SAT Chemistry Bonding - Sigma and Pi Bonds
SAT Chemistry Bonding - Sigma and Pi BondsWhen bonding occurs between s and p orbitals, each bond is identified by a special term. A sigma bond is a bond between s orbitals, or an s orbital and another orbital such as a p orbital. It includes bonding between hybrids of s orbitals such as sp, sp2, and sp3.
In the methane molecule, the sp3 orbitals are each bonded to hydrogen atoms. These are sigma bonds.
When two p orbitals share electrons in a covalent bond and the interaction is not sym-metrical about a line between the two nuclei, the result is a pi bond. Here is an example:
PROPERTIES OF IONIC SUBSTANCES
Laboratory experiments reveal that, in general, ionic substances are characterized by the fol-lowing properties:
- In the solid phase at room temperature they do not conduct appreciable electric current.
2. In the liquid phase they are relatively good conductors of electric current. The conductivity of ionic substances is much smaller than that of metallic substances.
- They have relatively high melting and boiling points. There is a wide variation in the properties of different ionic compounds. For example, potassium iodide (KI) melts at 686°C and boils at 1,330°C, while magnesium oxide (MgO) melts at 2,800°C and boils at 3,600°C. Both KI and MgO are ionic compounds.
- They have relatively low volatilities and low vapor pressures. In other words, they do not vaporize readily at room temperature.
- They are brittle and easily broken when stress is exerted on them.
- Those that are soluble in water form electrolytic solutions that are good conductors of electricity. There is, however, a wide range in the solubilities of ionic compounds. For example, at 25°C, 92 grams of sodium nitrate (NaNO3) dissolves in 100 grams of water, while only 0.0002 grams of barium sulfate (BaSO4) dissolves in the same mass of water.
Experiments have shown that these substances have the following general properties:
- Neither the liquids nor the solids conduct electric current appreciably.
- Many exist as gases at room temperature and atmospheric pressure, and many solids and liquids are relatively volatile.
- The melting points of solid crystals are relatively low.
- The boiling points of the liquids are relatively low.
- The solids are generally soft and have a waxy consistency.
- A large amount of energy is often required to decompose the substance chemically into simpler substances.