My teacher gave me a chart to fill in on The number of Nutrons in an atom. but I can't figure out how to do it, b/c I thought the number is always different. Do you think he means, fill out all the possible answers? or do you think he means to choose one and put that down? For example, one of the elements he gave me was sodium.

To calculate the number of neutrons in an element, use the periodic table and subtract the atomic number from the atomic mass.

For example, Carbon-12 has an atomic number of 6 and an atomic mass of 12.0115. We can round the 12.0115 to 12.

12 - 6 = 6

There are six neutrons in an atom of Carbon-12.

This system works because protons (shown in the atomic number) have the same mass as neutrons.

You may also be able to find some automatic reference table with Google or something, but that won't help come the test.

You see Yafa, the Atomic Number represented in the Periodic Table shows the number of protons;
while the atomic-mass shows the total weight.

We know that an atom consists of protons, neutrons and electrons (which are of negligible weight approximately 1/116 probably)

therefore, in order to obtain the # of neutrons we subtract the atomic number from the atomic mass.

With the concept behind this formula, I hope remembering this rule would be easy for you.
Pin~Jinx / anarchist

A bit high on the electron mass, Pin~Jinx.
An electron's mass is 1/1836th of a proton's mass.



The answers above will (usually) give you the most common isotope. I'm only replying to correct the mass of the electron and to throw in a little info about isotopes.

Different atoms of the same element can have different numbers of neutrons. The atoms with different numbers of neutrons are called different isotopes. The atomic mass is the average weight of all atoms of a certain element in the world (or a good approximation of it anyway). This is (the main reason) why the atomic mass is not a whole number.

Carbon, for example, has 6 protons.
98.9% of carbon atoms have 6 neutrons, giving them an atomic mass of 12.
1.1% of carbon atoms have 7 neutrons, giving them an atomic mass of 13.
A much smaller fraction of carbon atoms have other isotopes as well.

This all averages out to give an atomic mass of 12.011

For most elements, rounding the atomic mass and subtracting the atomic number will give you the most common isotope.

Sodium, by the way, has only one significant isotope.