During class,Ravi's teacher was teaching about Thomson's model of an atom,stating that an atom is electrically neutral,i.e.,it contains equal positive and negative charges,and that an atom consists of a sphere of positive charge in which electrons are embedded like plums in a pudding. Ravi wondered how his teacher related Thomson's model of an atom to a plum pudding,and he imagined that just as dry fruits are embedded in a Christmas pudding,electrons are embedded in a sphere of positive charge.
$(a)$ According to you,what kind of qualities does Ravi possess?
$(b)$ What are the limitations of Thomson's model?

Chemical formula of a compound is a symbolic representation of its composition. We should know the following points to write the chemical formulae of chemical compounds:
$(i)$ Symbols of the combining elements.
$(ii)$ Valency of the combining elements.
In order to know the valency,we must calculate the valence electrons present in the atoms.
The electrons present in the outermost shell or valence shell are called valence electrons. The number of electrons in the atoms of some elements are given below. The elements which have one valency are called monovalent.
$(a)$ What is a chemical formula?
$(b)$ Which main points we must know to write the chemical formula of compounds?
$(c)$ Which values do we find associated with the use of formula for various things in daily life?

Why is excess use of fertilizers detrimental for the environment?

Weeds affect the crop plants by:

Differentiate between manures and fertilizers.

If there is low rainfall in a village throughout the year,what measures will you suggest to the farmers for better cropping?