Element definition.

• A. A basic chemical substance that cannot be divided into a simpler substance.
• B. A mixture of two or more substances.
• C. A pure substance that can be split into simpler substances by physical means.
• D. An energy form in chemical reactions.

Answer: (A)

In chemistry, an element is a basic chemical substance that cannot be divided into simpler substances by chemical means. This means you can’t break an element down into other substances through ordinary chemical reactions; elements can only be separated into smaller parts by nuclear processes, which are outside typical chemical methods. Elements are defined by the number of protons in their nucleus (the atomic number) and can exist on their own or as parts of molecules (like oxygen as O2, which is two oxygen atoms bound together but still the same element). They combine with other elements to form compounds, yet the element itself remains a distinct substance.

The option that states this exactly matches the idea. The other descriptions don’t fit: a mixture is made of two or more substances, not a single element; a pure substance that can be split into simpler substances by physical means would imply it isn’t an element; and an energy form in chemical reactions isn’t a substance at all.