What cannot be broken down into other substances by chemical means?

An element is a fundamental substance that cannot be broken down into simpler substances through chemical means. Elements are the basic building blocks of matter, and each one is defined by the number of protons in its nucleus, which is known as its atomic number. For example, hydrogen and oxygen are both elements, and they cannot be decomposed into any simpler substances while retaining their essential properties.

In contrast, compounds can be chemically broken down into their constituent elements. A compound consists of two or more elements that are chemically combined in fixed ratios. Mixtures, on the other hand, are physical combinations of two or more substances that can be separated by physical means, such as filtration or distillation. Electrons, while fundamental particles that contribute to elements, cannot technically be substances themselves as they are not composed of smaller components and thus do not fit the same classification as elements, compounds, or mixtures. Therefore, among the options, the correct answer is indeed that an element cannot be broken down into other substances by chemical means.