Give reasons for the following:
a)Elements have generally fractional atomic masses.
b) Isobars always belong to differentelements.
c) Noble gaselementsdo not show any valency
d) Nucleus of an atom is heavy and positively charged
e) Atom as a whole is electrically neutral

a)  Atomic mass of any element depends upon the number of protons and neutrons present in it. Therefore, the atomic mass of a substance should always be a whole number.
 
However, there are some elements which deviate from this.
This is because; they exist in the form of isotopes in nature. Chlorine is an example of one such element. It is having a fractional atomic mass i.e., 35.5.
 
This is because; in nature chlorine exists in two isotopic forms. These are  . These two isotopes have a natural abundance ratio of 3:1. This means that 75.4% of chlorine found in nature exists as isotope, and the rest 24.6% exists as   isotope.
To calculate the average mass of the element, mass as well as the abundance of all isotopic forms are taken into consideration.
Hence, the mass of chlorine can be calculated as follows:
 
   e) Atoms are electrically neutral because they contain equal numbers of protons and electrons.An atomic nucleus is made of two kinds of particles, protons and neutrons. A proton has a positive charge and a neutron is electrically neutral. The atomic nucleus is surrounded by negatively charged particles called electrons.
A proton with positive charge is equal in magnitude to the negative charge of an electron, hence atom is neutral.  

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