 Hello everyone welcome back. So today we are gonna study regarding certain terms and terminologies which will help us understand an atom completely and we will be seeing regarding the mass and every detail concept we will be studying today which will help further in the more concept as well. So starting off first we need to see how we can represent any atom of an element for example the element is X for us. So now this X element we are talking about one atom of this element X. So if I take this one atom it will contain electrons and it will contain protons and neutrons. Now this proton and neutron will be present where will be present in the nucleus of the atom and this electron will be around the nucleus. Now how can we represent this particular atom of the element X? So we represent it as A in the superscript and Z in the subscript. So A denotes what? A denotes the total number of nucleons total number of nucleons that means it denotes N plus P, neutrons plus protons. How many neutrons plus how many protons are present in the particular atom of this element and Z represents the atomic number. What is atomic number? It represents the number of electrons that is present in the atom. Now this A is also known as mass number. Why mass number? Because in any element electrons mass is very very less. In any element you take the mass of the electron is almost negligible in comparison to proton and neutron and the mass of proton and neutron is almost comparable so almost equal. So whatever the mass of this X is it is solely because of the presence of protons and neutrons. So this A depicts what? This A depicts the amount of protons and neutrons and protons and neutrons will in turn is responsible for the mass of the element. So that's why it is also known as the mass number. Now coming to the important terms. First term that we need to know is atomic mass unit. AMU. AMU is nothing but atomic mass unit or it is also known as Dalton, one Dalton which is represented as DA or nowadays it is also known as only you small u which is known as unified mass. Unified mass. Now what is 1AMU? How it is defined? 1AMU is equal to or it's defined as 112th of the mass of 1 carbon 12 atom. That means 1 by 12. Now mass of 1 carbon 12 atom. What will be the mass of 1 carbon 12 atom? Now definitely if I am talking about carbon 12 I need to find out the mass of this particular 1 atom. So definitely how many total number of nucleons are there? This is the measure of N plus P, neutrons plus protons. So the mass will be the total number of nucleons that is 12 into our mass of 1 nucleon. Okay mass of 1 nucleon. So 1 by 12, so mass of 1 C12 atom will be 12 into mass of 1 nucleon. So 12 by 12 gets cancelled. So 1AMU is equal to mass of 1 nucleon which is equal to 1.66 into 10 to the power minus 24 grams or if we denote it in kilograms then it will be 1.66 into 10 to the power minus 27 kilograms. So this is the value of 1AMU or 1U. We need to remember this value. This will be very very important. Now the next particular term is again related to somehow to this AMU that is relative atomic mass r.a.m. Now this relative atomic mass r.a.m is equal to mass of mass of 1 atom of any element 1 atom of element upon upon 1 12th the mass of 1 C12 atom. Now this factor, this denominator factor just now we have seen that is nothing but our 1AMU correct. Now mass of 1 atom of any element. I am calculating this part now numerator part. So let's say the element is x any element we can take. So the mass of 1 atom of that specific element will be total number of nucleons in that element x total number of nucleons into mass of 1 nucleon. Okay, mass of 1 nucleon. Now suppose if you keep this as it is like 1 by 12 into mass of 1 C12 atom was what? 1 by 12 into 12 into mass of 1 nucleon. Here we have seen this particular term I am placing in the denominator of this r.a.m. Now what will happen? This 12 and 12 gets cancelled. Mass of 1 nucleon and mass of 1 nucleon gets cancelled. So what will be the value of our relative atomic mass? It will be nothing but the total number of nucleons that is present in any element. It will have no unit. Okay, it will have no unit 1 because it is relative atomic mass. So it states that how much heavier? Suppose we have 1 amu. Now if I say the relative atomic mass of any particular element is 19. So what am I referring to? I am referring to that this element x is 19 times heavier than 1 amu. Okay, as compared to 1 amu this particular element x is how many times heavier? 19 times heavier. So definitely it will have no units. Now, so we need to remember this particular thing from this term that relative atomic mass is what? Is equal to the total number of nucleons that are present. What is happening? Fine. Let it be uneven only. Correct. Now the next term. The next term is our atomic mass. The first one was relative atomic mass. Now it is atomic mass. Now what is atomic mass? Atomic mass is nothing but mass of 1 atom of the element. Now this term also you have seen somewhere. You have seen it in the definition of relative atomic mass. In relative atomic mass you can see the numerator part mass of 1 atom of the element. So that is nothing but our atomic mass. So here we can say that in relative atomic mass what did we see? Relative atomic mass is equal to this entire thing. Now one atom of an element which we can call as atomic mass divided by one twelfth the mass of one C12 atom which is known as 1 amu. What will be atomic mass from here? Atomic mass will be equal to relative atomic mass into 1 amu where our relative atomic mass is what? Total number of