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Atomic Mass - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

An element s atomic mass is the mass of one atom of that element, measured in atomic mass units (amu). An element s atomic mass in amu is equal to its molar mass in g/mol.

Alright so we're going talk about the atomic mass of an atom and what that actually means. So when you think of an atom what gives it the mass, well in the nucleus we have the proton and we also have the neutrons both of 1amu but those have mass surrounding the nucleus we have electrons which is 0amu or essentially weightless comparatively. So we're going to focus mainly on the protons and neutrons within an element which gives it its mass. So when you pull out the periodic table and extension of it we're going to look at potassium for example, we have bunch of information that we have to look through and decipher. The first thing we have is potassium which is obviously is a chemical name, then we have the number 19 and that [IB] atomic mass, sorry our atomic number which means that it has 19 protons for every potassium atom. We also have a chemical symbol in this case K, K for potassium which doesn't make sense but it's the latin root for potassium, and then we have this funny number at the bottom 39.098 what is that? Well that is the atomic mass of all the potassium atoms that exist. So you wonder like why is there a decimal place?

Well protons and neutrons all have round numbers of 1amu however they have varying numbers of neutrons within every different number of different potassium atoms in the world. So when you average them all together you can get this funny number of 39.098 it's the average atomic mass in atomic mass unit. That is the weighted average of all the isotopes of potassium in the world. So how do we calculate that? How do we get that answer? Well the bananas are great source of potassium, so we're going to talk about bananas. In my banana and in your banana and everybody's banana in the whole world if we pull out all potassium atoms and analyze them we find that 93.90% of potassium atoms in any banana has a mass number of 39. What that means is, there's 19 protons and 20 neutrons. 5.81% of potassium atoms that are in analyzed has a mass number of 41, in this case 19 protons which make sense and in this case 22 neutrons different. Lastly we have 0.02% of the potassium atom has a mass number of 40 again 19 protons and this case 21 neutrons. These guys are all isotopes of potassium, these are all isotopes of potassium that exist, so still how do we get those numbers to equal 39.098 which is what the periodic table told us.

Well we have to calculate the average atomic mass and everything, so in this case how we do that is change the percentages to decimal places. So we're going to say 93.90, we're going to say it's 0.9390 we're then going to multiply it by it's mass number in this case 39 and we're going to get 36.621. Next we're going to do the same thing for the next guy we're going to change 5.81 to decimal in this case 0.0581 multiplying it by its mass number 41 it's going to give us 2.381. Now lastly we're going to take the percentage of 0.02 change it to decimal 0.0002 and multiply that by 40 its mass number and we're going to get 0.008. Now keeping [IB] in mind this has four [IB] also, so we're going to get rid of one, this guys has three [IB] in this case we're going to get rid of this one to make it has three [IB], and this guy has one this guy has one so we're good. When you add them all together we get a number of actually I'm going to have to pick up my calculator real quick 39.010 we make sure they're correct number of [IB] this one has two so we make sure our decimal place two which is this is and this our atomic mass unit for potassium and so average atomic mass.

Notice that it is slightly different than what we have found in the periodic table, that's okay because our measurements are different and [IB] we had to take into account. So we have to make sure, so pretty much it's pretty much the same thing and that's the reason why it's a slightly different number due to measurements and things like that. But otherwise this is how you calculate the average atomic mass of a particular element.