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Atomic Structure

How to Balance Chemical Equations - Example 1.

When we are balancing chemical equations we first
need to know what the symbols and numbers mean.

Example 1 is the reaction between potassium and chlorine.

(1)    Cl2     KCl
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 reaction is not balanced
because atoms are not gained or lost in a chemical reaction.
The right side needs an extra chlorine.

A big number in front of the symbol
changes the number of elements in the formula that follows it.
2
K means two potassium ions. 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)
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)    2
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)    Cl2     KCl      unbalanced for chlorine.
(2)
Cl2     2KCl     unbalanced for potassium.
(3)    2
Cl2     2KCl                   Balanced!

See example 2.   The reaction of lithium with oxygen.