Earth Science: Matter – The Power of Chemical Bonds
Chemical bonds between atoms are formed when atoms come into contact with each other and their electron shells interact. The bonds may be covalent, ionic, or metallic.
Atoms bond upon contact with each other. For example, two hydrogen atoms that are far apart do not exert any force on each other. This changes when they make contact and the laws of electrostatics come into play.
The negatively charged electron shell of one atom attracts the positively charged nucleus of the other. The electron shells “coalesce” and a zone of negative charge is formed around the two nuclei.
Attraction and repulsion
This process has limits. The closer two atoms come, the greater the force of repulsion. The forces of attraction and repulsion combine, so that some distance remains between the atoms and a molecule is formed. Hydrogen chloride is formed by a covalent bond between chlorine and hydrogen.
The bonding electrons are more strongly attracted to the chlorine than the hydrogen. Such simple bonds are called monomers. When there are two monomers, the compound is a dimer; with three monomers, the compound is a trimer. The electrical charge in hydrogen chloride molecules is unequal; it has two poles. This bonding is called polar covalent.
Electronegativity (EN) is a measure of how much an atom in a molecule attracts electrons in a bond. This determines whether the bonding electrons are completely transferred to an atom or shared between the two atoms. When the EN-difference of the atoms is greater than 1.8, an ionic bond is formed.
When the EN-difference is very small, the bond s nonpolar covalent. Sometimes, one atom completely acquires the electrons of another, which results in the generation of ions. The force of attraction between ions is very high due to their opposite charges, and they build a very stable ionic lattice structure.
Metals, like ionic compounds, have a definite structure. The electrons are not bound to a particular atom. Instead, they form a sea of negative charge that binds the positive nuclei together. Most covalent molecules have only a few atoms. Carbon atoms are an exception.
They are able to form bonds with four other atoms, leading to complex molecules. Carbon chemistry thus forms its own branch of chemistry: organic chemistry.
STRUCTURES OF DIFFERENTLY BONDED MATERIALS
In covalent bonds, electrons are shared by the bonding atoms, and a stable connection—a molecule- is formed. The resulting molecules may be small, medium-sized, or even “gigantic,” depending on the number and type of atoms involved. Small compounds are generally volatile and gaseous (such as HCI, perfumes, and aromatic oils), with low melting and boiling points.
They generally do not conduct electricity well. Large molecules can form three-dimensional structures and chains (such as polymers and proteins). Solid ionic compounds are often powdery, salt-like substances, consisting of many small crystals (with a crystal- line lattice structure). They have very high melting and boiling points.
They are often soluble in water, where the positive cations and negative anions are good conductors of electricity.