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Mass Number

Numerical relationship between protons and neutrons.

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Periodic Pounds

Credit: wikimedia commons
License: CC BY-NC 3.0

Periodic Pounds [Figure1]

Very similar to human weight, the mass number is a fluctuating number in atoms. When you eat only healthy foods you should maintain healthy weights, and if you eat junk food your mass should increase exponentially. Similarly, when protons are in an atom, the mass stays the same, but a neutron when added increases the mass number.

In most stable atoms, the mass number is equal to two times the atomic number. This is because the number of protons must equal the number of neutrons to keep the protons from repulsing between themselves.

The mass number minus the atomic number is the amount of neutrons present in a nucleus. Typically the mass number can be found at the bottom of the element square in the periodic table of the elements.


1. What is the atomic number for Xenon?

2.What is the atomic number for Argon?

3. What is the atomic mass for Helium?

4. What is the atomic mass for Bromine?

5. How would you find the atomic mass for a non isotope atom with just its atomic number given? 

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  1. [1]^ Credit: wikimedia commons; License: CC BY-NC 3.0

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