The energy that holds molecules together is a form of electric energy, electrostatic potential energy. It arises from stationary, positive and negative electric charges with coulombic forces acting between them.
This means that when electric charges are within the reach, or the field, of electrostatic potential energy, a force described by Coulomb’s Law acts on the charges. This force grows with the magnitude of the two interacting charges and is diminished by the distance between them.
The result is that free positive charges move toward negative ones, and negative charges move toward positive ones.
When physically close charges are arranged so that positive charges and negative charges are separated, a dipole is formed.
In molecules, dipoles arise from polar covalent bonds in which one atom pulls more electrons to its region of the bond than does its bonding partner. This makes the atom with more electrons, or electron density, partially negative and the atom from which the electron density was taken, partially positive.
When polar bonds are close enough to each other, they will interact with each other. Consider HCl as an example of such a bond, with the electron density primarily associated with the chlorine atom. If two HCl molecules are close to each other, and if the positively charged H of one is closest to the negatively charged Cl of the other, the two molecules will draw closer.
Atoms and molecules of pure substances experience attractive forces that cluster them into what we perceive as matter in a number of ways that involve dipoles. These forces are collectively referred to as van der Waals forces.
One such force is hydrogen bonding in which N-H or O-H bonds – covalent polar bonds – draw together, often moving a larger structure with them. Hydrogen bonds play important roles in biological molecules by moving molecules – or portions of large molecules - together. Examples are the formation of double-stranded DNA from single stranded DNA or spherical proteins formed from linear chains of amino acids.
Another type of van der Walls force arises from polar bonds within a molecule that are arranged so that the entire structure they are in has positive charges on one end and negative charges on another – a large dipole. These are called polar molecules. The interaction of polar molecule with polar molecule is referred to as a dipole-dipole interaction.
Water is an example of both a polar molecule and one with hydrogen bonds, with partial positive charges on the hydrogens and partial negative charges on the oxygen.
Nonpolar molecules exhibit attractive forces too. They arise from the movement of electrons within bonds when they briefly sustain dipoles. These instantaneous dipoles can cause the electron cloud in adjacent molecules to form dipoles with a charge distribution that makes both dipoles move toward each other – so both molecules move toward each other too. Such intermolecular forces of attraction are called London dispersion forces or just dispersion forces.
Examples of these forces are found in organic molecules, such as alcohols or fats in which stronger dipole-dipole interactions contribute to higher boiling points or higher melting points.
So in answer to the question, it is electric energy and electric forces that bring molecules together in a variety of ways to form matter.
Everything that we see around us -- plants, water, animals, metals, etc. -- is all matter; that is, they occupy space and have mass. The matter is composed of molecules and these molecules are held together by forces known as intermolecular forces. These forces are attractive in nature and can be any of the following:
Dipole-dipole forces
London dispersion forces
hydrogen bonding forces
induced dipole forces
A great example of hydrogen bonding holding molecules together to form matter is water. Each water molecule contains an oxygen atom (with partial negative charge) and two hydrogen atoms (with a partial positive charge on each). Each of these atoms is attracted to an oppositely-charged atom of a neighboring water molecule, overall forming four hydrogen bonds with neighboring water molecules. All these molecules held together is what we see as water.
Interestingly, depending on the magnitude of these intermolecular forces and the kinetic energy of the molecules, the matter may be solid, liquid or gas. For example, if the intermolecular forces between the molecules are strong enough to overcome the kinetic energy of the molecules, the matter will likely be a liquid or a solid.
Hope this helps.
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