Agriculture Reference
In-Depth Information
'Chemistry' is a science that deals with the composition of matter and how that
composition changes. The two broad classes of chemistry are organic and inorganic.
Organic chemistry deals with matter that contains the element carbon , whereas
inorganic chemistry deals with matter that is mineral in origin. The term 'organic'
was used originally to mean compounds of plant or animal origin. However, now it
also includes many synthetic materials that have been developed through research.
Examples of such synthetic organic materials are plastics.
Plastics are chemical compounds made up of polymers. Polymers are materials
composed of molecules of very high molecular weight (MW). Polymers are formed by
a process called polymerisation, in which many basic units (mers) or monomers are
joined together. These large molecules are, in general, referred to as macromolecules .
Polymers are macromolecular structures that are generated synthetically or through
natural processes. Synthetic polymers are generated by addition (or chain growth
polymerisation) and condensation (or radical-initiated) polymerisation. In addition
polymerisation, the final molecule is a repeating sequence of blocks with the chemical
formula of the monomers. Condensation polymerisation occurs if the resulting polymers
have fewer atoms than those present in the monomers from which they are generated.
2.2.1 The Atom
Matter is anything that has mass (weight) and occupies space. Matter may exist in
the form of a solid, liquid or gas. According to chemical theory, the structures of
matter are composed of atoms which form elements such as hydrogen, oxygen and
nitrogen. These elements form molecules and, if two or molecules join together,
they form compounds. The atom is the smallest particle to which an element can be
reduced while retaining the properties of the element.
The atom may be broken down into basic subatomic particles known as neutrons ,
protons and electrons . The atoms of each element differ in the number of particles
they contain. Thus, no two elements will have identical atoms. The neutron is a
particle with no electrical charge. The proton is a positively charged particle. Both
occupy a dense compact central area of the atom called the nucleus . Protons are very
small particles. Electrons are negatively charged particles. They are located in orbit
or shells around the nucleus. Electrons are nearly three-times as large in diameter as
protons and are very much lighter.
The simplest atom is that of hydrogen. It has one proton (as its nucleus), one electron
and no neutrons. For all elements, the atom itself is electrically neutral. The negative
charge of the electrons moving around the nucleus is equal to the positive charge of
the protons in the nucleus. If an atom gains an electron, it becomes electronegative
 
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