Define-
1. Atom
2. Molecules
3. Elements
4. Compound
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1. Atom: Smallest unit for living matter

2. Molecules: A group of Atoms bounded together

3. Elements: The smallest substance that cannot be broken down into simpler substances

4. Compound: A group of Elements bounded together

Explanation:

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the smallest unit of ordinary matter that forms a chemical element

group of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction.

pure substance consisting only of atoms that all have the same numbers of protons in their nuclei.

chemical substance composed of many identical molecules composed of atoms from more than one element held together by chemical bonds.