Nature of Matter II: Molecules

Molecules, are groups of two or more atoms that form the smallest identifiable unit into which a pure substance can be divided and still retain the composition and chemical properties of that substance.








A molecule is the smallest particle in a chemical element or compound, having the chemical properties of the element or compound. They consist of atoms connected together by chemical bonds. These bonds are formed due to electron sharing or exchange between atoms. The atoms of certain elements easily combine with other atoms to form molecules. Examples of such elements are hydrogen and oxygen.

Atom and molecule

The size and complexity of molecules can vary greatly. The element helium is an atomic molecule. Some molecules consist of two atoms of the same element. For example, O2 is the most common oxygen molecule in the Earth’s atmosphere; it has two oxygen atoms. However, in some cases, the oxygen atom bonds synthesize triples (O3), forming molecules called ozone. Other familiar molecules include water, which consists of two hydrogen atoms and one oxygen atom(H2O), and carbon dioxide, which consists of one carbon atom bonded to two oxygen atoms (CO2).

Some molecules, especially certain proteins, contain hundreds of molecules that are always moving. In solids and liquids, they are tightly packed together. In solids, the movement of molecules can be compared to rapid vibration. In a liquid, molecules can move freely with each other in a sliding manner. In a gas, the density of molecules is usually less than in a liquid or solid of the same chemical compound, and they move even more freely than in a liquid.

For a particular compound in a particular state (solid, liquid, or gas), the speed of molecular movement increases with the increase of absolute high temperature.

Structure of Molecule


The valence force of a molecule brought together by atoms. A diatomic molecule contains two chemically bonded atoms. If the two atoms are the same, such as in, for example, an oxygen molecule (O2), they form a homo nuclear diatomic molecule, while if the atoms are different, such as in a carbon monoxide molecule (CO), they form a hetero nuclear diatomic molecule.

A Molecule containing more than two atoms are called polyatomic molecules, such as carbon dioxide (CO2)and water(H2O). Polymer molecules may contain thousands of component atoms. For molecules with complex three-dimensional structures, especially those involving atoms bonded to four different substituents, a simple molecular formula or even a semi-structural chemical formula may not be enough to completely specify the molecule.

In this case, you may need a graphical formula called a structural formula. Structural formulas can in turn be represented by one-dimensional chemical names, but this chemical naming requires many words and terms that do not belong to chemical formulas


A molecule consists of two or more atoms, whether it is the same element or two or more different elements, connected by one or more covalent bonds. According to the kinetic-molecular theory, the molecules of matter are constantly moving.

The size of the molecule depends on the number of atoms that make up the molecule. Most of them are too small to be seen with the naked eye. The smallest molecule is diatomic hydrogen(H2) with a bond length of 0.74 angstroms.

Macromolecules are macromolecules consisting of smaller subunits; this term from biochemistry refers to nucleic acids, proteins, carbohydrates, and lipids. Some macromolecules can be observed through a special microscope. The smallest molecule is diatomic hydrogen (H2) with a bond length of 0.74 Å


It is a theory based on a simplified molecular or particle description of a gas, from which many of the overall properties of a gas can be derived. The simplest kinetic model is based on the following assumptions:

(1) The gas is composed of a large number of the same molecules moving in a random direction, and the distance between them is large compared to its size

(2) the molecules experience a completely elastic collision between each other and the container wall (no energy loss), but do not interact in other ways

(3) the kinetic energy transfer between the molecules is heat.

Based on kinetic theory, the pressure on the wall of the container can be quantitatively attributed to the random collision of molecules, and its average energy depends on the gas temperature. Therefore, the gas pressure can be directly related to temperature and density. Many other general properties of the gas can be derived, such as viscosity, thermal and electrical conductivity, diffusion, heat capacity and fluidity.

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