Molecular substances

Molecular substances : Practically all substances that are gases or liquids at 25 ºC and at normal pressure are molecular.

Summary

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  • 1 Concept
  • 2 Molecule
  • 3 Properties
  • 4 Curiosity
  • 5 States of aggregation
  • 6 Sources

Concept

A molecular substance is a substance basically made up of molecules. The substances molecular generally have melting point and lower boiling than those ionic compounds, interactions between molecules are very small, consisting only of Van der Walls forces (intermolecular forces).

The great variety of substances present in nature is due to the ability of atoms to combine with each other, this property is called a chemical bond and can happen between atoms of the same element or different elements.

Molecule

Molecule is, therefore, the smallest combination of atoms that keep the composition of matter unchanged. Atoms can be linked through covalent, ionic or metallic chemical bonds.

The arrangements between molecules form molecular substances or molecular compounds. These substances are found at room temperature in the three physical states: solid, liquid and gaseous.

The water molecule is formed by the bond between two hydrogen atoms (H) and one oxygen atom (O). This molecule is in a liquid state and is represented by the H2O formula:

H – O – H

The methane molecule is formed by the bond between one carbon atom (C) and four hydrogen atoms (H), being represented by the formula CH4

Water and methane are molecular substances, since they are made up of molecules. In addition to these, we can cite as examples of molecular substances: sugar (solid state), alcohol (in liquid state) and gases in general (in gaseous state).

Properties

Intermolecular force: At the boiling temperature (TE) of a molecule it is influenced by the interaction between its atoms (intermolecular attractive forces): the more intense the attraction between the molecules, the higher the boiling temperature will be.

Molecule size: The size of a molecular compound is also influenced by its boiling point. The greater the substance, the greater its contact surface, due to the increased interactions between the molecules, consequently the boiling temperature will increase.

Curiosity

Molecular substances are made up of molecules, in which several atoms are joined together by covalent bonds. The molecules are joined together by intermolecular forces that can be:

  • Link by hydrogen bridges.
  • Van der Waals forces, which can be of two types:

o Dipolar interactions.

o Dispersive forces.

Aggregation states

Since the intermolecular forces are relatively small, the molecular substances are, in general, gaseous at room temperature and only in those cases in which the intermolecular forces are more intense are they in a liquid or gaseous state. In any case, the melting and boiling points are relatively low and can be explained taking into account intermolecular forces.

  • Hydrogen bonding: they appear when the molecule has hydrogen atoms linked by covalent bonding to fluorine, oxygen or nitrogen atoms. Substances with hydrogen bonding have especially high melting and boiling points.
  • Van der Waals forces: These weak interaction forces between molecules also manage to raise the melting and boiling points of substances, they can be of two types:

o Dipolar interactions: which increase with increasing polarity of the molecule.

o Dispersive forces: that increase with the molecular mass of the substance.

Dispersive forces always occur, and if the molecules are also polar, there will also be dipole interactions.

In general, molecular substances have low melting and boiling points, so they are in a gaseous state, but as the intermolecular forces are important, they will be in a liquid or gaseous state .

 

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