Q. 205.0( 4 Votes )

a. The following is a list of the electron distributions of atoms of unknown elements.



Choose an element from the list for each of the following descriptions.

i. It is a noble gas.

ii. It is a soft metal with a low density.

iii. It has giant covalent structure similar to diamond.

iv. It can form a negative ion of the type X3-.

b. How does electronic configurations of atoms change in a period with increase in atomic number?

Answer :

(a) (i) We know that noble gases have complete octet so element D is noble gas.


(ii) Element E is soft metal with a low density. Element E is Rubidium because its atomic number is 37.


(iii) Element B has giant covalent structure similar to diamond because element B has 4 valence shell electrons like carbon.


(iv) Element A can form a negative ion of the type X3- because element A has 5 electrons in outermost shell and it requires 3 electrons to acquire noble gas configuration.


(b) On increasing the atomic number, the number of electrons in valence (outermost) shell increases but number of shells will be same in all atoms of a period.


Ex- The elements of 2nd period are


Li3 = 2, 1


Be4 = 2, 2


B5 = 2, 3


C6 = 2, 4


N7 = 2, 5


O8 = 2,6


F9 = 2, 7


Ne10 = 2, 8


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