Atom vs Molecule- Definition, 12 Major Differences, Examples

Atom Definition

An atom is the smallest unit of matter composed of a nucleus and one or more electrons present around the nucleus.

• The atom is characteristic of the chemical element and is often considered the basic building block of chemistry.
• The atom consists of a large empty space while the rest is occupied by a nucleus and a cloud of negatively charged electrons. The nucleus consists of protons (positively charged) and neutrons (neutral), resulting in a positive charge.
• The nucleus is highly dense when compared to electrons that are the lightest charged particles in nature.
• The electrons are attracted by the protons in the nucleus by the electromagnetic force between the charged species.
• The number of protons in the atom refers to the atomic number, which defines the chemical element.
• The atomic weight of an atom is determined by the total mass of the nucleus, which in turn is the total weight of the protons and neutrons.
• The number of neutrons in a nucleus affects the mass of the atom but not the chemical properties. Thus, nuclei with the same number of protons and a different number of neutrons are termed isotopes.
• The electrons, neutrons, and protons are termed subatomic particles, and these cannot be separated by chemical reactions.
• All atoms have about the same size irrespective of the number of electrons present in the atom. The radius of an atom is about 1-2 Å.
• The atoms of different elements might or might not exist independently. Atoms of elements like argon and helium can exist independently, but those of oxygen, nitrogen sulfur cannot.
• Atoms of all elements, except the noble gases, are extremely unstable and often exist together with other atoms to reach a stable configuration.

Molecule Definition

A molecule is a group of two or more atoms that represent the smallest identifiable unit of a pure substance and retains the composition and chemical properties of the substance.

• Molecules are formed when atoms come close enough so that the electron cloud can interact with each other and with the nuclei.
• The atom involved in the interaction finally forms a chemical bond when the interaction leads to the lower total energy of the system.
• Molecules can be differentiated into monoatomic (single atom) and polyatomic (more than one) molecules depending on the number of atoms present in the molecule.
• Molecules can also contain the same (homoatomic) or different (heteroatomic) types of atoms.
• In a particular type of molecule, the ratio of the number of atoms that remain bonded is fixed.
• The bond between the atoms can be ionic and covalent, depending on the sharing or donation of electrons.
• The bonds usually are directional as the atoms tend to acquire certain positions that maximize the bond strengths. The arrangement of the bonds results in a definite, rigid structure of the molecules.
• The molecular weight of a molecule is the sum of the atomic weights of the atoms present in the molecule. The number of molecules in a mole is the same in all substances, and the number is termed as Avogadro’s number (6.022 ×10²³).
• The molecules of a compound can exist independently and are often much stable than the individual atoms.
• Molecules of a substance have designated molecular formulas that use chemical element symbols, numbers, and other special characters like parentheses, dashes, and brackets.

12 Key Differences (Atoms vs Molecules)

Example of atom

Oxygen atom

• Oxygen is a chemical element where each atom consists of eight electrons, protons, and neutrons.
• The oxygen atom is a highly reactive and unstable chemical species that is usually produced through chemical reactions.
• The oxygen atom contains six electrons in the outermost orbit which is why it tends to bond with other oxygen atoms to form a diatomic molecule and attain stability.
• Oxygen atoms are highly oxidative and can produce chemiluminescence with a wide variety of analytes.
• The atomic number of oxygen is 8, indicating the number of electrons and protons in the atom, and the atomic weight is 16 (mass of the nuclei).
• Oxygen atoms are one of the most abundant atoms in the biosphere that combine with other different atoms to form different compounds.

Example of molecule

Carbon dioxide molecule

• A carbon dioxide molecule is composed of one carbon atom and two oxygen atoms that are bonded by covalent linkages.
• Carbon dioxide is the fourth most abundant component of air that is released by most living organisms via respiration.
• In a carbon dioxide molecule, a single carbon atom is double-bonded to two separate oxygen atoms.
• The carbon dioxide molecule is a relatively simple molecular system, but it is important as carbon dioxide is one of the greenhouse gases that influence life on the planet.
• In the electronic ground state, a CO₂ molecule is linear, with the C-O bond length being 1.16Å. The carbon-oxygen bond is polar due to the differences in the electronegativities of the atoms involved.

References and Sources

• Cleaves H.J. (2011) Oxygen (Atomic). In: Gargaud M. et al. (eds) Encyclopedia of Astrobiology. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-11274-4_1137
• National Center for Biotechnology Information. “PubChem Compound Summary for CID 159832, Atomic oxygen” PubChem, https://pubchem.ncbi.nlm.nih.gov/compound/Atomic-oxygen. Accessed 22 February, 2021.
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• 1% – https://www.nuclear-power.net/nuclear-power/reactor-physics/atomic-nuclear-physics/atomic-nuclear-structure/atomic-number-proton-number/
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• <1% – https://www.quora.com/The-atoms-of-most-of-the-elements-exist-independently-Is-it-true-or-false
• <1% – https://www.periodic-table.org/oxygen-periodic-table/
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