A. nucleus
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Answer:

Explanation:
It is given that,
Initially, the electron is in n = 7 energy level. When it relaxes to a lower energy level, emitting light of 397 nm. We need to find the value of n for the level to which the electron relaxed. It can be calculate using the formula as :


R = Rydberg constant, 

Solving above equation we get the value of final n is,

or

So, it will relax in the n = 2. Hence, this is the required solution.
Answer:
metal> metalloids >nonmetals (Electrical conductivity)
Explanation:
Electrical conductivity of objects can be compared by the bonding energy of electrons in them.
Metals have less bonding energy of electrons, so even at room temperature their are significant number of free electrons to carry electrical current.
Nonmetals have a very high bonding energy of electrons, so at room temperature negligible number of free electrons are present so electrical conductivity is very low.
Metalloids have both metallic and non metallic features. The electron bonding energy falls in between that of metals and nonmetals. So electrical conductivity also lies in between metals and nonmetals.