We've already talked about atoms, or elements, and saw them
all listed in the periodic table. So what's an atom made up of? Let's look real closely at an atom and try to see what it consists of? What the physicists have found is that every
atom consists of the same building blocks. And the good news is that there are only three different types of building blocks. These three types of building blocks, or subatomic particles are:
Don't be scared, but suddenly you're talking to me about the three types of subatomic particles: protons, neutrons and electrons. Don't let it intimidate you, they could have been called Curly, Moe and Larry and you'd have no problem with that. They're just protons, neutrons and electrons. And every atom is made up of them.
So what do these really, really small particles look like. We can imagine them as being small round balls. The proton and neutron are about the same size, while the electron is much, much smaller. I was told once that as a comparison, if the electron was the size of the tip of your pencil, the proton and neutron would be the size of your classroom. So let's just say that the electron is really tiny compared to the proton and the neutron, OK?
Each one of these three subatomic particles also has it's own unique electrical charge. The proton has a single positive charge associated with it (Proton=Positive). The electron has a single negative charge associated with it (Electron=Negative). And the third particle, the neutron is neutral, it has no charge, it is neither positive nor negative (Neutron=Neutral, No Charge). Now that's not so hard, is it? The physicists discover these kinds of things, call them rules, and we just need to learn them.
Another thing the physicists have discovered about these three subatomic particles is that the two big ones, the proton and the neutron, stay together at the 'center' of the atom, while the electron circles them at a distance similar to how the earth would circle the Sun in its orbit. The center where the proton and neutron are found is called the nucleus of the atom. And, as we just said, circling the nucleus is the electron in it's 'orbital'. The earth travels around the Sun in it's orbit, and the electron circles the nucleus of the atom in it's orbital.
Here's the picture from the first page. It's a cartoon showing the Helium atom. As we will explain in the next paragraph, it has 2 protons (blue) and 2 neutrons (red) in it's nucleus and 2 tiny electrons (green) circling the nucleus.
Now I dare you to click onto this very nice web site. It explains just what we've been talking about, but in a much more 'textbook' fashion. Read through it, it will help. Besides, there's a cute joke waiting for you there.
First let me just throw out the first rule about reading the periodic table. Here goes: The atomic number of an atom will always be the same as the number of protons for that atom. Now let me make it a lot clearer by giving some quick examples. Click back to the previous page a look at the periodic table. You'll notice that Heluim, He, is atomic number=2. What that immediately tells you is that He has 2 protons. And where are those 2 protons, why in the nucleus of course. What about oxygen, how many protons does oxygen have? Well, from the periodic table it is atomic number=8. So, it has 8 protons. And what about carbon, how many protons does it have? Well, it is atomic number=6, so it has 6 protons. Very simple, yes?
Here's a second rule I'm gonna just throw out. For the atom to be electrically balanced, all the 'plus' charges must be balanced by an equal number of 'minus' charges. What that means is that in any atom, there will be the same number of protons (with their positive charge) and electrons (with their negative charge). So hear's the easy part about this rule, once you know the number of protons in that atom, you immediately know that there are the same number of electrons. Here's an example, how many electrons in an atom of Helium, He? Well, He has an atomic number of 2, so it has 2 protons, so it must also have 2 electrons. Ta-da!
How many electrons does an atom of carbon have? Well, let's see. Carbon is atomic number=6, so carbon has 6 protons in it's nucleus, and so it must have an equal number of electrons, and so carbon has 6 electrons. Ta-da!
For the next topic you'll want to click on this bigger and better periodic so you can find not only the atomic number of an atom, but also the atomic weight of an atom. After you click onto this site, keep moving down to find the periodic table.
And now the third and final rule for us to worry about. How many neutrons in the nucleus of an atom? Neutrons remember have no charge, but do have a weight or mass, 'cause their big like a proton. So you also find on the periodic table, for each and every atom, the 'atomic weight' or 'atomic mass' of that atom. That number tells you how much that atom weighs. Not much, but it's a useful number when trying to compare the sizes or weights of different atoms. What determines how much an atom weighs anyway? Well, what's there in that atom to give it weight? There's the proton, nice and big and round in the nucleus. And there's the neutron, nice and big and round also in the nucleus. And there's the tiny, tiny electron whizzing around in it's orbital. If we weigh this atom, we can ignore the weight of any electrons since they are so very, very small. For example, if the proton and the neutron are the size of elephants, and the electron is the size of a bug (even a heavy bug like a cockroach), if the atom had only one proton, one neutron and one electron, you would add the weights of one elephant (proton) plus another elephant (neutron) plus a bug (electron). The final weight would be something like 4 tons, so many pounds and so many ounces. Weigh it again without the bug (electron) and you'll essentially get the same weight, 4 tons, so many pounds and a little bit different amount of ounces. Overall, the weight of the electron (cockroach) is insignificant, so we can just simply ignore it. The weights of the protons and neutrons really determine the overall weight of the atom. So if Helium, He, weighs 4.0 units (and I just got that number by reading it off the periodic table), those 4.0 units of weight are because of the combined weight of the protons and neutrons. We already know, or can figure it out in a flash, that He has 2 protons (remember it's atomic number is two). So if Helium's overall weight is 4 units, how many neutrons does helium have? Well,
[overall weight (4)] = [# of protons (2)] + [# of neutrons (?)]
So there must be 2 neutrons in the nucleus of a helium atom. So what about the 2 electrons you know helium has? Remember, we just ignore their weight because they are so very, very small.
