Final answer:
A substance composed of atoms with the same atomic number is an element, not a heterogeneous mixture. Elements are pure substances and include examples like gold and selenium, whereas heterogeneous mixtures have a non-uniform composition.
Step-by-step explanation:
The statement that a substance composed only of atoms with the same atomic number is classified as a heterogeneous mixture is incorrect. A substance consisting solely of atoms with the same atomic number is known as an element. Elements are pure substances that cannot be broken down into simpler substances by chemical means. For example, gold and selenium are elements because they consist of only one type of atom. On the other hand, a compound is a substance made from two or more different types of atoms chemically bonded together, such as carbon dioxide or water.
A homogeneous mixture, also known as a solution, has a uniform composition throughout, like salt water or filtered tea. Lastly, heterogeneous mixtures, such as soil or freshly squeezed orange juice, have a non-uniform composition and different components that are visually distinct.