Final answer:
A chemical equation is balanced when there is an equal number of atoms on both sides, fulfilling the law of conservation of mass, and the total charge is also balanced for ionic species.
Step-by-step explanation:
The condition that must be met in order for a chemical equation to be balanced is a) Equal number of atoms on both sides of the equation. This ensures that the chemical equation abides by the law of conservation of mass. To achieve this, one may need to insert coefficients in front of the reactants or products in a chemical equation until there are the same number and types of atoms on both sides of the arrow.
Moreover, it's important to factor in the diatomic nature of certain elements (H2, N2, O2, F2, Cl2, Br2, I2) and use subscripts where appropriate to indicate their molecular forms. In addition to the atoms being equal, the total charge must also be balanced, this is particularly important when dealing with ionic species.