Video transcript
- [Instructor] So my apologies. I just had a deviatedseptum surgery yesterday. What we're going totalk about in this video is the notion of an ion. So before we talk about ions we're just going to talkabout the idea of an element. For example, if I havecarbon, carbon is an element. And let's say I have one atom of carbon. What do we know aboutthat one atom of carbon? Well, by definition an atomof carbon has six protons. So it's going to have six protons. And if it is neutral carbon it is going to have thesame number of electrons. It is going to have six electrons and that's what makes it neutral. You have the six positive charges and the six negative charges. Now you could have a carbon ion, although they aren't that typical. The way you get an ionis if you don't have an equal amount of protons and electrons. So for example, if you had six protons and five electrons what would that be? Well, we still have six protons. And remember, protons arewhat define what element we're dealing with. So we're still dealing with carbon, but now we have one more positive charge than we have a negative charge. So this will be carbon, youcan write it with a one plus charge like that or you couldeven write it like this. And this you would justview as a carbon atom. This you would now call an ion because it has that net charge. It has a different numberof protons and electrons. What if you were to gothe other way around? What if you were to have five protons, five protons and six electrons? What would this be? Well remember, protons define what element you're dealing with, so now if you look at whatelement has five protons we're dealing with boron. So this is going to be boron. Neutral boron would have fiveprotons and five electrons. But this one has one extra electron, so it has one extra negative charge. So you can write it like this, one minus. Or you could just say ithas a negative charge. So this is a boron ion right over here. As soon as you have an imbalance between protons andelectrons you no longer would call it an atom, you wouldcall it an actual ion. Now let's do an examplequestion dealing with this. So our question tells us... Our question ... our question tells us ... An atom of platinum hasa mass number of 195. So let's just look up platinumon our periodic table. Platinum is sitting rightover here if you can see it. So an atom of platinumhas a mass number of 195. And 195 looks prettyclose to that atomic mass we have there. And it contains 74 electrons. 74 electrons. How many protons andneutrons does it contain and what is its charge? Alright, so let's thinkabout this a little bit. So we're dealing with platinum. So by definition platinum has78 protons, so we know that. It has 78 protons. They're telling us it has 74 electrons. 74 electrons. So just from that we seethat we have four more protons than electrons. So you're going to havea positive four charge. Four more of the positive thing than you have of the negative things. So you could write this asplatinum with a plus four charge. This is a platinum ion,a positive platinum ion. The general term when we'retalking about a positive ion, we're talking about a cation. That is a positive ion. Up there when we talkedabout boron being negative, a negative ion, that is an anion. This is just to get ourselves used to some of the terminology. But we're not done answering the question. They say an atom of platinumhas a mass number of 195 and contains 74 electrons. How many protons andneutrons does it contain and what is its charge? We figured out its charge. We know the protons. By definition platinum has 78 protons. But what about its neutrons? Well protons plus neutrons is going to be equal to our mass number. It's going to be equal to 195. So we have 78 protons plusI'll say N for neutrons is equal to 195. Subtract 78 from bothsides and what do you get? The number of neutrons is equal to 1 ... let's see if I subtract 70 I'll get 125, minus eight I have 117 neutrons. So 117 neutrons, and we're done. But the big idea is, and wesay atom when we have the same number of electrons and protons, that's when we are neutral. But as soon as we become non-neutral, we have either moreelectrons or more protons, and this is true of an atom or a molecule, we will then call it an ion. And if you wanna be alittle bit more specific, a positive ion is called a cation and a negative ion is called an anion.