Let's try another example. How many neutrons in an atom of Oxygen? Oxygen is atomic number 8, so oxygen has 8 protons (and 8 electrons). The weight of oxygen from the periodic table is 16.0 atomic units. So for a total weight of 16, and 8 of those 16 are due to the 8 protons, so how many neutrons must that leave? Well, 8 neutrons plus 8 protons makes 16 for the weight. Ta-da! Now don't start whinning about the eletrons, I told you, they don't count when you calculate the weight, they're too small to matter.
One more tricker example. What about Hydrogen? How many protons, and electrons and neutrons does Hydrogen have? Firstly, we look on the periodic table and see it is atomic number=1. So the Hydrogen atom has one proton in it's nucleus. And since it has one proton, it must have one electron to balance it out in it's orbital. Now, what about the neutrons? We look at the periodic table and see that Hydrogen has an atomic weight, or mass, of 1. Hydrogen has one proton and an weight of one. Hummmm. So how many neutrons does hydrogen have? Well, Hydrogen has zero neutrons. Can't have any neutrons, the entire weight of the atom is due to the one proton. So, in fact, hydrogen is the smallest, simpliest atom there can be, just a single proton with its electron.
This is great. Given a periodic table, we can determine all the protons, neutrons and electrons of any atom. We even know where they're found, the proton and neutron in the nucleus and the electron(s) out in their orbitals. Now that brings up one other thing about where to find the electrons. What about Helium and it's electrons, for example. Helium is atomic number=2, so it has 2 protons, and so 2 electrons. Where are those 2 electrons exactly? They both circle the nucleus like the earth circles the sun. But as you know, the earth has it's own orbit that only earth occupies. Atoms with their electrons are not this way. Just like our solar system, there are several planets with separate orbits circling our Sun. Our planets will always stay in their orbits. Likewise with an atom. There are several orbitals that are found circling the nucleus of an atom. Not so bad, just envision an atom like our solar system with orbitals like the orbits of the planets: separate and always a certain distance away. Just like Mercury is the closest planet to the Sun and so travels in the closest orbit to the Sun, there is the closest orbital to the nucleus of an atom.
But, unlike the planets in our solar system, the first, closest orbital to the nucleus of an atom can actually contain 2 electrons. That would be like having 2 planets in the same orbit. We don't find that happening in our solar system, but there it is with our atoms. The closest orbital to the nucleus of every atom can hold 2 electrons. Usually those 2 electrons are at opposite ends of the orbital since they both have negative charges are so are repulsed. Now the physicists who identified the orbitals simply call this closest orbital to the nucleus the first orbital, or orbital #1. Easy enough. Now I agree, it's an odd 'rule' that orbital #1 can hold 2 electrons, but that's the way it is.
Just outside the first orbital, orbital #1, is the second orbital, called (as you might guess) orbital #2. The 'rule' about orbital #2 is that there is room for 8 electrons. Orbital #2 can hold up to 8 electrons. It would be like having 8 planets in the same orbit. Wierd only because we don't see it that way in our solar system, but that's how it is for all of our atoms. And if you really want to know, the third orbital out from the nucleus, orbital #3, also will hold 8 electrons. So let's summarize the 'rules' about where we find the electrons of an atom:
Orbital #1 holds 2 electrons;
Orbital #2 holds 8 electrons;
Orbital #3 holds 8 electrons;
There's more orbitals, but we needn't worry about them for this class.
Let's get back to our original question: where do we expect to find the two electrons of Helium? Well, they're in the first orbital. OK, let's locate the electrons for the very next atom, Lithium (atomic number=3). Lithium has 3 protons, and so 3 electrons. Where are those 3 electrons located exactly? Electrons #1 and #2 go into the first orbital, orbital #1. Now remember our rule, the first orbital can hold 2 electrons, and then it's full. What that means is that the third electron then must go into the next orbital outward, into the second orbital, orbital #2. Ta-da!
Let's do another example of finding out where the electrons of an atom are located. Let's do the next atom, Beryllium, atomic number=4. This should go quickly by now. It must then have 4 protons and so 4 electrons. The first 2 of those 4 electrons fill-up the first orbital, leaving the last 2 electrons to go into the second orbital. Ta-da!
Let's try Oxygen. Where are Oxygen's electrons? Let's see, Oxygen is atomic number=8, so it has 8 protons and 8 electrons. The first 2 of those 8 electrons go into the first orbital and fill it up. That means that the remaining electrons, electrons #3,4,5,6,7 and 8 go into the second orbital. Can this be? Is that OK? Can electrons #3,4,5,6,7 and 8 all fit into the second orbital? Well, remember our 'rule', the second orbital can hold up to 8 electrons. So even though there's room for 8, we're adding only 6 electrons into the second orbital. Let's check our math, 8 total electrons for Oxygen; 2 in the first orbital and the other 6 in the second orbital. Ta-da!
Let's try Neon (the atom, not the sub-compact). It is atomic number=10, so it has 10 protons and so 10 electrons. Where do we find these electrons? Electrons #1 and 2 are in the first orbital, filling it up. So the remaining 8 electrons go into the second orbital, filling up the second orbital. Now it's full of electrons too. What's going to happen if we have any more electrons? For example, the next atom is Sodium, Na, with atomic number=11. Sodium is going to have 11 electrons. The first 2 go into the first orbital, the next 8 go into the second orbital. That's 10 electrons (2 + 8), but Sodium has 11 total electrons. Where does that last 11th electron go? Well, just as we did before, we move out to the next orbital, orbital #3, and put it into this orbital now. Eventually the third orbital will be filled when we get to 8 electrons in it. Here's a good question to see if you're paying attention: What element has it's first, second and third orbitals full of electrons? (Hint: it rhymes with 'jargon')
Whew, we've covered a lot here. Now try to contain your exuberance and calmly click onto the next page so we can talk about chemical bonding.