nucleons. So for example let's say let's do a chart itself. Let's see what the relative atomic mass and atomic mass will be. For example we are taking elements here and here relative atomic mass and here atomic mass. So suppose we have Na sodium 2311. So this 2311 if you ask me ma'am how to remember this. So definitely you have studied periodic table in 10 grade. So for now at least you should remember till atomic number 20. So from there only you have to memorize all this. And then let's say for example another one I am taking calcium 4020. So for these two we will see. So relative atomic mass is nothing but the total number of nucleons. And total number of nucleons is our superscript one that is our A. This is A for sodium and this is A for calcium. So if I write the relative atomic mass it will be just 23 and for calcium it will be 40. Now atomic mass atomic mass just now we have written that atomic mass is equal to relative atomic mass into 1 amu. So definitely for sodium it will be 23 into 1 amu so it will be 23 amu and here it will be 40 amu. Now this we have found out the atomic mass in atomic mass unit or Dalton. But suppose in the question it is asked find out the atomic mass for example one more we will do. Let's say fluorine 19 9. So you have to find the atomic mass of fluorine in grams atomic mass of fluorine in grams. So how to find out first you know that relative atomic mass will be how much total number of nucleons that is nothing but our 19. And then what will be our atomic mass in atomic mass atomic mass will be our 19 amu. Now we know that 1 amu is equal to 1.66 into 10 to the power minus 24 grams. So 19 amu will be equal to 19 into 1.66 into 10 to the power minus 24 grams. So this particular multiplication term will give you a certain value which will be the atomic mass of fluorine in grams. And this is the atomic mass of fluorine only atomic mass that is relative atomic mass into 1 amu. So for this also if we are asked to find out in grams it will be 23 into 1.66 into 10 to the power minus 24 grams. And for calcium it will be 40 into 1.66 into 10 to the power minus 24 grams. I hope this part is entirely clear to you. Moving on next term that we have is moles, fourth term moles. What is a mole? Mole is a quantity with which we measure any amount of substance. And mole represents n a number of entities, number of entities. What does that mean? Entities can be anything atoms, molecules, ions. Okay, anything it can be. If I say 1 mole of pencil then that means it contains that 1 mole of pencil means how many total pencils are there? N a number of pencils are there. Now what is the value of this N a? This N a value will be equal to this is known as Avogadro's number. That same person who has given that Avogadro's law, what was Avogadro's law? Volume of any gas equal volume of all gases contains equal number of molecules not atoms, molecules. So that same person has given this N a. This is a very special number which you need to remember in your entire 11th and 12th grade. Okay, so N a is equal to Avogadro's number. Now remember this magical number 6.023 into 10 to the power 23. So when I say 1 mole of pencils, it means 6.023 into 10 to the power 23 pencils. When I say 1 mole of water bottle, I mean 6.023 into 10 to the power 23 water bottles. So like that 1 mole atoms will contain N a atoms, 1 mole molecules will contain N a molecules and so on. Now this thing is very very important. Please mark it. You have to remember this value. Now coming on to the, we will talk more about this moles later. We will be seeing how to calculate moles and stuff but before that some terminologies are left. We will complete that first. Next is our gram atomic mass. Gram atomic mass. What is gram atomic mass? Now atomic mass was mass of 1 atom of element. Gram atomic mass is the mass of or atomic mass of 1 mole of atoms or we can also say or atomic mass atoms. N a atoms is nothing but 1 mole of atoms only. So atomic mass of 1 mole of atoms or atomic mass of N a atoms. Now for example if let's start with the question itself. For example the question is find gram atomic mass of oxygen, nitrogen. So how to represent an atom of oxygen? 16, 8 and if I represent nitrogen it will be 14, 7. Again I am telling this is coming from periodic table. Don't get confused. You have already studied this and again. This is our mass number. This is our atomic number. Now relative atomic mass was what? Relative atomic mass for oxygen will be 16. For the atomic mass will be what? Atomic mass will be 16 into 1 amu that is 16 amu. Now I am finding out this atomic mass in grams. This is an amu. So if I convert this into grams it should be multiplied with 1.66 into 10 to the power minus 24 grams. This is the amount in atomic mass of oxygen in grams. So this is the mass of 1 atom of oxygen. So gram atomic mass that is our mass of N a atoms of what? 1 atom of oxygen mass is this much. So for N a atoms it will be N a into 16 into 1.66 into 10 to the power minus 24 grams. Now we need to focus on two things one is this N a another one is this 1 amu value. Now this value 1.66 into 10 to the power minus 24. This value is equivalent to 1 upon N a. When we will do mathematically 6 1 upon 6.023 into 10 to the power 23 we will get what? 1.66 into 10 to the power minus 24 and this will come in grams. Don't confuse this will come in grams okay not kg don't put minus 27 kgs. So when you are taking avocados number whatever value this is coming this will be in grams. So now definitely in this value place I can write as 1 upon N a. Let's see it will be N a into 16 into 1 upon N a. Grams