How is a balanced chemical equation defined?

A balanced chemical equation is defined by the same amount of each type of atom on each side. This means that the number of atoms for each element involved in the reaction must be equal in both the reactants and the products. This principle is based on the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction. Therefore, balancing an equation ensures that the mass and the number of atoms remain consistent throughout the reaction, thus reflecting the true nature of the transformation that is occurring.

While the other choices bring up interesting points, they do not accurately capture the definition of a balanced equation. For instance, the presence of an equal number of protons does not necessarily correlate with balancing the equation, as it focuses solely on the subatomic structure rather than the atomic quantities. Likewise, the equal mass of reactants and products is a result of balancing but is not itself the definition—it's about the count of atoms. Lastly, the phases of matter being the same for all compounds is not a requirement for a balanced equation, as reactions may involve reactants and products in different phases (solid, liquid, gas).