When compounds are formed, the identity of the atoms, which is associated with the number of protons in the nucleus, does not change. For example, oxygen atoms are still oxygen atoms whether they are part of oxygen gas molecules, water molecules, carbon dioxide molecules, etc., because the number of protons is unchanged.
But unlike mixtures, where two or more substances are mixed together but there is little or no interaction among the atoms of the various substances, there is interaction among the atoms within molecules. New compounds are formed when the atoms within the molecule form a chemical bond. These bonds are a sort of "glue" that hold the atoms together within the molecule. Bonds involve only the outermost electrons of the atoms, that is, those in the highest shell or energy level. It is this change in electron arrangements that is responsible for the new properties observed when compounds are formed. There are two major types of bonds, ionic and covalent.
An ionic bond forms when the outermost electrons are transferred from one atom to another. One atom loses one or more electrons and another atom gains these electrons. Sodium metal is a soft, shiny, and very reactive metal that is stored under kerosene to keep it from reacting with the oxygen in the air. Sodium atoms have only one electron at the highest energy level and would be more stable if they got rid of this electron. Chlorine is a very poisonous green gas involved in the purifying process of swimming pools and responsible for the characteristic smell around them. Chlorine has seven electrons at the highest energy level and would be much more stable with eight electrons at this level. When sodium and chlorine come in contact with each other, there is an instantaneous reaction. Neutral atoms of sodium metal give up one electron with their negative charge and form particles, called ions, with net charges of +1. Neutral atoms of chlorine gain negatively-charged electrons from sodium atoms and form particles, also called ions, with net charges of -1.
Throughout these changes, the number of protons and neutrons in the nucleus and all of the innermost electrons around the nucleus stay the same. Only an insignificant amount of the mass of the atom is associated with electrons, so the mass of the atom also stays essentially the same. However, a chemical change has occurred and the original properties of the atoms have changed because of the new arrangement of the electrons. The newly-formed sodium ion and the chloride ion are electrically attracted or bonded to each other because opposite charges attract each other. The substance formed is ordinary table salt which is a white, salty, crystalline solid, properties that are very different from the original elements of sodium and chlorine. The bond that formed is called an ionic bond and sodium chloride is called an ionic compound. An ionic bond is formed when the "glue" between atoms is the force of attraction between opposite charges. In fact, salt crystals are formed by the very neat and orderly arrangement of alternating sodium and chloride ions. Ionic bonds are most often formed between atoms of metals and atoms of non-metals.
Often, the outermost electrons of an atom are shared with the outermost electrons of another atom. Electrons move around the nucleus of an atom at very high speeds and, when electrons are shared between two atoms, the nuclei move so close together that the shared electrons spend part of their time near both nuclei simultaneously. This sharing of electrons by the nuclei of two atoms simultaneously is the "glue" that is holding them together within the molecule. This type of bond is called a covalent bond. Electrons that are shared can be contributed by either or both atoms involved in the bond formation.
Carbon atoms have four outermost electrons and need eight to be more stable. If one carbon atom shares each of its outermost electrons with a hydrogen atom, which needs only two electrons to become more stable, a molecule of methane is formed. The formula for the new substance formed is CH4. Methane is often called marsh gas because it forms in swamps and marshes from the underwater decomposition of plant and animal material. It is widely distributed in nature and about 85% of natural gas is methane.
When things are shared, like sharing the sofa with a friend, they are not always shared equally. This is also true of electrons involved in covalent bonds. At times the electrons involved in bonding are shared equally between the nuclei of two atoms and the bond is called a pure covalent bond. More often, however, the sharing is unequal and the electrons spend more time around the nucleus of one atom than of the other. The bond formed is called a polar covalent bond. Usually covalent bonds (both pure covalent and polar covalent) form when atoms of non-metallic elements bond to atoms of other non-metallic elements.
The "glue" or bonding that holds atoms of metals close to each other is usually referred to as a metallic bond. It is formed because the outermost electrons of the metal atoms form a sort of "sea" of electrons as they move freely around the nuclei of all the metal atoms in the crystal.These mobile electrons are responsible for the electrical and heat conductivity of the metals.
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Leona B. Bronstein