons (which is 16 for Oxygen). The atomic number gives the number of protons in the atom. The number of nucleons gives the total number of protons plus neutrons in the atom. The number of nucleons is always very close to the atomic weight of the atom (which is 15.9994 for Oxygen).
Here a simple way to remember this:
Now you can figure out how many electrons, protons and neutrons Oxygen has: Atomic number (which is always the smaller of the two numbers) is 8. Hence 8 protons. Because the atom is neutral, there are also 8 electrons. Number of nucleons (which is the larger of the two numbers because it counts all nucleons, not just protons): 16. This is the total number of protons and neutrons. Subtract the number of protons (8) and you get the number of neutrons, which is also 8.
Another example: Iron, which is 26 Fe 56. It has 26 protons, 26 electrons, and 56-26=30 neutrons.
To find the atomic and group numbers of many other elements, you can look at the following Web site: http://www.webelements.com/
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