Introduction to Chemistry - Part I

What we will study in this chapter   

1. Atomic Structure

2. Chemical Symbols

3. Chemical Formulae  

4. Valency

5. Chemical Reactions

1. Atomic Structure Each atom consists of a central core with negatively charged electrons revolving around the core. The core consists of positively charged protons and uncharged neutrons. The core is called the nucleus of an atom. An atom as such is electrically neutral. Atoms of different elements have different and unique characteristics. Although all atoms are made up of protons, neutrons and electrons, it is the way these subatomic particles are arranged that gives each of the atom its unique characteristics.
	Atoms Of Hydrogen, Helium, Carbon
Mass of protons, neutrons and electrons are very small. A proton is 1.672 6  x 10-27 kg, a mass of neutron is 1.674 x 10-27 kg.  A neutron therefore is slightly heavier than a proton. An electron weighs 9.1.93 x 10-31 kg. Thus a mass of an electron is about 1830 times smaller than a proton. Now you can imagine how small an atom is!!

The dimension of an atom is of the order of 10^-10 m. This unit of measurement is called  an Angstrom and is written as A°. That is 10^-10 m  = 1 A°. On a 1 cm marking on your ruler, you can place 10 crore Hydrogen atoms side by side!! A nucleus of an atom is more compact and its approximate size is of the order of 10^-15 m. This unit of measurement is called as Fermi and is written as fm. That is 10^-15m= 1fm. It has to be noted here that both the nucleus as well as the atom is not a point like particle, but has a certain definite size or dimensions.

From our above discussion, we can see that an atom of any element would  weigh very less. It has been an established convention in Physics, to say that the atomic weight of an element is a number representing the sum of the constituent protons and neutrons. The number of protons is represented as Z, the number of neutrons is represented as N and the atomic weight is represented as A (A= Z+N).  

Table below lists some of the atomic weights of elements.  

  An atom X is represented as _ZX^A or _Z^AX_N.

Atomic weight is a pure number. It is observed in nature that the actual weights of the atoms is a little less than A [1]. The number of protons Z is called the atomic number. Physical and chemical characteristics of elements depend on the number of protons Z.  Thus Hydrogen with 1 proton is vastly different from Oxygen which has 8 protons. 92 elements occurring in nature have Z going from 1 to 92. The heaviest element is Uranium with 92 protons. Neutrons, since they are electrically neutral, very indirectly affect the properties of elements, although their contribution to the nuclear structure is significant. Some elements have same Z, but different N and hence A. For example, most of the Oxygen that is in air has Z=8, N=8, A= 16. But a very small proportion of Oxygen in air also has  Oxygen atoms that have A= 18.  The heavier Oxygen has 10 neutrons!! Elements which display more than one atomic mass (A) are called isotopes.  

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