To be technical, the chemical mole is defined as the number of atoms in 12 grams of the most common form of carbon - carbon 12. The National Institute of Standards and Technology says that's around
But the mole doesn't have to just measure atoms - you could buy a mole of donuts instead of a dozen. You could, that is, if the bakery had a bag 5 times as big as the Moon. Counting one donut every second, it would take the baker something like 19,000,000,000,000,000 years to fill your order.
A mole of donuts is hard to get but just breathe for two or three minutes and you will move a mole of air molecules in and out of your lungs. The mole is really a unit designed to measure small things like atoms, molecules and ions1.
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Grab two aluminum beverage cans and you'll have over a mole of aluminum atoms in your hand. WOW! Atoms must be small.2 The two cans pictured were washed out and dried before I put them on the balance. I was hoping they would weigh 26.982 grams because that would be exactly a mole of aluminum atoms. Oh well, part of the other .423 grams is paint. How did I know that we need 26.982 grams of aluminum to have 602000000000000000000000 (6.02 X 1023) atoms? - One of the numbers in the Periodic Table of the Elements told me.
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Three Periods of the Periodic Table of the Elements |
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| 1 H 1.008 | 2 He 4.003 |
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| 3 Li 6.941 | 4 Be 9.012 | 5 B 10.811 | 6 C 12.011 | 7 N 14.007 | 8 O 15.999 | 9 F 18.998 | 10 Ne 20.180 |
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| 11 Na 22.990 | 12 Mg 24.305 | 13 Al 26.982 | 14 Si 28.086 | 15 P 30.974 | 16 S 32.066 | 17 Cl 35.453 | 18 Ar 39.948 |
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Our material world is made of elements and the millions of compounds formed when atoms of different elements bond together. That's why elements are important enough to have their own table. There are over a hundred elements but only a couple of dozen are very common.
The table above shows a few of the elements. Each square contains the element's symbol and two numbers. To understand these numbers and especially the number that tell us how to weigh out a mole of atoms, I need to tell you a little about atoms themselves.
An atom contains protons (particles with a positive charge) and neutrons (neutral particles) in its nucleus (center). All atoms of an element have an identical number of protons in their centers but the number of neutrons may vary a little from atom to atom of the same element. Even though the number of neutrons vary, all atoms of the same element act just alike when forming compounds.
Electrons (particles with a negative charge) are found around the nucleus. Compared with protons and neutrons, there is not much to an electron but it carries a charge that will balance out that of a proton. Atoms may sometimes lose or gain electrons and thus acquire a positive or a negative electric charge. These charged atoms are called ions. Gaining or losing electrons has no measurable effect on the mass of an atom and does not change its identity as a particular element. You still need 6.02 X 1023 ions to get a mole.
Above an element's symbol in a square of the periodic table is a number called the atomic number. This number tells us how many protons are in the nucleus of every atom of that element. If the atom is neutral (no overall charge), the atomic number also tells us the number of electrons around the atom's nucleus.
Below each element is a number called the atomic mass and it allows us to compare the masses of atoms of different elements. Since the number of neutrons in atoms of an element varies a little, some atoms of the same element have slightly different masses. The atomic mass is a special average of the masses of all the atoms of an element. The unit that goes with the atomic mass is called the atomic mass unit (amu). You can quickly see, for example, that argon (Ar) has atoms whose average mass is 39.948 amu and that argon has more massive atoms than say beryllium (Be). Beryllium's atoms have an average mass of only 9.012 amu each.
A neat thing here is that if we take the atomic mass of any element and weigh out that number of grams of the element, we get 6.02 X 1023 atoms - one mole. If someone wants a mole of lithium (Li) atoms, they just weigh out 6.941 grams of lithium. Do you see in the periodic table where I got my information about the number of grams of aluminum (Al) needed for a mole of aluminum atoms?
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Here is a mole of water molecules. To weigh it out, the "Tare" button was pushed with an empty graduated cylinder on the balance. This eliminated the weight of the container. Water was added to the cylinder until we had a mole's worth - 18.015 grams. How did I know to weigh out 18.015 grams of water to get 6.02X1023 water molecules?
Water is H2O.
Please notice on the graduated cylinder that 18 grams of water takes up about 18 milliliters of volume. Is this just some coincidence? No, metric system planners set up the gram so that, at a certain temperature, it is a milliliter of pure water.
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It's more than a spoonful but still it's not a lot considering there are 602000000000000000000000 sweet molecules of sugar in the bag! How much C12H22O11 (sucrose) needs to be weighed out to get a mole?
(Click here for the answer.)
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The mole is valuable to chemists because atoms of different elements bond in whole number ratios to form compounds. For example, sodium (Na) and chlorine (Cl) combine in a 1:1 ratio. One atom of sodium needs one atom of chlorine to form sodium chloride3. Chemists can't count out individually 1 sodium atom and 1 chlorine atom to get this ratio but they can keep the 1:1 ratio by using moles. One mole of sodium (22.990 grams) and one mole of chlorine (35.453 grams) provide 6.02X1023 atoms of each element. This is more than one atom of each element but it is a 1:1 ratio.
Other compounds may require a different ratio of the atoms. Two atoms of aluminum will combine with three atoms of oxygen to form aluminum oxide. Chemists can combine two moles of aluminum atoms (2 X 26.982 = 53.964 grams) and three moles of oxygen atoms (3 X 15.999 = 47.997 grams) to get the 2:3 ratio and insure that all chemicals are used up.
You may have noticed that two of the elements mentioned above (chlorine and oxygen) are gasses at room temperature. It's not easy to measure the mass of a gas directly on a balance but there are ways, other than weighing, to get the number of moles needed for chemical reactions.
Knowing how to measure out substances in moles allows chemists to prepare compounds without having too much of one substance and not enough of another. Our Chemical Mole lets chemists correctly blend substances so chemicals are not wasted and purer products result.
Created and maintained by George Leonberger