Atomic Number, Mass Number and Related Terms

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle before we start. Take carbon monoxide, C O, and nitrogen gas, N two. One is made of a carbon and an oxygen, the other of two nitrogens. Totally different, right? And yet, in one important way, these two are exact twins. What could they have in common? Come on, think about it. By the end of this video, you will answer it in a few seconds. And look, JEE questions often test protons, neutrons and electrons, and these iso words: isotopes, isobars and more. So, let's begin.

0:38Reading an atom's symbol

First up, how do we write an atom? Capital X here stands for the symbol of the element, okay? And notice, it carries two numbers on its left: capital A on top, as a superscript, and capital Z at the bottom, as a subscript. Every question today starts by reading these two numbers. Clear? So, the bottom number, capital Z: what does it count? Yes, the protons! Z is called the atomic number, and it equals the number of protons, okay? And in a neutral atom, with no charge overall, the electrons balance the protons. So the number of electrons also equals Z, okay? But only in a neutral atom! For ions this changes, and example one shows you how. Now the top number, capital A, the mass number. It counts protons plus neutrons, both together. Electrons are not counted in it. So think. If A counts protons plus neutrons, and Z counts only the protons, how do you get the neutrons? Simple, subtract! The number of neutrons, capital N, equals A minus Z, that's it. Z, A and N: everything today is built on these three letters. Got it?

1:52Same what? Isotopes, isobars, isotones

Alright, now a family of words that all start with iso. Each one says that something is the same. The question is always: same what? First, isotopes: same Z, but different A, okay? Like hydrogen, with mass numbers one, two and three written on top. All hydrogen, so all the same Z, but different mass numbers. With me? Now think. Isotopes have the same Z, so a neutral atom of each has the same number of electrons. Will they behave the same way in chemistry? Yes, exactly! Same electrons, so the same chemical properties. But the masses differ, so the physical properties, like mass and density, are different. Okay? Next, look at these two: argon forty, with Z eighteen at the bottom, and calcium forty, with Z twenty. Both have mass number forty. So, same mass number, same element, right? No, no, careful! This is the trap. Look at the bottom numbers: eighteen and twenty. Different Z, so different elements, argon and calcium. Same A, different Z: these are isobars. Clear now? Next, isotones: the same number of neutrons, capital N, okay? Carbon fourteen: fourteen minus six, eight neutrons. Oxygen sixteen: sixteen minus eight, eight again. Different elements, different mass numbers, but the same N, so they are isotones. Now, a topper's trick to never mix these up. Look at the bold letters. Isotopes, with the letter P: same protons. Isobars, with the letter A: same capital A, the mass number. Isotones, with the letter N: same neutrons. Really easy to remember, right?

3:36Isoelectronic species and isodiaphers

Next, isoelectronic: the same number of electrons. Now the species can be completely different, atoms or ions, okay? Only the electron count matters. Look at this famous family: N three minus, O two minus, F minus, neon, N a plus, M g two plus, and aluminium three plus. The little plus and minus signs on top, with their numbers, are the charges, okay? All seven have ten electrons each. The negative ions have gained electrons, the positive ions have lost them, and all of them land on ten. Got it? And for molecules and polyatomic ions? Add up the electrons of all the atoms, then adjust for the charge: a minus charge adds electrons, a plus charge removes them. Remember this; example two needs it. Last one: isodiaphers, the same N minus Z, neutrons minus protons. Uranium two hundred and thirty five, with Z ninety two, and thorium two hundred and thirty one, with Z ninety. Check it yourself: same N minus Z? Uranium: two hundred and thirty five minus ninety two gives one hundred and forty three neutrons; minus ninety two, that is fifty one. Thorium: two hundred and thirty one minus ninety, one hundred and forty one; minus ninety, again fifty one. Yes, the same! Isodiaphers.

4:58Example 1: protons, neutrons, electrons in Cl- and Fe3+

Now, example one. Find the numbers of protons, neutrons and electrons in chlorine thirty five, C l minus, with Z seventeen, and in iron fifty six, F e three plus, with Z twenty six. Pause the video, and try it first. Done? Okay, let's check. Chlorine first. Small p for protons: p equals Z, seventeen. Small n for neutrons: A minus Z, thirty five minus seventeen, eighteen. Electrons, small e. The minus sign on top means one extra electron. So seventeen plus one, eighteen electrons. With me? Now iron. Protons: twenty six. Neutrons: fifty six minus twenty six, thirty. Now the electrons. The charge is three plus, and plus means add, doesn't it? So twenty six plus three, twenty nine electrons, right? No, no, careful! This is the JEE trap. Three plus means the atom has lost three electrons. So twenty six minus three: twenty three electrons. Remember: plus charge, fewer electrons; minus charge, more electrons. Got it?

6:10Example 2: which species are isoelectronic with CO

Now a harder one, JEE level, and it brings back our puzzle. Which of N two, O two, C N minus and N O plus are isoelectronic with C O? The small two at the bottom means two atoms, okay? Pause the video, and try it first. Done? Okay, let's check. Start with C O: carbon gives six electrons, oxygen gives eight. Six plus eight, fourteen electrons. Now N two: two nitrogens, seven each. Seven plus seven, fourteen. The tick mark means yes! And look, our puzzle is solved: C O and N two are the twins. Both have fourteen electrons, so they are isoelectronic! O two: eight plus eight, sixteen. Not fourteen, so the cross mark: it is out. Now C N minus. Carbon six, nitrogen seven. And the minus charge: do we add one electron, or remove one? Yes, add! Minus means one extra electron. Six plus seven plus one, fourteen. Tick! Now you do N O plus. Quick, how many electrons? Nitrogen seven, plus oxygen eight, minus one for the plus charge: fourteen. Tick! If you got that, you are absolutely right. So three of the four are isoelectronic with C O: N two, C N minus and N O plus. Only O two is out. See? Just a few seconds, as I promised at the start.

7:39Quick revision: Z, A, N and the iso family

So, quick revision. Z counts the protons, A counts protons plus neutrons, and N equals A minus Z, okay? For ions, a minus charge adds electrons, and a plus charge removes them. And the iso family: isotopes, same Z; isobars, same A; isotones, same N; isoelectronic, same electrons. Now go and try the practice problems on this page. You will find them easy, I promise. All the best!