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3.6 Orbitals

The region in the space around the nucleus where the probability of finding an electron is maximum is known as an orbital of that electron.

Actually, according to the concepts of wave mechanics (electron behaving as a wave), the negative charge of the electron is spread around the nucleus which is referred to as ’cloud of negative charge.’ The charge density can vary and it is maximum in particular regions. This region of higher probability of electron cloud is called as orbital.

Types of orbitals :

There are four different types of orbitals s, p, d and f which are used to express the electronic configuration (arrangement of electrons in different orbitals) of elements. All these have a particular energy and three dimensional shape.

Quantum numbers :

The state of the electron in the atom is described by its location and energy level. These states are governed by the laws of quantum mechanics. The numbers used to identify these states are termed as quantum numbers. They specify the location and the energy of an electron.


Principle quantum number (symbolized n) :

It gives the main energy level to which the electron belongs. It is related to the distance of the electron cloud from the nucleus and hence indicates the size of the electron cloud.

It is represented by integer and letter designation. It can have any integral from 1 to infinity but only 1 to 7 have been established for known elements.

If n = 1 it corresponds to the lowest energy level and is designated as ’K’ level or shell. The subsequent letters L, M, N, ... etc. denote progressive higher energy levels.

The total number of electrons in the same shell is given by 2n2

Subsidiary or orbital quantum number ( symbolized’l’ ) :

It indicates sublevel or subshell in which the electron is present. It also indicates the shape of the electron cloud.

The values of ’ l ’ range from 0 to n-1

’ l ’

=

0

1

2

3

letter designation

=

s

p

d

f

shape

=

spherical

dumb-bell

four-lobe planar

complicated

No.of orbitals

=

1

3

5

7

Max.no.of electrons

=

2

6

10

14

 

The maximum number of electrons each orbital can accommodate is two.

A set of electron orbitals with the same values ’n’ and ’l’ is called a subshell which is represented by the following notation:

Click here to enlarge

Figure 5 subshell notation

Electronic configuration of various atoms are given in following table with valence subshell.

Table 4

Atomic No.

Element

Atomic wt.

Eletronic configuration

Valence subshell

Common valences

n=1

n=2

n=3

1

Hydrogen

1

1s1

1s1

+1, -1

2

Helium

4

1s2

1s2

0

3

Lithium

7

1s2

2s1

2s1

+1

4

Beryllium

9

1s2

2s2

2s2

+2

5

Boron

11

1s2

2s22p1

2p1

+3

6

Carbon

12

1s2

2s22p2

2p2

+4, +2, -4

7

Nitrogen

14

1s2

2s22p3

2p3

+5, +3, -3

8

Oxygen

16

1s2

2s22p4

2p4

-2

9

Fluorine

19

1s2

2s22p5

2p5

-1

10

Neon

20

1s2

2s22p6

2p6

0

11

Sodium

23

1s2

2s22p6

3s1

3s1

+1

12

Magnesium

24

1s2

2s22p6

3s2

3s2

+2

13

Aluminium

27

1s2

2s23s2

3p1

3p1

+3

14

Silicon

28

1s2

2s22p6

3s23p2

3p2

+4

15

Phosphorous

31

1s2

2s22p6

3s23p3

3p3

+5 , +3 , -3

[next chapter]

Index

3.1 The Atom
3.2 Isotopes
3.3 Atomic Weight
3.4 Radioactivity
3.5 Icons
3.6 Orbitals
Chapter 4





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