Welcome to the bustling cafeteria of Bright Minds School! The air was filled with the delicious aroma of hot food and the happy chatter of students. But did you know, amidst all the fun, there were secret “Roti Rules” at play?

Forget boring chemical equations! Let’s learn the rules of oxidation numbers with a story straight out of your school cafeteria! We call them the ‘Roti Rules.’ Our baseline assumption being, everyone needs 2 rotis to be satisfied.

Why Oxidation Numbers?

But before we enter our café, let’s quickly understand what an oxidation number is and why it matters?

Oxidation number is simply a hypothetical charge assigned to an atom in a molecule or an ion. It tells us how many electrons an atom has gained or lost. Similar to how many roti’s a student has shared. These are your roti rules. If you give your oxidation number is positive and if you take your oxidation number is negative. So for example: give 2 samosas and your oxidation number is +2 but if you take   3 laddoos your oxidation number is -3. And if you don’t share, your oxidation number stays at zero

So, in our cafeteria; imagine students to be atoms and rotis they share as electrons shared. Using these roti rules lets crack how these oxidation numbers are assigned. So, let’s start

First, we have Rohan, who’s a total loner. He’s brought his own tiffin, and he’s perfectly content. He doesn’t bother anyone, and nobody bothers him. This guy is your free element. He’s not bonded to anyone else, so his score is a perfect zero. He’s chill.

In Free Elements the oxidation number of an atom in its elemental form (like pure Oxygen, O₂, or pure Sodium, Na) is always 0.

Then there’s Dev, who just has a sabzi or just a roti. He’s comfortable, but he feels like he’s either in surplus or deficit by one item. He’s your monoatomic ion, with a simple charge of +1 or -1.

For a simple Monatomic Ions made of only one atom (like Cl⁻ or Na⁺), its oxidation number is equal to its charge. So, Cl⁻ has an oxidation number of -1, and Na⁺ has +1.

Now, let’s talk about the givers! This student, Priya, always, always brings one extra roti and is happy to share with anyone who needs it. These are your Alkali Metals—the Group 1 donors. Their generosity gives them a permanent score of plus one!

Group 1 Metals (Alkali Metals) like (Li, Na, K, etc.)  In compounds, always have an oxidation number of +1.

And then there’s Akash, the truly generous one, who brings two extra rotis every single day! These are your Alkaline Earth Metals, with a permanent score of plus two!

Group 2 Metals (Alkaline Earth Metals) like (Be, Mg, Ca, etc.)  In compounds, always have an oxidation number of +2.

But not everyone is so nice. There’s Karan, a bully who will demand food from you and sometimes even snatch it! These are your hungry Halogens! They’re so greedy, their score is almost always minus one. UNLESS… they run into a bigger thug. Veer, your Fluorine

Halogens (Group 17 elements – Cl, Br, I) are usually -1 in binary compounds with less electronegative elements (e.g., NaCl). They can have positive oxidation numbers when bonded to more electronegative elements like oxygen or fluorine (e.g., Cl in HClO₄).

This is Veer, your fluorine, the undisputed don of the cafeteria. He’s the greediest of them all and will snatch from anyone. Even his friends. So his score is always -1!

In all its compounds, Fluorine (F) always has an oxidation number of -1. It’s the most “greedy” for electrons!

And then we have Rahul, this one chirpy guy. He usually has one roti extra and is happy to give it away. But when he hangs out with the rich kids who have fancy burgers, he’s more than happy to borrow one instead! This is your Hydrogen. Usually plus one, but can be minus one when he’s with the really rich kids—the metals!

Hydrogen is usually +1 when bonded to non-metals (e.g., H₂O, HCl) but is -1 when bonded to metals (these are called metal hydrides, e.g., NaH).

And finally, Pooja, a dog lover, is always short of 2 rotis and keeps borrowing. Normally, she gets two. But sometimes, when she sees a stray dog, she gives away both those rotis and goes home hungry! This is your Oxygen! She’s usually minus two. But sometimes, she gets just one, making her minus one in peroxides. And when she’s with that don, Veer, she’s willing to give both rotis away, giving her a score of plus two!

Oxygen is usually -2 in most compounds (e.g., H₂O, CO₂). but is -1 in peroxides (compounds with an O-O bond, e.g., H₂O₂, Na₂O₂). it becomes +2 when bonded to fluorine (e.g., OF₂), as fluorine is more electronegative.

And lastly, let’s talk about groups. The twins, Maya and Tara, sitting together are a perfectly content group. Whatever they have, they’ve already shared among themselves, so their total score is a perfect zero! For a neutral molecule or compound, the sum of the oxidation numbers of all atoms is zero.

But the sports team, they are the polyatomic ions. Their total score, or charge, is the sum total of all their individual rotis! So, we add up all their individual scores to find the group’s overall charge!

For example in a polyatomic ion like SO₄²⁻ which is made of multiple atoms, the sum of the oxidation numbers of all atoms equals the charge of the ion. So in this case it is -2