Question

Which element has:
A.Two shells, both of which are completely filled with electrons?
B.The electronic configuration $2,8,2$?
C.A total of three shells, with four electrons in its valence shell?
D.A total of two shells, with three electrons in its valence shell?
E.Twice as many electrons in its second shell as it is in the first shell?

Answer 1

Every element has a unique number of electrons and protons present in it known as its atomic number. According to Bohr’s model of atom, for $nth$ shell in an atom, the maximum number of electrons $N$ in that particular shell is given by:
$N=2{{n}^{2}}$
This formula decides the order of filling of electrons in the shells of an atom which is known as electronic configuration of an atom, which is unique to every element.

Complete step by step answer:
A.Since two shells are completely filled,
For $n=1,N=2{{(1)}^{2}}=2$
For $n=2,N=2{{(2)}^{2}}=8$
The atom with completely filled 2 shells has atomic number $2+8=10$
Thus the atom is Neon $Ne$
B.The atomic with electronic configuration $2,8,2$ is $2+8+2=12$
The atom is Magnesium $Mg$

C.Given that the atom has 3 shells.
For $n=1,N=2{{(1)}^{2}}=2$
For $n=2,N=2{{(2)}^{2}}=8$
The outermost shell is the valence shell. Thus no of electrons in the third shell is $4$.
Thus atomic number is $2+8+4=14$
The element is silicon $Si$.

D.Given that the atom has $2$ shells.
For$n=1,N=2{{(1)}^{2}}=2$
Outermost shell has $2$ atoms in its valence shell.
Thus atomic number is $2+2=4$
The element is Beryllium $Be$.
E.The second shell cannot be filled unless the first shell is filled.
Thus first shell has$n=1,N=2{{(1)}^{2}}=2$ electrons
Given that the second shell has double the electrons of the first shell, thus the second shell has $2\times 2=4$ electrons.
Thus the atomic number=$2+4=6$ electrons
The atom is carbon $C$.

Additional Information:
The maximum number of electrons which can be accommodated in any energy level or shell was given by Bohr and Bury.
According to Bohr-Bury scheme:
1. The maximum number of electrons which can be accommodated in any energy level of the atom is given by rule $N=2{{n}^{2}}$.
2. The outermost shell of an atom cannot accommodate more than 8 electrons, even if it has the capacity to accommodate more electrons.
3. Electrons in an atom do not occupy a new shell unless all the inner shells are completely filled with electrons.

Note:
It is worthy to note that except for the first shell, the outermost shell cannot accommodate more than 8 electrons, even if it can accommodate more.
For example, consider$Ca$ atom (atomic number 20).
Its atomic configuration will be $2,8,8,2$ and not $2,8,10$ as the valence shell cannot have more than 8 electrons. Thus the third shell occupies only 8 electrons and the rest 2 electrons move to the fourth shell.