If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains ***.kastatic.org** and ***.kasandbox.org** are unblocked.

Main content

Current time:0:00Total duration:9:44

NGSS.HS:

HS‑PS1.A.1

so the atomic number is symbolized by Z and it refers to the number of protons in a nucleus and you can find the atomic number on the periodic table so we're going to talk about hydrogen in this video so for hydrogen hydrogen's atomic number is 1 so it's right here so there's one proton in the nucleus of a hydrogen atom in a neutral atom the number of protons is equal to the number of electrons because in a neutral atom there's no overall charge and the positive charges of the protons is completely balanced with the negative charges of the electrons so let's go ahead and draw an atom of hydrogen so we know the atomic number of hydrogen is 1 so there's one proton in the nucleus so there's my 1 proton in the nucleus and we're talking about a neutral hydrogen atom so there's one electron I'm gonna draw that one electron somewhere outside the nucleus and I'm going to use the oversimplified Bohr model alright so this isn't actually what an atom looks like but it's a very simple view that helps you get started so this is this is one this is one version of hydrogen so this is one isotope of hydrogen so this is called protium so let me go ahead and write that here so this is protium and let's talk about isotopes an isotope isotopes are atoms of a single element so we're talking about hydrogen here that have that differ and the number of neutrons in their nuclei and so let's talk about the next isotope of hydrogen so this is called deuterium let me go ahead and write deuterium here deuterium is hydrogen so it must have one proton in the nucleus and it must have one electron outside the nucleus but if you look at the definition for isotopes atoms of a single element that different in the number of neutrons protium has zero neutrons in the nucleus deuterium has one so let me go ahead and draw in deuterium's one Neutron I'll use blue here so neutrons are going to be blue so deuterium has one Neutron and since neutrons have mass right deuterium has more mass than protium so isotopes have different masses because they differ in terms of number of neutrons notice though that they have the same atomic number they have the same number of protons in the nucleus right it's one proton in the nucleus and that's important because if you change the number of protons you're changing the element and that's not what we're doing here we're talking about atoms of a single element deuterium is still hydrogen it's an isotope and finally our last isotope which is tritium so tritium has one proton in the nucleus right one electron outside the nucleus and we draw that in here and it must differ in terms of number of neutrons so tritium has two neutrons I'm go ahead and draw the two neutrons here in the nucleus and so those are the isotopes of hydrogen so how do we how do we distinguish between the different isotopes all right well we're gonna write little symbols to represent these isotopes and so the symbol that will draw here for protium is going to have the element symbol which is of course hydrogen and then down here we're going to write the atomic number so the subscript is the atomic number which is one because there's one proton in the nucleus and then for the superscript we're going to write in the mass number so let me move down here so we can look at the definition for the mass number the mass number is the combined number of protons and neutrons in a nucleus so it's protons and neutrons and it's symbolized by a so a is the mass number which is equal to the number of protons that's the the atomic number which we symbolize by Z and I plus the number of neutrons so a is equal to Z plus N and for protium let's look at protium here right so in the nucleus there's only one proton and zero neutrons so one plus zero gives us a mass number of one and I'll use I'll use red here for mass number so we can distinguish alright so mass number is red and let me use let me use a different color here for for the atomic number let me use magenta here so the subscript right is the atomic number and that's Z and the superscript is the mass number and that's a alright so this symbol represents the protium isotope so let's let's draw one for deuterium all right so it's hydrogen so we put an H here there's still one proton in the nucleus right one proton the nucleus so we put an atomic number of one the mass number is a superscript it's the combined number of protons and neutrons so we look in the nucleus here there's one proton and one Neutron so one plus one is equal to two so we put a two here for the superscript and finally for tritium it's still hydrogen so we put hydrogen here all right there's one there's one proton in the nucleus atomic number of one so we put a one here and then the combined number of protons and neutrons that would be three all right so one proton plus two neutrons gives us three so there's the symbol for tritium so here are the isotopes of hydrogen and using these symbols allows us to differentiate between them alright let's let's take what we've learned and do a few more practice problems here so let's look at let's look at a symbol for carbon so here we have carbon with subscript six superscript 12 alright and if we want to know how many protons electrons and neutrons there are so let's first think about protons well we know that the subscript is the atomic number and the atomic number is equal to the number of protons so there are six protons in this atom of carbon and if it's a neutral atom of carbon the number of electrons must be equal to the number of protons so if there are six protons there must also be six electrons and finally how do we figure out the number of neutrons well let's go head in write down write down the formula we discussed the mass number is equal to the atomic number plus the number of neutrons all right so the the mass number was right here right that's 12 so we can put in a 12 the atomic number was 6 right here so we put in a 6 right plus the number of neutrons plus the number of neutrons so the number of neutrons is just equal to 12 minus 6 which is of course 6 so there are 6 neutrons so just subtract the atomic number from the mass number and you'll get the number of neutrons in your atom all right let's do another one so this is carbon and this time we have a superscript of 13 all right so the atomic number doesn't change when you're talking about an isotope if you change the atomic number you change the element so there's still 6 protons in the nucleus of this atom and in a neutral atom there must be the equal number of electrons so 6 electrons and then finally how many neutrons are there well just like we did before we we subtract the atomic number from the mass number all right so we just have to do 13 minus 6 to figure out the number of neutrons so 13 minus 6 is of course is 7 so there are 7 neutrons in this atom all right another way to represent isotopes let's say we want to represent this isotope in a different way sometimes you'll see it where you write the name of the element so this is carbon and then you put a hyphen here and then you put the mass number so carbon - 13 refers to this isotope of carbon and this is called - notation so let me go ahead and write this - notation alright let's do one more one more example here let's do one that looks a little bit scarier so let's do let's do uranium alright so use uranium the atomic number of uranium is 92 the mass number for this isotope is 235 so how many protons electrons and neutrons in this atom of uranium alright so once again for protons we look at the atomic number that's 92 so there must be there must be 92 protons in a neutral atom the number of electrons is equal to the number of protons so there are 92 electrons and then finally to figure out the number of neutrons all right we subtract this number from the mass number so we just need to do 235 235 - 92 and that gives us a hundred and forty-three alright so there are a hundred and forty-three neutrons