The atomic number represents the number of protons in an atom's nucleus.
When referring to a molecule, such as Br2 (a molecule of two bromine atoms), the atomic number does not change. Each bromine atom within the Br2 molecule still has an atomic number of 35. So, the atomic number of Br2 is still 35.
Now, let's consider Fe2O3, which represents a molecule of iron(III) oxide. In this case, Fe2O3 consists of two iron (Fe) atoms and three oxygen (O) atoms. To find the atomic number for Fe2O3, we need to consider the atomic numbers of the constituent elements.
The atomic number of iron (Fe) is 26, so for two Fe atoms, we have a total atomic number of 26 × 2 = 52.
The atomic number of oxygen (O) is 8, so for three O atoms, we have a total atomic number of 8 × 3 = 24.
To find the total atomic number for Fe2O3, we sum up the atomic numbers of the constituent atoms:
Total atomic number = Atomic number of Fe × number of Fe atoms + Atomic number of O × number of O atoms = 52 + 24 = 76
Therefore, the atomic number for Fe2O3 is 76.