Two or more atoms may be joined together by chemical bonds to form a molecule. If the atoms in the molecule come from different elements, it forms a compound.
Covalent Bonds
Covalent bonds are strong bonds. They are formed when a pair of atoms shares electrons. Atoms will enter covelent bonds in order to complete the complement of electrons in their outermost electron ring.
For example, each atom of hydrogen has only one electron. Its K electron energy level will be 'filled' with two electrons. If two hydrogen atoms can be positioned so that they can share electrons, both atoms will be 'satisfied'. A covalent bond is produced when the pair of electrons [one from each atom] is shared. The resulting molecule is a diatomic molecule of hydrogen, H2.
Covalent bonds can be indicated in a number of ways. One common method is to use electron dot diagrams. In these diagrams, the electrons in the outermost energy level [the ones that may be involved in chemical bonds] are indicated by a series of dots around the atom. Pairs of dots between the symbols for two atoms indicate a bond between them. The formation of the hydrogen molecule could therefore be diagramed like this:
Another way to show the bond is to simply connect the symbols of the two atoms with a bar.
In complex diagrams of molecules, it is not even necessary to indicate the bonds by bars; they are simply assumed to be present.
Covalent bonds can also form between two atoms of different elements. Carbon, with four electrons in its outermost L electron energy level, will seek to share in four covalent bonds. Methane, CH4, is a simple molecule made up of carbon and hydrogen.
Complete the electron dot diagram of the methane molecule.
Knowing the number of electrons present in an atom, and the number of those electrons available in the outermost energy ring, will allow you to determine how many covalent bonds a given atom can be engaged in. Hydrogen, oxygen, nitrogen, and oxygen are the four elements that are most common in living organisms. These are the elements that we will be most concerned with. Use the table below to calculate how many covalent bonds each of these atoms will form.
Element Atomic number # electrons # electrons # of covalent
in K-electron in L-electron bonds
ring ring
hydrogen
oxygen
nitrogen
carbon
In some instances, two atoms can form more than one covalent bond with each other. Double bonds can be formed when a pair of atoms shares two pairs of electrons. Triple bonds occasionally form when three pairs of electrons are shared between two atoms.
The diatomic molecule of oxygen is a good example of a molecule produced by a double bond between two atoms.
With electron dot diagrams, this can be shown as:
The two pairs of shared electrons are shown between the two atoms, and each atom has 'access' to a total of 8 electrons in its outer ring.
A double bond is usually indicated simply by using two connecting bars between the atoms.
2 O --> O = O
In many formulas, double bonds are not specifically indicated, but are simply assumed. One common molecule is carbon dioxide, CO2 . Using electron dot diagrams, show the reaction that produces carbon dioxide.
Polarized covalent bonds
In most cases, the bond that is formed is described as a non-polar covalent bond. This indicates that the two atoms involved in the bond sahre the electrons equally. Neither atom can exert a stronger 'pull' or attraction on the electrons.
The two hydrogen atoms in a hydrogen molecule, for example, are equal partners. The shared electrons are just as likely to be drawn toward either of the two nuclei. The same can be said for the carbon and hydrogen atoms involved in the methane molecule.
However, some atoms exert a stronger attraction for the electrons. These elements are described as being electronegative. Electronegativity is related to both the number of protons in the nucleus and the size of the atom [roughly speaking, to the number of electron energy rings that are occupied]. In the elements common in living systems, both oxygen and nitrogen are electronegative. They will pull the electrons of small atoms, such as hydrogen, closer to themselves, creating polar covalent bonds.
In a polar bond, one atom in the bond monopolizes the shared electrons and carries a slight negative charge. The other atom in the bond has less 'access' to the electrons, and thus has a slight positive charge.
Water is a good example of a molecule with polarization. Complete the electron dot diagram for the formation of water.
Expect polarized bonds to form anywhere that oxygen or nitrogen atoms are covalently bonded to hydrogen atoms.
Ionic Bonds
Ionic bonds are formed between ions. When an atom has gained or lost electrons, it becomes a charged particle, either a positive ion or a negative ion. There is a electrostatic attraction between oppositely charged particles. Positively charged ions are attracted to negatively charged ions. Ionic bonds are fairly strong in the solid state but are relatively weak in aqueous [water] solutions.
Sodium and chloride ions combine to form sodium chloride, common table salt, through the formation of ionic bonds.
Ionic compounds are readily dissociated in aqueous solution, releasing their charged ions.
Hydrogen bonds
Hydrogen bonds are the third major category of chemical bonds. Like ionic bonds, they are electrostatic interactions between regions of opposite charge.
Hydrogen bonds form between areas of unlike charge in polarized bonds.
In water, for example, each molecule has two polarized bonds. The positively charged 'hydrogen end' of one water molecule is electrostatically attracted to the negatively charged 'oxygen end' of another water molecule, forming a hydrogen bond between the two molecules. Each water molecule can enter as many as four hydrogen bonds with other molecules.
Hydrogen bonds are typically indicated by dotted lines.
Hydrogen bonds are considered to be weak and easily broken bonds. They do, however, play critical roles in the stabilization of molecules. Many of the unique characteristics of water are due to the hydrogen bonding between water molecules. Large organic molecules, such as proteins, polysaccharides, and nucleic acids owe their characteristic shapes to hydrogen bonding between different regions of the molecules.
Van der Waals forces
Van der Waals forces are specializes stablilizing interactions that occur between non-polar and uncharged regions of molecules. Strictly speaking, they are not chemical bonds, but like hydrogen bonds they provide stabilizing forces that help to reinforce the structure of molecules, and provide a low intensity attractive force between uncharged molecules.
Practice
Using the rules for covalent bonding, construct several molecules, showing their structural formulas.
1. molecular oxygen [O2]
2. carbon dioxide [CO2]
3. hydrogen peroxide [H2O2]
4. formaldehyde [CH2O]
5. ethanol [C2H5OH]
6. water [H2O]
7. glycerol [C3H8O3]