Question

For the elements: Cl, Br, F, Al, C, Li, Cs and Xe, choose the best option. The element having 8 electrons in their outermost shell is:
A.Xe
B.Cs
C.Br
D.C

Answer 1

All noble gases, except helium have outer electronic configuration of ${\text{n}}{{\text{s}}^{\text{2}}}{\text{n}}{{\text{p}}^{\text{6}}}$ where ‘n’ is the valence shell, i.e., 8 electrons in their valence shell. The noble gas among the given elements is having the atomic number 54.

Complete step by step answer:
Cl is the element symbol of the element chlorine. Chlorine is a p – block element and it belongs to the seventeenth group. We know that p – block elements have outer electronic configuration of ${\text{n}}{{\text{s}}^{\text{2}}}{\text{n}}{{\text{p}}^5}$ . Since chlorine has atomic number 17, its electronic configuration is ${\text{1}}{{\text{s}}^2}{\text{2}}{{\text{s}}^2}{\text{2}}{{\text{p}}^6}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^5}$ . Hence, chlorine has 7 electrons in its outermost shell.
Br is the element symbol of the element bromine. Like chlorine, bromine is also a p – block element and it belongs to the seventeenth group. Since bromine has the atomic number 35, its electronic configuration is ${\text{1}}{{\text{s}}^2}{\text{2}}{{\text{s}}^2}{\text{2}}{{\text{p}}^6}{\text{3}}{{\text{s}}^2}{\text{3}}{{\text{p}}^6}{\text{3}}{{\text{d}}^{10}}{\text{4}}{{\text{s}}^{\text{2}}}{\text{4}}{{\text{p}}^5}$ . Hence, bromine also has 7 electrons in its outermost shell.
Similarly, F is the element symbol of fluorine which is also an element of the group 17. It has the atomic number 9 and so its electronic configuration is ${\text{1}}{{\text{s}}^2}{\text{2}}{{\text{s}}^2}{\text{2}}{{\text{p}}^5}$ . Hence, fluorine also has 7 electrons in its outermost shell.
Al or aluminium is an s-block element of group 2 and has the atomic number 12. So, its electronic configuration is ${\text{1}}{{\text{s}}^2}{\text{2}}{{\text{s}}^2}{\text{2}}{{\text{p}}^6}{\text{3}}{{\text{s}}^2}$ . Hence, aluminium has 2 electrons in its outermost shell.
C or carbon is a p-block element of group 14 with the atomic number 6. So, its electronic configuration is ${\text{1}}{{\text{s}}^2}{\text{2}}{{\text{s}}^2}{\text{2}}{{\text{p}}^2}$ . Hence, carbon has 4 electrons in its outermost shell.
Li or lithium and Cs or caesium are both s-block elements of group 1 and their atomic numbers are 3 and 55 respectively. So, their electronic configurations are ${\text{1}}{{\text{s}}^2}{\text{2}}{{\text{s}}^1}$ and ${\text{1}}{{\text{s}}^2}{\text{2}}{{\text{s}}^2}{\text{2}}{{\text{p}}^6}{\text{3}}{{\text{s}}^2}{\text{3}}{{\text{p}}^6}{\text{3}}{{\text{d}}^{10}}{\text{4}}{{\text{s}}^{\text{2}}}{\text{4}}{{\text{p}}^6}{\text{4}}{{\text{d}}^{10}}{\text{5}}{{\text{s}}^{\text{2}}}{\text{5}}{{\text{p}}^6}6{{\text{s}}^1}$ respectively. Hence, both of them have 1 electron in their outermost shells.
Xe or xenon is an element of group 18, i.e., it is a noble gas and has the atomic number 54. So, its electronic configuration is ${\text{1}}{{\text{s}}^2}{\text{2}}{{\text{s}}^2}{\text{2}}{{\text{p}}^6}{\text{3}}{{\text{s}}^2}{\text{3}}{{\text{p}}^6}{\text{3}}{{\text{d}}^{10}}{\text{4}}{{\text{s}}^{\text{2}}}{\text{4}}{{\text{p}}^6}{\text{4}}{{\text{d}}^{10}}{\text{5}}{{\text{s}}^{\text{2}}}{\text{5}}{{\text{p}}^6}$ . Hence, it has 8 electrons in its outermost shell.

Thus, option A is correct.

Note:
All the noble gases are colorless, odourless and monatomic. The monoatomic nature of the noble gases is because of the stable outer electronic configuration of their atoms. As a result, noble gases do not enter into chemical combinations even amongst themselves. Their monoatomic nature is further proved by the facts that the ratio of their molar heats at constant pressure and constant volume is $1.67$ and $22.4$ litres of each noble gas at NTP weighs equal to the atomic masses of the gas in grams.