Question

Why is Carbon dioxide written as co2 whhy not 02c?

Answer 1

The positive ion(s) in any compound is to appear first, which is followed by the negative ion(s). In this case, even though Carbon isn't positive, it has a slightly less electronegativity than the two Oxygen atoms altogether, making it partly positive. Therefore, Carbon has a partial positive charge and Oxygen a partial negative charge (dipole-dipole moment). Partial charges are denoted as δ+ (delta plus) and δ- (delta minus). This is something termed as polarity in chemistry. A polar covalent compound consists of partially charged particles. 

Remember:
The positive ion goes to the left-hand side.
The negative ion goes to the right-hand side. 

To sum up,
as Carbon has a positive charge, it is written first. 
And, the double Oxygen which bears a negative charge appears later.

The image (attachment) below may promote better understanding of the concept.

Hope this helps you! :-)

Answer 2

The positive ion(s) in any compound is to appear first, which is followed by the negative ion(s).