Though the periodic table has only 118 or so elements, there are obviously more substances in nature than 118 pure elements. This is because atoms can react with one another to form new substances called compounds (see our Chemical Reactions module). Formed when two or more atoms chemically bond together, the resulting compound is unique both chemically and physically from its parent atoms.
Let's look at an example. The element sodium is a silver-colored metal that reacts so violently with water that flames are produced when sodium gets wet. The element chlorine is a greenish-colored gas that is so poisonous that it was used as a weapon in World War I. When chemically bonded together, these two dangerous substances form the compound sodium chloride, a compound so safe that we eat it every day - common table salt!
In 1916, the American chemist Gilbert Newton Lewis proposed that chemical bonds are formed between atoms because -19 coulombs and a mass of 9.11 × 10-31 kg. Electrons are generally found around the nucleus of an atom, but may be gained or lost during ion formation. Compare to the proton.');">electrons from the atoms interact with each other. Lewis had observed that many elements are most stable when they contain eight electrons in their valence shell. He suggested that atoms with fewer than eight 2 2s2 2p6 3s1; the 3s electron is the only valence electron in the atom. Valence electrons determine the chemical properties of an atom and are the only electrons that participate in chemical bonding.');">valence electrons bond together to share electrons and complete their 2O, oxygen has a valence of two; in CH4, carbon has a valence of four.');">valence shells.
While some of Lewis' predictions have since been proven incorrect (he suggested that -19 coulombs and a mass of 9.11 × 10-31 kg. Electrons are generally found around the nucleus of an atom, but may be gained or lost during ion formation. Compare to the proton.');">electrons occupy cube-shaped orbitals), his work established the basis of what is known today about chemical bonding. We now know that there are two main types of chemical bonding; ionic bonding and covalent bonding.
Ionic Bonding
In ionic bonding, -19 coulombs and a mass of 9.11 × 10-31 kg. Electrons are generally found around the nucleus of an atom, but may be gained or lost during ion formation. Compare to the proton.');">electrons are completely transferred from one atom to another. In the process of either losing or gaining negatively charged electrons, the reacting atoms form ions. The oppositely charged ions are attracted to each other by electrostatic forces, which are the basis of the ionic bond.
For example, during the reaction of sodium with chlorine:
Notice that when sodium loses its one 2O, oxygen has a valence of two; in CH4, carbon has a valence of four.');">valence -19 coulombs and a mass of 9.11 × 10-31 kg. Electrons are generally found around the nucleus of an atom, but may be gained or lost during ion formation. Compare to the proton.');">electron it gets smaller in size, while chlorine grows larger when it gains an additional 2 2s2 2p6 3s1; the 3s electron is the only valence electron in the atom. Valence electrons determine the chemical properties of an atom and are the only electrons that participate in chemical bonding.');">valence electron. This is typical of the relative sizes of ions to atoms. Positive ions tend to be smaller than their parent atoms while negative ions tend to be larger than their parent. After the reaction takes place, the charged Na+ and Cl- ions are held together by electrostatic forces, thus forming an ionic bond. Ionic compounds share many features in common:
- Ionic bonds form between metals and nonmetals.
- In naming simple ionic compounds, the metal is always first, the nonmetal second (e.g., sodium chloride).
- Ionic compounds dissolve easily in water and other polar solvents.
- In solution, ionic compounds easily conduct electricity.
- Ionic compounds tend to form crystalline solids with high melting temperatures.
This last feature, the fact that ionic compounds are solids, results from the intermolecular forces (forces between molecules) in ionic solids. If we consider a solid crystal of sodium chloride, the solid is made up of many positively charged sodium ions (pictured below as small gray spheres) and an equal number of negatively charged chlorine ions (green spheres). Due to the interaction of the charged ions, the sodium and chlorine ions are arranged in an alternating fashion as demonstrated in the schematic. Each sodium ion is attracted equally to all of its neighboring chlorine ions, and likewise for the chlorine to sodium attraction. The concept of a single molecule becomes blurred in ionic crystals because the solid exists as one continuous system. Forces between molecules are comparable to the forces within the molecule, and ionic compounds tend to form crystal solids with high melting points as a result.
Covalent Bonding
The second major type of atomic bonding occurs when atoms share -19 coulombs and a mass of 9.11 × 10-31 kg. Electrons are generally found around the nucleus of an atom, but may be gained or lost during ion formation. Compare to the proton.');">electrons. As opposed to ionic bonding in which a complete transfer of electrons occurs, covalent bonding occurs when two (or more) elements share electrons. Covalent bonding occurs because the atoms in the compound have a similar tendency for electrons (generally to gain electrons). This most commonly occurs when two nonmetals bond together. Because both of the nonmetals will want to gain electrons, the elements involved will share electrons in an effort to fill their valence shells. A good example of a covalent bond is that which occurs between two hydrogen atoms. Atoms of hydrogen (H) have one 2 2s2 2p6 3s1; the 3s electron is the only valence electron in the atom. Valence electrons determine the chemical properties of an atom and are the only electrons that participate in chemical bonding.');">valence electron in their first electron shell. Since the capacity of this shell is two electrons, each hydrogen atom will "want" to pick up a second electron. In an effort to pick up a second electron, hydrogen atoms will react with nearby hydrogen (H) atoms to form the compound H2. Because the hydrogen compound is a combination of equally matched atoms, the atoms will share each other's single electron, forming one covalent bond. In this way, both atoms share the stability of a full 2O, oxygen has a valence of two; in CH4, carbon has a valence of four.');">valence shell.
