Q&A for How to Calculate Atomic Mass

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How do I find the mass number of an atom?

Community Answer

Add the protons and neutrons together to find the mass, or add the masses of the isotopes multiplied by the atom's natural abundance.

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If 1 amu is 1/12 of a carbon 12 atom, why is it that when I add the masses of the individual parts of a carbon 12 atom, I get more than 12 amu?

Community Answer

1 u = 1/12 the mass of carbon 12 by definition. You're adding the masses of uncombined protons and neutrons, 1.0073 u and 1.0087 u respectively. But when those particles fuse together to form an atom, some of the mass is converted into energy according to E=mc^2. The lost mass is called the "mass defect", and the equivalent amount of energy is the "binding energy."

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How can I find the mass of any atom to convert it in a.m.u.?

Community Answer

We can find the mass of any atom by adding the electrons, protons and neutrons.

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How can I calculate the atomic weight of fluorine?

Community Answer

Atomic weight basically means number of neutrons plus number of protons. In the fluorine atom the number of protons is 9 and the number of neutrons is 10, so the atomic mass of fluorine is 19.

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Question

Can I round off the relative atomic mass of an element to a whole number?

Community Answer

In most cases, yes, but it depends on what you need it for. It also depends on how exact your answer has to be.

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Question

How do I calculate the relative atomic mass without the percentage?

Community Answer

You add the protons and the electrons to find your atomic mass. For example: the carbon atom has 6 protons, 6 neutrons and 6 electrons.

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How can I compare the relative atomic mass of carbon with any other atom?

Community Answer

You can compare how many protons, electrons and neutrons both have.

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Question

How can find the atomic mass of an element if 60.4% of the atoms have a mass of 68.9257 and the rest have a mass of 70.9249?

Community Answer

You have to round the numbers to the second significant figure. Then add them all together and there you have it. It won't be the exact number but it'll be close.

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What does a.m.u. stand for when calculating atomic mass?

Community Answer

A.M.U. stands for ”atomic mass unit”. This is used when calculating atomic mass because even mg are not small enough of a unit.

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Question

How do I get the number of neutrons?

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Community Answer

The neutrons is the atomic mass minus the atomic number.

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