Periodic table is the representation of elements in an order
The history of periodic table can be summarized as
a) 1829 : Johann Dobereiner proposed the triad rule. According to him we can classify the elements in a triad where the atomic mass of middle element is average of atomic mass of the near by two atoms.
For example : Li, Na and K is a triad. Where atomic mass of Na is average of atomic mass of K
atomic mass of Na = 7+ 39 / 2 = 23
However, with the further discovery of elements the law was rejected.
b) 1864 : John Newland proposed the law of octave. According to him we can arrange the elements in a set of seven elements where the property of second set of seven elements will resemble the properties of first of seven elements or property will be repeated after seven elements. This was also rejected due to its limited applicability.
c) Meyer : He arranged some 28 elements into six different families based on their atomic masses. The members of each familiy shared some common properties.
d) 1869 : Mendleev : He actually developed a periodic table based on atomic mass of elements. He arranged the elements into groups and periods. He even left space for some undiscovered elements, which were later on discovered.
e) 1916 : Henry Moseley : He finally arranged elements based on their atomic number based on X-ray studies. He proposed the modern periodic law that the periodic properties of elements are due to atomic number of elements.
Lets look at the example
Cu(SO4)2
SO4 means 1 atom of S and 4 atoms of O
(SO4)2 means 2*1 atom of S and 2*4 atoms of O, so we get
2 atoms of S and 8 atoms of O.
Answer: Cr2NiO7 is the molecular formula for nickel II dichromate.
B. not fully absorbed by the atmosphere.
Explanation:
Visible light, which has a wavelength of between 380nm and 760nm is usually not fully absorbed by the atmosphere.
The earth's atmosphere is transparent to visible light emanating from the sun.
When visible light is radiated out from the sun, on reaching the atmospheric cover of the earth, it goes through without restraints.
- Little to no interaction occurs between the atmospheric cover of earth and visible light.
- This is why the surface is bright and receives sunlight.
- Other more energetic radiations interacts with atmosphere.
- Some of them gets cut off and some partially penetrate through.
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"The reaction is exothermic and ΔH is negative" can be understood about the reaction and the enthalpy change (ΔH) during the reaction.
<u>Option: D</u>
<u>Explanation:</u>
When the reaction is positive, the process becomes endothermic, i.e. heat appears to be consumed by the system because the reaction products are more enthalpic than the reactants. When the reaction is negative, on the other hand, the process is exothermic, which is the total decrease in enthalpy is caused by heat production. Here the initial temperature is 21.0 C but increase in final temperature to 38.8 C, because if some processes require heat, others must give off heat when they take place.
