Dictionary P (Part 3 of 3)

Presence of Light

The presence of light is necessary to initiate many chemical reactions which propagate by a free radical mechanism. The initiation of the reaction is due to the formation of free radicals in the presence of light.

Prevention of Corrosion

Prevention of corrosion is achieved by controlling the mechanisms by which corrosion is caused.

Primary Alcohol

A primary alcohol is an alcohol where the hydroxyl group is attached to a carbon atom that is attached to two hydrogen atoms. Thus, it has the general structure, RCH2OH, where R is a hydrogen atom or an alkyl group.

Primary Cell

A primary cell is an electrochemical cell for the production of electricity. A primary cell is used to power portable electrical devices. A primary cell cannot be recharged.

Primary Standard

A primary standard is a substances which exists in a high state of purity and can be weighed out accurately and dissolved to give a solution whose strength is then known accurately.

Primary Treatment

Primary treatment is the process for the reduction of the pollution load in an effluent or sewage, by allowing the suspended solids to settle out under the influence of gravity. The suspended solids usually consist of colloidal organic matter in suspension that contribute significantly to the biochemical oxygen demand .

The primary treatment may be enhanced by the use of chemical coagulants to promote flocculation and settlement of the suspended solids.

Principal Quantum Group

The principal quantum group of an orbital determines the maximum number of electrons which can occupy that orbital and is represented by the expression 2*n^2, where n is the principal quantum number for the level. The overall allocation of electrons to the shells and sub-shells is as follows.


nShell Maximum       Sub-orbitals Distribution of electrons
number of                  in the sub-levels
electrons
1s,p,d,f,                                                             1K       
2s                (l = 0)              2                              2L   
8s,             p (l = 0,1)2           6                              3M     
18s,p,          d (I = 0,1,2)2         6      10                      4N    
32s,p,d,        f (I = 0,1,2,3)2       6      10             14       5O    
32**s,p,d,      f (I = 0,1,2,3)2       6      10             14       6P    
32**s,p,d,      f (I = 0,1,2,3)2       6      10             14**

The number of elements do not require further orbitals to account for the electronic configuration of the known elements. The sequence in which orbitals are filled depends on the energy levels of the sub-orbitals, which is not exactly the same sequence as that of the main orbitals. For example, the 4s sub-orbital has lower energy than the 3d sub-orbital, and the former is filled before the latter.

Topics Quantum Numbers

Principal Quantum Number

The principal quantum number n describes the general overall energy level and size of the orbital. The atomic orbitals (n=1, 2, 3, 4, etc.) were originally known as electron shells (K Shell, L Shell, M Shell, N Shell, etc.).


                   n = 1           First level         K Shell 
                   n = 2           Second level        L Shell 
                   n = 3           Third level         M Shell 
                   n = 4           Fourth level        N Shell

As the principal quantum number increases, the size and energy level of the orbital increases, and the number of electrons which the orbital can hold also increases.


 Principal Quantum           Shell 	 Electrons capacity    
Number (Level) Original       Name         of the Orbital                                          
             1                 K                 2     
             2                 L                 8     
             3                 M                16    
             4                 N                32    
             5                 O                32    
             6                 P                32

Topics Quantum Numbers

Products of a Chemical Reaction

The products of a chemical reaction are the materials created in a chemical reaction, and are formed by a rearrangement of the atoms which are present in the reactants (i.e. the initial materials).

Propagation

Propagation is the step in a free radical reaction, where new monomer molecules are added to the growing polymer chain in the free radical process.

Propane

Propane is the third member of the alkane series of hydrocarbons.

Proton

A proton is an atomic particle. It resides in the nucleus, which is the central core of the atom. It has unit positive charge and unit atomic mass.

Purification

Purification is the physical or chemical process of removing contaminants from a compound. The physical processes may include sublimination, distillation, filtration, crystallisation, or extraction. The chemical processes may involve formation of a derivative, purification of the derivative and recovery of the original material in a pure form of the derivative.

Purification of Ores

The purification of ores are important industrial processes and are the first steps to the extraction of the metals. Normally, the ore is concentrated by separating it from the clay body in which it occurs either by gravity, sedimentation, or by a floatation process, before the extraction of the metal from the ore is started. This allows operations with less inert materials during the extraction process.

Start of Hypertext .... Elements .... Compounds .... Index