Test: Distribution Of Electrons In Atoms - Class 9 MCQ

The fourth shell of an atom can hold a maximum of 32 electrons, depending on the element. There are four subshells - 4s, 4p, 4d, and 4f - that can hold a maximum of 2, 6, 10, and 14 electrons, respectively, for a total of 32.

The maximum number of electrons present in a shell = 2n2

Where n = shell number

Value of n for K shell = 1

Therefore, Maximum number of electrons in K shell = 2n2 = 2(1) = 2

Value of n for L shell = 2

Therefore, Maximum number of electrons in L shell = 2(2)2 = 8

Thus, total no. of electrons = 2 + 8 = 10

Magnesium has Atomic Number 12, It will be distributed in K, L, M shell in the following way:
K shell can accommodate a maximum of 2 electrons.
L shell can accommodate a maximum of 8 electrons and
M shell will accommodate 2 electrons.
So, the configuration becomes 2,8,2.

According to Rutherford’s model of atom, The electrons revolve around the nucleus in well-defined orbits.but it has a drawback:The orbital revolution of the electron is not expected to be stable. Any particle in a circular orbit would undergo acceleration .During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know. 

The electronic configuration of the element with atomic number 19 , is 2,8,8,1
The element is potassium. It is an alkali metal with one valence electron. 

Helium only has 2 electrons and therefore it has a configuration of 1s². Because the 1s orbital is full with 2 electrons and any additional electrons would go in a new energy level. The electron configuration for Helium shows a full outer shell and is Helium is therefore called a Nobel Gas. This means it will not react with other atoms.

As we know , the  element having atomic no. 7 is nitrogen. The first shell is filled with 2 electrons. Remaining are 5 electrons, to fill second shell it need 3 more electrons and will become stable. Therefore, the valency of nitrogen is 3.

Bohr’s Model of Atom

Neils Bohr, a Danish physicist, in 1913 proposed model of atom which rectified the problems left by Rutherford’s Model. He proposed that

(a) Electrons revolve round the nucleus in a fixed orbit.

(b) He called these orbits as ‘stationary orbit’.

(c) Each stationary orbit is associated with fixed amount of energy, thus electrons do not radiate energy as long as they keep on revolving around the nucleus in fixed orbit.

The K shell corresponds to the principal quantum no. ( n) = 1 ,   L, (n) = 2, M, ( n) = 3 
And   N ( n) = 4

The atomic number of Helium is Z = 2. The electronic configuration is, 2. Therefore, Helium is a noble gas with two valence electrons.

Bohr-Bury Scheme

According to Bohr’s model, electrons occupy certain stable orbits or shells. Each shell has a definite energy.
These orbits or shells are represented by the letters K, L, M, N… or the numbers 1, 2, 3, 4…
The maximum number of electrons present in the shell is given by the formula (2n2), where n is the orbit number or shell number.

If Z = 6, that means atomic number of the element is 6.The electronic configuration of the element is 2,4. Since it has 4 electrons in its outermost (valence) shell so, the valency of the element is 4.

The outer shell of any atom is called the valence shell. When the valence electron in any atom gains sufficient energy from some outside force, it can break away from the parent atom and become what is called a free electron.

Atomic number of potassium atom is 19. Its electronic configuration is 2,8,8,1 so, its valency is 1.