- Introduction to chemistry
- Preparing to study chemistry
- Elements and atoms
- Average atomic mass
- Worked example: Atomic weight calculation
- The mole and Avogadro's number
- Atomic number, mass number, and isotopes
- Worked example: Identifying isotopes and ions
- Isotope composition: Counting protons, electrons, and neutrons
Atomic number, mass number, and isotopes
Each atom has a charged sub-structure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons. The number of protons and the mass number of an atom define the type of atom. Atoms of the same element with different mass numbers are called isotopes. Created by Jay.
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- This is probably a very stupid Question, but I have to ask it.
Can Elements have an Infinite number if isotopes?(353 votes)
- There isn't any set number of isotopes an atom can have. As the number of neutrons in an atom increases or decreases, the isotopes tend to become more and more unstable until they get to the point where they decay faster than neutrons can change. According to the International Atomic Energy Agency, Mercury currently has the most at 45 identified isotopes.(386 votes)
- Hydrogen has its own set of isotopes. Does that mean that all elements in the periodic table have their own names for their isotopes? Also, do all elements have only three isotopes each?(37 votes)
- For the most part, only H-1, H-2 and H-3 have unique names. However, every element has isotopes. And the majority of them have more than three, including Hydrogen. Mercury currently has 45 known isotopes.(48 votes)
- Are the names protium, deutrium, and tritium only meant for hydrogen with different neutrons?(22 votes)
- Yes, these are the names of the hydrogen isotopes. Protium is hydrogen-1, deuterium is hydrogen-2, and tritium is hydrogen-3(27 votes)
- How come the symbol for Atomic weight is Z?(13 votes)
- The conventional symbol Z possibly comes from the German word Atomzahl (atomic number). However, prior to 1915, the word Zahl (simply "number") was used for an element's assigned number in the periodic table.(28 votes)
- What is a neutral atom? I still dont get it(11 votes)
- Every atom is made up of protons (that are positively charged), neutrons (that have no charge) and electrons (that have a negative charge). When we look at an atom, we find that it has the same about of protons as it does electrons. So, if we look at oxygen, we see that its atomic number is 8, meaning that it has 8 protons. We can also assume that it has 8 electrons. So, oxygen has eight positive particles plus eight negative particles. Think of this as 8+(-8) or 8-8. Eight minus eight equals zero, which is neutral. Basically, a "neutral atom" is an atom that has the same amount of protons as it does electrons. Later you will learn about ions, which have unequal amounts of protons and electrons.(33 votes)
- if protium [hydrogen w/ no neutrons] has the mass [weight] of only the single proton, and protons and neutrons have the same mass [weight], does that mean that deuterium is twice as massive [heavy] as protium? the same question would apply regarding tritium since it has only one proton but two neutrons.
if so then since water generally weighs about six pounds / gallon, does that mean a gallon of deuterium would weigh about twelve pounds ...?(12 votes)
- 2/26 of H2O is hydrogen atoms. Replacing H with deuterium will not make 6 lbs. of H2O become 12 lbs. of H2O because you are only doubling the H portion of the H2O or 2/26 of it.(11 votes)
- What causes isotopes to form? Do they ever lose the extra neutron(s) or gain new ones?(7 votes)
- With the exception of the hydrogen and helium that were formed shortly after the Big Bang event, elements mainly form in the following ways:
1. Fusion in the core of a star.
2. Fusion during a supernovae.
3. Decay of a radioactive isotope.
4. Being changed from one type of atom to another by high energy nuclear reactions, such as having a neutron slam into the nucleus at an extremely high velocity.
There are a few other ways, but they are not all that significant.
Each isotope (more properly called a nuclide) has its own way(s) in which it is formed. There is not one set way they form. So, it is not possible to give you a general answer other than to say that the isotopes of an element form in one or more of the above ways, but each isotope of an element may or may not form in a similar way as other isotopes.
Except as listed above, an atom that is not radioactive never changes its number of neutrons. So you do not usually have one isotope changing into another isotope of the same element. There are ways it can happen, but they are not the normal course of events.(8 votes)
- How do we know which isotopes are radioactive(7 votes)
- So throughout this entire video he was using "neutral atoms". But are non-neutral atoms common? When and on what elements do they occur?(5 votes)
- Any atom can gain or lose electrons and become a charged version of that atom, which we call an ion.(6 votes)
- do most elements on the periodic table have a neutral charge?(3 votes)
- All elements have a neutral charge in their natural state(7 votes)
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 one. 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 completely balance with the negative charges of the electrons. So let's go ahead and draw an atom of hydrogen. We know the atomic number of hydrogen is one, so there's one proton in the nucleus. So there's my one proton in the nucleus, and we're talking about a neutral hydrogen atom, so there's one electron. I'm going to draw that one electron somewhere outside the nucleus and I'm going to use the oversimplified Bohr model. So this isn't actually what an atom looks like, but it's a very simply view that helps you get started. So this is one, this one version of hydrogen. This is one isotope of hydrogen. So this is called protium. 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 differ in the number of neutrons in their nuclei. So let's talk about the next isotope of hydrogen. So this is called deuteriums. 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 differ 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, 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. Finally, our last isotope, which is tritium. So tritium has one proton in the nucleus, 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. Let me go ahead and draw the two neutrons here in the nucleus. And so those are the isotopes of hydrogen. How do we distinguish between the different isotopes? Well we're going to write little symbols to represent these isotopes. And so the symbol that we'll 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 atomic number which we symbolized by Z, plus the number of neutrons. So A is equal to Z plus N. And for protium, let's look at protium here. 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 red here for mass number so we can distinguish. Alright, so mass number is red and let me use a different color here for the atomic number. Let me use magenta here. So the subscript is the atomic number and that's Z, and the superscript is the mass number and that's A. So this symbol represents the protium isotope. Let's draw one for deuterium. So it's hydrogen so we put an H here. There is still one proton in the nucleus, right one proton in the nucleus, so we put an atomic number of one. The mass number is the 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. There's one proton in the nucleus, atomic number of one, so we put a one here. And then the combined numbers of protons and neutrons, that would be three. 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. So let's take what we've learned and do a few more practice problems here. So let's look at a symbol for carbon. So here we have carbon with subscript six, superscript 12. 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 ahead and write down the formula we discussed. The mass number is equal to the atomic number plus the number of neutrons. So the mass number was right here, that's 12. So we can put in a 12. The atomic number was six, right here. So we put in a six. Plus the number of neutrons. Plus the number of neutrons. So the number of neutrons is just equal to 12 minus six, which is, of course, six. So there are six neutrons. So just subtract the atomic number from the mass number and you'll get the number of neutrons in your atom. Let's do another one. This is carbon and this time we have a superscript of 13. 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 six protons in the nucleus of this atom and in a neutral atom, there must be the equal number of electrons. So six electrons and then finally, how many neutrons are there? Well just like we did before, we subtract the atomic number from the mass number. So we just have to 13 minus six to figure out the number of neutrons. So 13 minus six is, of course, seven. So there are seven neutrons in this atom. Another way to represent isotopes, let's say we wanted 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 hyphen 13 refers to this isotope of carbon and this is called hyphen notation. So let me go ahead and write this hyphen notation. Alright, let's do one more example here. Let's do one that looks a little bit scarier. So let's do uranium. So U is 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? So once again for protons, we look at the atomic number, that's 92. So 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, we subtract this number from the mass number. So we just need to do 235 minus 92. And that gives us 143. So there are 143 neutrons.