Aspect | Atom | Element | Molecule | Compound |
|---|---|---|---|---|
Definition | An atom is the smallest unit of matter consisting of a central nucleus with protons and neutrons and an outer electron cloud. Example: Carbon atom, Oxygen Atom, Hydrogen atom | An element is a substance consisting of only one type of atom, that is, all the atoms having the same atomic number. Example: He, Ne, Ar, Fe | Covalent bonding of two or more atoms of the same or different types forms an electrically neutral molecule. For example, O2, O3, H2O, CO2 are molecules | A substance consisting of two or more different elements chemically bonded by ionic or covalent bonding in a fixed ratio is a compound. The properties of a compound differ significantly from those of the constituent elements (parent elements) from which it forms. For example, C6H12O6, NaCl, H2SO4, CH3OH are all compounds. |
Note: Monoatomic and Diatomic refers to atomicity - the number of atoms per molecule of that element. Some elements naturally exist as single, individual atoms like inert gases. Some element exists as pairs of atoms bonded together to form molecules like O2, F2, Cl2, etc. Mnemonic for the 7 common diatomic elements – "Have No Fear Of Ice Cold Beer" | ||||
Number of element types
| One (by definition) An atom is the smallest unit of an element.
| One (The type of an atom identifies an element)
| One or more (homoatomic or heteroatomic)
| Two or more (always different elements)
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Discrete units?
| Yes (single atom, like He atom, Na atom, C atom)
| Varies: single atoms (like in Ne), discrete molecules (like in O2), or giant networks (like in Diamond)
| Yes (discrete, finite groups)
| Varies: discrete molecules (covalent, like in C6H12O6) or giant lattices (ionic lattices like NaCl, MgO or network covalent like in SiO2)
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| Note: Both C6H12O6 and CO2 are assigned terms, molecules, and compounds. In contrast, NaCl is a compound, not a molecule. | |
Bonding
| No bonding (single particle) | None (monatomic) or covalent (in molecular forms)
| Usually covalent (discrete units)
| Covalent (molecular) or ionic (lattice)
|
Can be broken down chemically?
| No (Only nuclear reactions are capable of altering the proton number, changing the identity of an atom) | No Cannot be broken down chemically.
| Yes (into atoms) For example, UV radiation or photolysis can break Cl2 into unstable 2Cl atoms or radicals. | Yes (into elements) For example, electrolysis of water liberates H2 and O2 gas.
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Examples (basic)
| He (helium atom), Na (sodium atom), C (carbon atom)
| He (helium gas), Au (gold metal), Fe (iron)
| H₂ (hydrogen gas), O₂ (oxygen gas), H₂O (water)
| H₂O (water), CO₂ (carbon dioxide), NaCl (salt)
|
Examples (homoatomic/elemental)
| —
| N₂, O₂, Cl₂ (diatomic elements), P₄, S₈
| O₂, N₂, O₃ (ozone), C₆₀ (buckyball)
| —
|
Examples (advanced/tricky)
| Single atom in noble gas or isolated ion (theoretical)
| Diamond (C, network covalent), Graphite (C, layered network), Na metal (metallic lattice)
| C₆₀ (buckminsterfullerene, discrete nanoscale), Glucose (C₆H₁₂O₆, large covalent molecule)
| SiO₂ (silica/quartz, giant covalent network), MgO (ionic lattice), PVC (polymer, macromolecular chains)
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Is it always a pure substance?
| Yes (single atom context)
| Yes
| Yes
| Yes
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Common Classification
| Building block of everything else
| Appears on periodic table; can be monatomic (He), diatomic (O₂), or polyatomic/network (Diamond)
| All elemental molecules are molecules, but not compounds
| All molecular compounds are molecules; ionic/network compounds are often not called molecules
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Key catchphrase
| "The smallest indivisible unit of matter"
| "One kind of atom only. An element identifies the type of atom"
| "Covalently bonded atoms (same or different) forming discrete molecules"
| "Different elements bonded – ionic/network solid or covalent"
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Take the Quiz
Beginner Quiz on Identifying Element, Molecule, or a Compound
Intermediate Quiz on Identifying Element, Molecule, or a Compound
Advanced Quiz on Identifying Element, Molecule, or a Compound