Final answer:
A molecular formula indicates the number of each type of atom in a molecule, with subscripts used to show quantity; coefficients are used for balancing chemical equations by altering the number of molecules, not by changing subscripts.
Step-by-step explanation:
In chemistry, understanding molecules and their composition is fundamental. The molecular formula such as MgCl2 for magnesium chloride tells us the types of atoms and how many of each are in a molecule. The number of atoms of each element is indicated by a subscript. The subscript '2' after Cl in MgCl2 indicates there are two chlorine ions needed to balance the charge of one magnesium ion with a +2 oxidation state. If there is no subscript written, it is understood to be '1', as in the case for magnesium here. To balance chemical equations, we use coefficients to indicate how many molecules of a substance are involved, while subscripts refer to the number of atoms within a single molecule of that substance. One must not change the subscripts to balance a chemical equation; balance must be achieved by adjusting the coefficients.
When asked for the chemical formula of a compound like dinitrogen trioxide, it is written as N2O3. Here, the subscripts indicate two nitrogen atoms and three oxygen atoms are present in each molecule of the compound. In ionic compounds, such as sodium iodide (NaI), calcium bromide (CaBr2), or potassium chloride (KCl), the atoms gain or lose electrons to form ions, which are represented in their Lewis dot diagrams.