Which of the following best defines an element?

• A. A substance with atoms that have the same number of neutrons
• B. A substance composed of molecules containing different types of atoms
• C. A substance that can be broken down into simpler substances by chemical means
• D. A substance composed of atoms that all have the same number of protons

Answer: (D)

Verified step by step guidance

1. Recall the definition of an element in chemistry: an element is a pure substance consisting entirely of one type of atom.
2. Understand that atoms of an element all have the same number of protons, which is called the atomic number. This distinguishes one element from another.
3. Note that the number of neutrons can vary within atoms of the same element, leading to different isotopes, so having the same number of neutrons is not a defining feature of an element.
4. Recognize that a substance composed of molecules containing different types of atoms is a compound, not an element.
5. Remember that an element cannot be broken down into simpler substances by chemical means, which differentiates it from compounds and mixtures.