Cell notation is like writing a story about how chemical reactions happen inside a battery. The battery has two parts, a negative part and a positive part. The negative part is where electrons come from, and the positive part is where they go. So, we write this story like this:
"Negative part | Positive part"
Here's an example using a real battery:
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
Imagine you're playing with blocks. You see a block of Zinc on the left side, and a block of Copper on the right side. You know that Zinc likes to give away electrons, and Copper likes to take them. So, you know that the Zinc block is the NEGATIVE part, and the Copper block is the POSITIVE part.
Now, we need to write down what's happening to the Zinc as it gives away electrons, and what's happening to the Copper as it takes them. We write this down using special letters, called symbols.
For example, Zn(s) means "Zinc as a solid," and Zn2+(aq) means "Zinc as an ion floating around in water." The two vertical lines (||) in the middle of the notation tell us that there's a TINY LITTLE GAP between the two parts, where the electrons are zooming from the Zinc to the Copper.
Now, let's write the story. On the left side, we have:
Zinc as a solid (Zn(s))
This means we have a block of Zinc.
On the right side, we have:
Copper as a solid (Cu(s))
This means we have a block of Copper.
In the middle, we have a tiny gap, like a bridge:
||
This is where the electrons are zooming across from the Zinc to the Copper.
And finally, we write down what's happening to the Zinc and the Copper using arrows and special letters:
Zinc (Zn) is giving away two electrons (e-) to become Zinc ions (Zn2+), and Copper ions (Cu2+) are taking those electrons to become Copper (Cu) again.
So, the entire cell notation looks like this:
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
This is how we write down the story of a battery using cell notation.