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Atomic Structure

Balancing Equations.

The formula of a compound cannot be changed.
The formula is shown by small numbers written after the element.

KCl  is an ionic compound.
KCl  means one potassium ion with one chlorine ion
(the number one is never written, KCl  = K₁Cl₁).
Na₂O  means two sodium ions with one oxygen ion.
Al₂O₃ means two aluminium ions with three oxygen ions.

The reaction between potassium and chlorine.

(1)    K  +  Cl₂     KCl
The above reaction is not balanced.
On the left side of the arrow, there is 1K and 2Cl.
The two chlorines in Cl₂ cannot be changed,
since Cl₂ is the formula of chlorine.
On the right side of the arrow, there is 1K and 1Cl.
The right side needs an extra chlorine.

A big number changes the number of elements
in the formula which follows it.
2K means two potassium atoms. 2Cl₂ means four chlorine atoms.
2KCl means two potassium ions with two chlorine ions.

To get an extra chlorine on the right side, we can put a 2 before KCl.
The equation becomes
(2)    K  +  Cl₂     2KCl
Now the equation is balanced for chlorine, 2 on each side.
But the equation has now become unbalanced for potassium.
There is 1 on the left and 2 on the right.
To get an extra potassium on the left, we can put a 2 before K.
(3)    2K  +  Cl₂     2KCl
The equation is now balanced.
On the left there are 2 potassiums and 2 chlorines.
On the right there are 2 potassiums and 2 chlorines.

Showing the three steps together, we have

(1)    K  +  Cl₂     KCl      unbalanced for chlorine.
(2)    K  +  Cl₂     2KCl     unbalanced for potassium.
(3)    2K  +  Cl₂     2KCl                   Balanced!

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