Assalam O Alaikum! My name is Ghulam Rasool And the topic on which we are going to discuss is RELATIVE ATOMIC MASS The Word Relative means, we are relating with something. Now with what, we are going to relate it to? We will relate it with some Standard. Gone above your head?? Didn't Understand What I said??? OK, I will make it understandable [For Example] You went to a vegetable shop. You Picked up a piece of Cabbage. You didn't knew its mass You placed a Standard weight on one side [of Weight Balance] And You placed the Cabbage on other side and then you compared both masses and found which mass is higher and which one is lower Or Which mass is equal. Then you adjusted both in such a way [the both get balanced] and you found that Ok!! The mass of this Cabbage is 1 Kg. Understood?? It was an example from Daily life It will be very easy for you if you understood [the example] Listen again You went to buy cabbage Or instead you went to buy any other vegetable. i.e potatoes You went to buy potatoes You got there and started adding potatoes [on the balance] You need 1 kg Potatoes but you don't know their mass So with any standard weight whose mass you already know Like 1 kg or 2kg weight We call it 2 kg because we know that it is exactly 2 kg Now, what is the use of this in chemistry? because this example was related to everyday life. how would we adjust this example in chemistry? This is exactly the same as Relative Atomic Mass Now i will tell you, The definition written in books is: 1/12th part of the carbon-12 isotope Now You didn't understand a single word? It passed over your head? I will explain it to you, Easy! Carbon-12 Is an isotope Isotope Means? That it also has other forms. It includes carbon 13 and 14 Carbon-12 is the most abundant Carbon-13 is less abundant And carbon-14 is radioactive Radioactive as I described in previous lectures, means that it emits rays [alpha & beta etc] Both of these are not our topic today. our topic is discussion is Carbon-12 The line in the book states that: I/12th part of carbon-12 isotope These things are not linking right now but Now they will link. Stay Tuned ! This is a circle If we say this Circle as 1 And if i divide this circle from center into Two parts then I can call it as 1/2 And if i divide this circle into four parts then I can call it as 1/4 And if i divide this circle like this then I can call it as 1/8 Getting my point? It means that I am dividing this one thing into small sections. Now what actually is Relative Atomic Mass? We took carbon-12 isotope An atom of Carbon-12 And we divided that atom into 12 equal parts After Dividing this into 12 parts. We then separated one of those 12 parts [1/12th] part Now we set this 1/12th part as a standard after separating What standard? That this 1/12th part of carbon-12 is a standard weight Now we have to relate this with another specie to find its weight And what we will call it? We will call it Relative Atomic Mass Now after that what we did? We said that this 1/12th part is equal to 1amu So what is atomic mass unit? Atomic Mass Unit is the Mass of 1/12th part of Carbon-12 isotope We defined the separated part as a Unit