Periodic Classification of Elements - Class 10th Science

Electronic Configuration

Extra Topic

How to write the Electronic Configuration of Elements

Before writing the electronic configuration of atoms, some basic concept about atom is necessary to understand.

Structure of Atom

An atom comprises of electron, proton and neutron.

Shape of an Atom: An atom has spherical shape.

Nucleus: In centre there is nucleus in every atom and there are shells around the nucleus. Nucleus contains proton and neutron.

Orbit or Shell: There are shells around the nucleus. Shells are called orbit also. Orbits are denoted by Arabic Letters: K, L, M, N, ..........

K = I^st Orbit or shell (n = 1)

L = 2^nd Orbit or shell (n=2)

M = 3^rd Orbit or shell (n=3)

N =4 ^th Orbit or shell (n=4)

O =5 ^th Orbit or shell (n=5)

And so on.

Orbit number are generally denoted by letter 'n'.

Orbital or Sub-shell: Every orbit is divided into orbitals. Orbitals are known as sub-shells also.

There are four types of Orbitals or Sub-shells.

These orbitals are denoted by 's, p, d, and f'

Number of Orbitals in an Orbit

1^st (K) Orbit has only only orbital, i.e. 's-orbital'

2^nd (L) Orbit has two orbitals, i.e. 's-orbital' and 'p-orbital'

3^rd (M) Orbit has three orbitals, i.e. 's-orbital, p-orbital and d-orbital'

4^th (N) Orbit has four orbitals, i.e. 's-orbital, p-orbital, d-orbital and f-orbital'

Maximum Number of electrons in an Orbit

The maximum number of electrons is decided by a formula 2n² where 'n' is the orbit number.

Maximum number of electrons in 1^st orbit, i.e. in K orbit.

Here, orbit number n = 1

Therefore, maximum number of electrons = 2n² = 2(1)²= 2 × 1=2

Thus, 1^st orbit can have maximum 2 (two) electrons.

Maximum number of electrons in 2^nd orbit, i.e. in L orbit.

Here, n = 2

Therefore, maximum number of electrons in 2^nd = 2 n² = 2 × (2)²=2 × 4 = 8

Thus, 2^nd orbit can have maximum 8 electrons.

Maximum number of electrons in 3^rd orbit, i.e. in M orbit.

Here, orbit number = 3, i.e. n = 3

Thus, by applying formula 2n²

=2(3)²= 2 × 9 = 18

Thus, 3^rd orbit can have maximum 18 electrons.

Maximum number of electrons in 4^th orbit, i.e. in N orbit.

Here, number of orbit (n) = 4.

Thus, by applying formula 2n² to calculate maximum number of electrons

= 2(4)² = 2 × 14 = 32

Thus, 4^th orbit can have maximum 32 electrons.

Thus, by using formula 2n² (where n = orbit number) maximum number of electrons in any orbit can be calculated.

Maximum Number of electrons in an Orbital or Sub – shell

Maximum number of electrons in s – orbital

1^st orbit, i.e. K orbit has only one orbital that is s – orbital.

Since, maximum number of electrons in 1^st orbit = 2

Thus, s – orbital can have maximum 2 electrons.

Thus, maximum number of electrons in s – orbital = 2

Maximum number of electrons in p – orbital

2^nd orbit has two orbitals, i.e. s – orbital and p – orbital

Since, maximum number of electrons in 2^nd orbit = 8

And, maximum number of electrons in s – orbital = 2

Thus, maximum number of electrons in p – orbital

= maximum number of electrons in 2^ndorbit – maximum number of electrons in s – orbital

= 8 – 2 = 6

Thus, maximum number of electrons in p–orbital = 6

Maximum number of electrons in d – orbital

3^rd orbit has three orbitals, i.e. s – orbital, p – orbital and d – orbital

Since, maximum number of electrons in 3^nd orbit = 18

And, maximum number of electrons in s – orbital = 2

And, maximum number of electrons in p – orbital = 6

Thus, maximum number of electrons in d – orbital

=maximum number of electrons in 2^ndorbit – maximum number of electrons in s – orbital – maximum number of electrons in p – orbital

=18 – 2 – 6=18 – 8=10

Thus, maximum number of electrons in d – orbital =10

Maximum number of electrons in f – orbital

3^rd orbit has three orbitals, i.e. s – orbital, p – orbital, d – orbital and f – orbital

Since, maximum number of electrons in 4^nd orbit = 32

And, maximum number of electrons in s – orbital = 2

And, maximum number of electrons in p – orbital = 6

And, maximum number of electrons in d – orbital = 10

Thus, maximum number of electrons in f – orbital

=maximum number of electrons in 4^thorbit – maximum number of electrons in s – orbital – maximum number of electrons in p – orbital– maximum number of electrons in d – orbital

= 32( – 2 – 6 – 10)

= 32 – 18 = 14

Thus, maximum number of electrons in f – orbital =14

Aufabu's Principle:

Electrons orbiting the nucleus of an atom goes or fill first of all in orbitals having lower energy level only then electrons goes or fill the orbitals having higher energy level.

In other words, Aubau's states that electrons orbiting one or more atoms fill the lowest available energy levels before filling higher energy levels. (Reference Wikipedia.org)

Explanation

While filling electrons in an atom, electrons fill that energy level first which has the lowest energy. And only then the electron will fill up the orbital having a higher energy level.

The energy level of an orbital in an orbit can be easily understood by the diagram given here.

It is easy to know which orbital has a lower energy level. To know this write the orbitals prefixing the orbit number as follows

And now cut each orbital using an arrow as follows

Now, the order of striking the orbital by arrow gives the order of energy level of the orbital in ascending order.

Thus, the order of energy level of orbitals is as obtained

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s and so on.

This means 1s has less energy than 2s

And, 2p has less energy than 3s

And, 4s has less energy than 3d

And so on.

Hund's Rule

Every orbital in a sublevel is first singly occupied by electrons then only electrons start pairing up.

Fully half-filled or fully-filled orbitals are more stable than not fully half-filled or not fully occupied orbitals.

All of the electrons in singly occupied orbitals have the same spin.

This rule is applied in the electronic configurations of many atoms, such as chromium (Cr), Copper (Cu), etc.

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