N a in a gets cancelled. So what will be the gram atomic mass of oxygen? It will be 16 grams it will be 16 grams. So I want you to try for this particular substance nitrogen. This will be this you do alone. Okay so definitely you will get it as 14 grams. Right now what important point here is they don't get confused look at the question. Read the question what they're asking if they say atomic mass in grams. Then it will be what 16 into 1.66 into 10 to the power minus 24 grams if I specifically talk about oxygen over here. Okay so if they're asking atomic mass in grams then it will be 16 into 1.66 into 10 to the power minus 24 grams. But if they're asking gram atomic mass then it will be what for oxygen only if I say it will be 16 grams. Remember this this is very very important don't get confused in these two. Atomic mass in grams and gram atomic mass. Gram atomic mass means the mass of one mole of oxygen atom. But here in this case they are asking regarding the mass of one atom only in grams. The unit just you have to change. Right now coming on to the next term that is our molecule. What is the molecule? Now we will write certain points regarding molecules. We know that atoms will combine with each other to form molecules. We have studied regarding the laws of chemical combination and we have seen there that atoms do combine gets combined in fixed whole number ratio not whole number ratio fixed composition. Whole number ratio that is different thing. Now the atoms are getting combined in fixed proportion. Proportion is a correct word fixed proportion to form certain molecules and compounds. Now so we can say that a molecule it is the smallest particle which has a free existence which will have a free existence. So it is the smallest particle which will have a free existence by its own. And definitely a particular molecule can be we can break it further into atoms. A molecule we can break it further into atoms by different chemical and physical methods. And atomicity I have already discussed before. What is atomicity? Atomicity is nothing but number of atoms present in a molecule. Number of atoms present in a molecule. For example H2O. How many atoms are there? 2 hydrogen atoms plus 1 oxygen atoms. So atomicity is 3. For example H2SO4. So it has 2 atoms of hydrogen, 1 atom of sulphur and 4 atoms of oxygen. So the atomicity will be total 2 plus 1 plus 4 that is 7. For example O3 it contains 3 oxygen atoms. So the atomicity will be 3. So this is how we calculate atomicity. I had already discussed this under Avogadro's law. So I hope you have understand this atomicity part. Now coming on to the next term that is our... Now there are 2 terms that are left we will study together on that will be helpful. Molecular mass, gram molecular mass. Molecular mass and gram molecular mass. So if I talk about molecular mass. Molecular mass means mass of 1 molecule. And what is gram molecular mass? It is the mass of 1 mole of molecules. Or we can say mass of any number of molecules. Any number of molecules. So we will see certain molecular masses and gram molecular masses of certain molecules. So for example I am taking here molecules. Here I will write the molecular mass and here the gram molecular mass. So for example H2. So what will be the molecular mass of H2? It will be... There are 2 hydrogen atoms here. So 1 plus 1. 1 hydrogen atom weighs 1 amu. So this molecular mass will be expressed in amu. Gram molecular mass will be expressed in grams. So it will be 2 amu. And this one will be 2 grams. See we have already calculated in case of atomic mass and gram atomic mass. That whenever they are asking atomic mass. Then it will be the total number of nucleons. The numeral value will be the total number of nucleons. And whenever they are saying atomic mass or molecular mass. It will be expressed in amu. And if they are talking about gram atomic mass or gram molecular mass. Then it will be grams. And the numeral value will be the total number of nucleons that are present. So in molecules what happens? We add the total number of nucleons of each atom that is present in the molecule. For example let's say H2. There are 2 hydrogen atoms and 1 oxygen atoms. So 2 hydrogen means how many nucleons? 2 and 1 oxygen means how many nucleons? 16. So molecular mass will be total number of nucleons. And the unit will be amu. So it will be 18 amu. And gram molecular mass will be nothing but 18 grams. Now let's say one more example we will take. H2SO4. 2 hydrogen total number of nucleons is 2. Plus 1 sulphur. 1 sulphur how many nucleons it contains? Sulphur contains 32 nucleons. And there are 4 oxygen. So 1 oxygen contains 16. 1 oxygen contains 16 nucleons. So 4 oxygen will contain 4 into 16. So it will be 64. So molecular mass will be 2 plus 32 plus 64. These are the total nucleons of all the atoms that are present in H2SO4. So this will be the answer that is our 98 amu. And what will be gram molecular mass? It will be 98 grams. So I hope this particular gram molecular mass and molecular mass is clear to you. So this difference again here that important term will come. That molecular mass if they are asking in grams. Suppose this example, what are example I am taking. So they are asking molecular mass in grams. Then we will just multiply 18 into 1.66 into 10 to the power minus 24 grams. But if they say gram molecular mass it will be 18 grams only. So this particular point you have to take note of. So we have covered all the terms. And next topic will be our calculation of moles which we will cover in the next video. Thank you so much. See you in the next class.