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.
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)
|Definition||An atom is the smallest unit of matter composed of a nucleus and one or more electrons present around the nucleus.||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.|
|Size||Atoms are much smaller, and the size usually ranges between 1-2Å.||Molecules are larger in size, but the size depends on the chemical structure of the substance.|
|Consists of||An atom consists of sub-atomic particles like protons, neutrons, and electrons.||A molecule consists of two or more atoms that can be either of the same or different elements.|
|Stability||Atoms are less stable.||Molecules are more stable|
|Division||Atoms cannot be separated into subatomic particles by chemical means.||Molecules can be separated into atoms by chemical means.|
|Existence||Atoms might or might not exist independently, depending on the nature of the element.||Molecules can exist independently.|
|Bonding||The subatomic particles in an atom are not linked together by a chemical bond.||The atoms in a molecule are linked together by a chemical bond in the form of inter-molecular or intra-molecular forces.|
|Properties||Atoms might or might not exhibit the properties of the chemical substance.||Molecules always exhibit properties of the chemical substance.|
|Nature||Atoms of a substance are of the same type.||Molecules of a substance might contain one (homoatomic) or different (heteroatomic) type of atoms.|
|Symbol||The symbol presents the symbol of the chemical element to which the atom belongs.||The molecular formula of the substance indicates the symbols of the atoms present in the molecule.|
|Mass||The mass of the atom is represented by atomic mass.||The mass of the molecule is represented by molecular mass.|
|Examples||Atoms of oxygen, nitrogen are some examples.||Molecules of carbon dioxide, ammonia are some examples.|
Example of 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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