Thanks to Phil Krause for this interesting post.
How many atoms makes one mole? Because atoms are so very tiny it is difficult for us to imagine in our macro world. It turns out that you need around 600 thousand million million million or 6 x 10^23 in scientific notation. One mole is actually defined as 6.0221417930 x 10^23 . This quantity is also called Avogadro’s number. Thats why its mole day today. If you write today’s date in the American way it would be 10/23. Mole day is an unofficial holiday celebrated among chemists on October 23rd between 6: 02 AM and 6:02 PM. This date can be written as 6:02 10/23. Happy mole day everyone!
There are 7 base units recognized by the International System of Units within the metric system. All other units can be derived from these base units which are;
- meter – length
- kilogram – mass
- second – time
- ampere – electric current
- kelvin – thermodynamic temperature
- candela – luminous intensity
- mole – amount of substance
Considering that these are the only base units that all other units can be derived from, the mole is the one that most people have not come across other than a blemish on your skin or a small rodent that digs your garden up. This is the unit that relates measurements on a human scale to the atomic level of which all mater consists.
Elements are described by the number of protons that they contain in their nucleus ranging from one upwards. The element with just one proton is called hydrogen and is the lightest of all the elements. The next lightest element is helium with two protons. As protons are electrically charged with protons having a positive charge, if more than one proton are together in the nucleus they will repel each other just the same as similar magnetic poles do.
When several protons are held together in a nucleus by the strong nuclear force then one or more will spontaneously transform into a similar particle that we call a neutron. We call it a neutron because it has no electric charge. When a proton changes into a neutron it absorbs an electron which has a negative charge along with another particle that we call an anti-electron neutrino. Thus the positive charge of the proton is cancelled out by the negative charge of the electron and it becomes an uncharged neutron. So if two protons come together in a nucleus then one will spontaneously transform into a neutron and the nucleus will contain one proton and one neutron. As it still has only one proton it is still hydrogen, albeit hydrogen that is just over twice the mass of normal hydrogen as it still has a single positive charge.
This heavy hydrogen is called an isotope of hydrogen and is given its own name, Duterium. If four protons are thrust together into a nucleus, two will change into neutrons while two will remain as protons. It turns out that although there are two positively charged protons held close together by the strong nuclear force, the presence of the two neutrons separate them enough to become stable. We call this this helium. There are also isotopes of helium just like hydrogen, some of which are stable and some of which will spontaneously decay into something else. Neutrons can also spontaneously decay into protons by emission of an electron and an anti-electron neutrino. It takes roughy as many neutrons in a nucleus as there are protons for it to remain stable. So helium is roughly four times the mass of hydrogen because protons and neutrons have similar mass.
To relate the number of atoms to a quantity that we can see and measure easily we use the mole. If we collect lots of hydrogen atoms together in sufficient quantity to weigh one gram then we are said to have one mole of hydrogen. Because protons are all identical with each other one gram of hydrogen always contains the same quantity of hydrogen atoms defined as a mole. If we now collect exactly the same quantity of helium atoms together so that we have one mole of helium, what will this weigh?
Well because helium has two protons and two neutrons which all have similar mass then we would have four grams of helium in one mole.
The element carbon has six protons and six neutrons in its nucleus so one mole of carbon would weigh 12 grams. As carbon is normally solid in a crystalline structure it is easier to measure than hydrogen gas so we define one mole as the quantity of carbon atoms required to have a mass of twelve grams
Thus the quantity of one mole can be used to measure all the elements as well as compounds. A compound is a substance containing more than one element, for example water contains oxygen and hydrogen. There are twice as many hydrogen atoms as there are oxygen atoms so we call it H2O. Oxygen contains eight protons and eight neutrons so how much would one mole of water weigh?
Well, we have to count the total amount of nucleons (nucleons is a collective name for protons and neutrons, whatever is contained in the nucleus). There are 16 in oxygen and one in each hydrogen making 18 so one mole of water would weigh 18 grams.