Unique Nature of Carbon. Carbon has two properties that enable it to form such an extensive range of compounds: 1. Catenation – the ability to form chains of atoms. 2. The ability to form multiple bonds. Catenation.
Carbon has two properties that enable it to form such an extensive range of compounds:
1. Catenation – the ability to form chains of atoms.
2. The ability to form multiple bonds.
The elements nitrogen and oxygen do not undergo extensive catenation. Compounds that contain –O-O- bonds (peroxides) are typically unstable and explode.
Likewise, compounds containing -N-N- bonds are often explosive. An example is the azide ion, N31-.
Carbon readily forms long chains of bonds with itself. This property is called catenation, and is fairly unique. It results for several reasons:
1. Carbon can make up to 4 bonds.
2. The carbon-carbon bond is generally as
strong as bonds between carbon and other
3. The catenated compounds are inert.
Silicon can also make long chains within its compounds, but, since the silicon oxygen bond is much stronger than that between two silicon atoms, the chains typically contain –O-Si-O-Si- type links, rather than -Si-Si- bonds.
Silicon also has empty low-lying d orbitals which make its compounds more reactive.
Since carbon can undergo extensive catenation and make as many as four bonds, the array of compounds is limitless.
The simplest compounds, those with carbon and hydrogen, are used as the basic structure of all molecules.
One of the clearest differences between the two elements is in their oxides. Carbon dioxide is a non-polar molecular substance with double bonds between the carbon and the oxygens.
Since silicon doesn’t readily make double bonds, and the silicon-oxygen bond is so stable, the oxide of silicon is a network solid, in which each silicon atom is bonded to four oxygen atoms which are, in turn, bonded to other silicon atoms.
The oxide of silicon is found in quartz and sand.
Silicates, ions containing silicon and oxygen, are found in most rocks, soils and clays. Their structures also are based on interconnected SiO4 tetrahedral units.
Glass contains silicates in a more random pattern than found in quartz.
The elements of group 5A may form three, five or six covalent bonds, except for nitrogen which cannot expand its “octet.”
Due to its small size, nitrogen readily forms π bonds. Thus elemental nitrogen, N2, has a triple bond. The other elements exist as larger molecules containing single bonds.
Elemental nitrogen is an extremely stable molecule due to the triple bond. As a result, many nitrogen containing compounds decompose exothermically (and sometimes explosively) to form nitrogen gas.
Nitrogen based explosives such as nitroglycerin, will rapidly decompose when ignited or exposed to a sudden impact.
4 C3H5(NO3)3(l) 6 N2(g) + 12 CO2(g) + 10 H2O(g) + O2(g) + energy
Note the large number of moles of gaseous products. Explosives typically involve a very large volume change, producing many moles of small gaseous molecules.
Trinitrotoluene, TNT, is another nitrogen based explosive.
2C7H5(NO3)3(l) 12 CO2(g) + 5 H2(g) + 3N2(g) + 2C(s) + energy
Sodium azide, NaN3(s), is used in air bags in automobiles. A small amount of sodium azide (100g) yields 56L of nitrogen gas at 25oC and 1 atm.
2 NaN3(s) 2Na(l) + 3 N2(g)
This reaction takes place in about 40ms. Other components are put in the air bag so that the molten sodium metal is deactivated into glassy silicates.
10 Na(l) +2KNO3(s) K2O(s) +5Na2O(s)+ N2(g)
2 K2O(s) + SiO2(s) K4SiO4(s)
2 Na2O(s) + SiO2(s) Na4SiO4(s)
Phosphorus was initially isolated in an attempt to extract gold from urine.
The element emits light and glows when exposed to oxygen.
Elemental phosphorus exists as several allotropes. All differ greatly in structure from nitrogen due to a lack of multiple bonding between the larger phosphorus atoms. Phosphorous can also use d orbitals to expand its bonding.
White phosphorus exists as discrete P4 molecules. It is a waxy white solid that is very poisonous and reactive. It burns vigorously in air, and is stored under water.
The element gets its name from the phosphorescent glow emitted by white phosphorus when it is exposed to air in the dark.
White phosphorus has been used in weaponry. The pieces of phosphorus in bombs and grenades get embedded in the skin, where they burn.
Red Phosphorus is a polymeric chain of P4 units. It is stable in air to a temperature of 400oC. Red phosphorus is used in “safety” matches on the striking surface.
Red phosphorus is used in “safety” matches on the striking surface.
Black phosphorus is the most stable of the allotropes. It is formed from white phosphorus that is heated under very high pressures.
Oxygen contains a double bond that is much stronger than a single bond (494 kJ/mol vs. 142 kJ/mol). The lower elements in the group form much weaker π bonds due to their larger atomic size and greater bond length.
Oxygen is a colorless, odorless gas that forms a pale blue liquid. The molecule is paramagnetic due to the presence of two unpaired electrons, and is attracted to a magnetic field.
Ozone, O3, is an allotrope of oxygen. It occurs naturally in the upper atmosphere of earth. The ozone layer absorbs ultraviolet light and serves to help screen out harmful, cancer causing, radiation.
Sulfur is found in large deposits as the free element, or in a variety of ores. Elemental sulfur has a variety of forms and structures. At room temperature, the most stable form is rhombic sulfur, S8 rings.
Sulfur has a tendency to bond with itself. This is called catenation. The sulfur-sulfur bonds are stable, despite lone pairs, since the bond length is relatively long.
If molten sulfur is cooled slowly, the eight- membered rings stack into monoclinic sulfur, which has a needle-like appearance.