Universe is composed of matter,during vedic period in india ,maharshi kanada proposed that matter is composed of smallest particles such as "anu" and "paramanu". later a greek philospher proposed that matter consists of smallest particles called "Atoms" meaning indivisible , Atoms combine to form "compound atoms",which later on called as molecules. According to modern theories Atoms contain positive charged nucleus and negatively charged electrons. Nucleus further contain protons which are positively charged and neutrons which are neutral, combinely both protons and neutrons are called Nucleons. Mass of electron is negligable while mass of proton is 1837 times of electrons. J.J Thomson is the person who discovered the cathode rays(electrons which are travelling from cathode to anode and these are invisible.G.J Stoney named these cathode rays as "electrons".
Two types of forces are present in atom a)force of attraction between the electrons and protons(Attractive force).This force pulls the electrons towards the nuclues. second type of force is b)the revolving electrons experience a centrifugal force directed away from the moving path of electrons.This force pulls away the electrons from nucleus ,since these two forces are equal and opposite ATOM is stable.
IF THESE ATOMS ARE STABLE WHY THEY COMBINE?
It is known that inert gases like Ne(10),Ar(18),Kr(36) have stable electronic configuration and have no tendency to combine with other atoms.Elements like fluorine(9),chlorine(17) are very reactive as they are short of one or two.Similarly atoms of Alkali metals like Li(3),Na(11),K(19) are very reactive and form numerous compounds.
WHY SOME ARE REACTIVE AND SOME OTHER NOT?
In the above paragraph i used the word "electronic configuration",it is defined as systematic arrangement of electrons in the atomic orbits.The maximum number of electrons present in each orbit is generally given by 2nsquare.(n=number of orbit) i.e 1st orbit -2,2nd orbit-8,3rd orbit-18...
*Electrons present in the outer most orbit of an atom are known as valence electrons and based on these atoms are said to be reactive or not.Now observe electronic configuration of inert gases they have 8 electrons in their outermost orbit except helium(2) all are stable in inert gases.
*Halogens have 7 electrons in outermost orbit.Normally every atom has tendency to attain nearest noble gas configuration and become stable,As they are 1 electron short they combine with another atom to have 8 electrons which provides them with much stability.
This is perfect reason for "chemical bonding".Atoms combine to form molecules so "molecules are much stable than atoms".
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