The key difference between molecule and atom is that molecule is a combination of two or more atoms via chemical bonding whereas atom is an individual chemical species that can combine with each other to form molecules and ions.
All matter in the world is made up of atoms and molecules hence we can consider them as the building blocks of everything on this planet, including us. In layman’s terms, we can say that the most basic and also the smallest unit of any chemical element is an atom. A chemical element is a species of atoms. For example, the smallest unit of oxygen gas is the atom of oxygen; we can represent it by letter O. However, this atom of oxygen does not exist independently, and it is only when it combines chemically with another atom of oxygen that it becomes stable. It then becomes a molecule, and we can represent it by the chemical formula O2.
CONTENTS
1. Overview and Key Difference
2. What is a Molecule
3. What is an Atom
4. Side by Side Comparison – Molecule vs Atom in Tabular Form
5. Summary
What is a Molecule?
A molecule is a group of atoms that combine with each other via chemical bonds. These atoms can be of the same chemical element or different chemical elements. A homonuclear molecule forms when atoms of the same chemical element make up the molecule. A heteronuclear molecule forms when atoms of different chemical elements combine with each other.
The atoms in a molecule can bind with each other via covalent bonds or ionic bonds. A covalent chemical bond forms when atoms share their electrons with each other to complete the octet of electron configuration. Ionic bonds form when the electrons completely exchange between atoms. This forms cations (positively charged ions) and anions (negatively charged ions) which are held together by electrostatic attraction forces or ionic bonds.
Figure 01: Formation of a Hydrogen Molecule
To understand the atoms present in a molecule, we can use its chemical formula. The chemical formula is a set of symbols that give the atoms and the ratios between them that has combines to form the molecule. Sometimes other than the chemical symbol of atoms and numbers, we use some other symbols as well; parentheses, dashes, brackets, and plus (+) and minus (−) signs. We can calculate the molecular mass of the molecule using this chemical formula. Moreover, we can use a structural formula that gives the arrangement of atoms in the molecule.
Different molecules have different geometries. Consequently, the geometry displays the spatial arrangement of atoms. It thus gives the bond lengths and bond angles between these atoms. Some molecules have a symmetrical geometry while other have non-symmetrical geometries.
What is an Atom?
An atom is the smallest repeating unit that makes up all matter. A chemical element is a species of atoms; thus, atoms have both chemical and physical properties of its particular chemical element. An atom is extremely small; the size is around 100 pm. It consists of an atomic nucleus containing protons and neutrons. This nucleus is surrounded by a cloud of electrons. Protons, neutrons and electrons are subatomic particles of an atom.
Figure 02: Bohr’s Model of An Atom
Typically the number of protons equals the number of electrons and neutrons. Sometimes there are atoms which have a high or a low number of neutrons than the number of protons, hence, we name them as isotopes of the same chemical element. Furthermore, the term nucleons refer to the units of protons and neutrons. These nucleons determine the mass of the atom because the mass of an electron is negligible compared to protons and neutrons. There are many theories that came into the stage in order to show the structure of atoms. i.e. Dalton’s theory, J.J. Thompson’s plum pudding theory, Bohr’s atomic structure and modern atomic theory.
What is the Difference Between Molecule and Atom?
Molecules are made up of atoms. Hence, the key difference between molecule and atom is that a molecule is a combination of two or more atoms via chemical bonding whereas an atom is an individual chemical species which can combine with each other to form molecules and ions. Moreover, we can use chemical symbols to represent a molecule; we name it as a chemical formula. Accordingly, a chemical formula shows the symbols of atoms present in the molecule. However, for atoms, an English letter shows the symbol of a chemical element to which the atom belongs. Hence, the chemical formula of a molecule has a set of English letters along with the number and some other symbols such as parentheses, dashes, brackets, and plus (+) and minus (−) signs. A single atom cannot exist independently except for noble gases while molecules exist independently because they have low energy.
Moreover, based on the existence, the difference between molecule and atom is that the molecule has a stable existence whereas, the individual atoms are unstable. The below infographic provides more facts on the difference between molecule and atom.
Summary – Molecule vs Atom
Molecules and ions form from atoms. Therefore, the key difference between molecule and atom is that a molecule is a combination of two or more atoms via chemical boding whereas an atom is an individual chemical species which can combine with each other to form molecules and ions.
Reference:
1. Helmenstine, Anne Marie, Ph.D. “What Is a Molecule?” ThoughtCo, Jun. 11, 2018. Available here
2. “Atom.” Wikipedia, Wikimedia Foundation, 10 Oct. 2018. Available here
Image Courtesy:
1.”Covalent bond hydrogen”By Jacek FH – Own work, (CC BY-SA 3.0) via Commons Wikimedia
2.”Bohr atom animation 2″By Kurzon – Own work, (CC BY-SA 3.0) via Commons Wikimedia