As we have discussed, molecular compounds are composed of nonmetals and do not contain metals, ions or ionic bonds. Instead, they are held together by what is known as a covalent bond. Let’s examine the bonding in the simplest molecule, H2.
Each hydrogen atom has 1 proton and one electron. Hydrogen atoms need two electrons to fill their 1s orbital and become stable. In the diagram, each hydrogen nucleus is attracted to each electron. This mutual attraction is known as a covalent bond. The electrons are described as being shared by the two hydrogen atoms. A shared pair of electrons is called a single covalent bond. In a covalent bond each atom is considered to possess each of the two electrons being shared in the covalent bond. Therefore, each single covalent bond increases the number of valence electrons in each atom by one. Each hydrogen atom in the H2 molecule above is considered to have 2 valence electrons.
Let’s consider another example, Cl2. Each chlorine atom has 7 valence electrons, but needs eight to have an octet. In the Cl2 molecule there is one covalent bond or one shared pair of electrons. This bond increases the number of valence electrons for each chlorine atom by one giving them both an octet.
Most molecules are formed from atoms of different elements. For example, in hydrofluoric acid, HF, both atoms need one electron to become stable and will for a single covalent bond. This will give hydrogen 2 valence electrons and fluorine an octet.
Some atoms must form more than one covalent bond to satisfy the octet rule. For example, oxygen has six valence electrons and must form two covalent bonds to achieve and octet. Therefore a hydrogen-oxygen compound will have a 2:1 ratio, H2O.
Here are some other examples of atoms that require several bonds to achieve octets:
Remember, valence electrons are common within a group/family, therefore we can assume elements within a group will bond in the same manner.
When atoms share electrons in H2, each atom is identical and the electrons are shared equally or each nucleus has the same attraction for the electrons. This is described as nonpolar covalent bonding. This will not be true in the hydrofluoric acid molecule, HF.
In this molecule the fluorine atom will have a greater attraction for the shared electrons due to its larger nuclear charge (H is 1+ and F is 9+). This unequal attraction creates a slightly negative area around the fluorine atom and a slightly positive area around the hydrogen atom.
The charge is a fraction of the charge of a proton/electron and is symbolized by the Greek letter sigma, δ. The arrow symbol is called a dipole and points toward the negative area I the molecule. This bond is known as a polar covalent bond because it has a positive and negative pole which can attract or repel other polar molecules.
The ability of an atom to attract electrons in covalent bonds is measured by a unitless number known as electronegativity(EN). The larger the EN value, the greater the atoms attraction for electrons in the bond. EN is a periodic trend and generally increases from the lower left to the upper right part of the table.
Bonds between atoms with the same or very similar EN values will be nonpolar. Bonds with very different EN values will be ionic. In between these extremes is where polar covalent bonding occurs. The greater the EN difference the more polar the bond. For example, H-H < C-H < N-H < O-H < F-H where the F—H bond is the most polar and H—H is nonpolar.
It will be helpful to remember that bonds involving O,N,F and Cl will usually be polar because they have high EN values and that C-H bonds are generally considered nonpolar.
Write formulas for the following compounds:
1. Dinitrogen pentoxide 2. Sulfurous acid
3. Ammonium dichromate 4. Octane
5. Copper (II) nitrate 6. Methane
7. Sulfur trioxide 8. Hydrophosphoric acid
9. Hydrosulfuric acid 10. Aluminum sulfate
11. Nitric acid 12. Sodium bicarbonate
13. Hexane 14. Potassium thiosulfate
15. Bromic Acid 16. Carbonic Acid
17. Tetraphosphorus hexoxide 18. Silicon Dioxide
19. Hypochlorous acid 20. Perchloric acid
Name the following compounds:
1. HNO2 2. KOH 3. H2SO4 4. HCl 5. SiCl4
6. XeF2 7. NH4SCN 8. Co(SCN)3 9. AgCN 10. C7H16