Atom vs Element vs Molecule vs Compound vs Mixture

Definition: An atom is the smallest unit of an element that retains its chemical properties. It consists of a nucleus (made up of protons and neutrons) and electrons that orbit the nucleus.
Example: A hydrogen atom (H) or an oxygen atom (O).
Key Point: Atoms are the basic building blocks of matter and cannot be broken down into simpler substances by chemical means.

Definition: An element is a pure substance made up entirely of one type of atom. Each element has a unique number of protons, called the atomic number.
Example: Oxygen (O), carbon (C), or gold (Au).
Key Point: Elements are listed in the periodic table and cannot be chemically broken down into simpler substances.

Definition: A molecule is a group of two or more atoms chemically bonded together. Molecules can be made of the same element or different elements.
Example: Oxygen gas (O₂) consists of two oxygen atoms bonded together, while water (H₂O) consists of two hydrogen atoms and one oxygen atom bonded together.
Key Point: Molecules are the smallest units of compounds or elements that retain their chemical properties.

Definition: A compound is a substance formed when two or more different elements chemically bond in a fixed ratio. Compounds have properties different from the individual elements that form them.
Example: Water (H₂O) is a compound made from hydrogen and oxygen; sodium chloride (NaCl) is a compound made from sodium and chlorine.
Key Point: Compounds can be broken down into simpler substances (elements or simpler compounds) through chemical reactions.

Definition: A mixture is a combination of two or more substances (elements, compounds, or both) that are physically mixed but not chemically bonded. The components of a mixture retain their individual properties.
Example: Air is a mixture of gases (like oxygen, nitrogen, and carbon dioxide), and seawater is a mixture of water, salts, and other substances.
Key Point: Mixtures can be separated into their components by physical means, like filtration or distillation.

Summary Table of Basic Science Terms often confusing

Term	Definition	Examples	Key Characteristics
Atom	Smallest unit of an element	H, O, C atoms	Basic building block of matter
Element	Pure substance of one type of atom	O (Oxygen), C (Carbon)	Unique number of protons, cannot be broken down
Molecule	Two or more atoms bonded together	O₂, H₂O	Smallest unit of a compound or molecular element
Compound	Substance with two or more elements chemically bonded	H₂O, NaCl	Fixed ratio, distinct properties from elements
Mixture	Physical blend of substances	Air, seawater	Components retain properties, separable by physical methods