**ORBITALS OF THE ATOM**

The region in space within which there is maximum possibility of finding an electron in an atom is called an ATOMIC ORBITAL. We have s, p, d, and f orbitals.

Shapes of s and p orbitals are as follows**: **** **

** s-orbital **

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**p _{x} orbital**

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** p _{y} orbital p_{z }orbital**

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**EVALUATION**

- Define atomic orbital
- State the shape of (i) S-orbital (ii) P-orbital.

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**ELECTRONIC STRUCTURE OF THE ATOM**

With the knowledge of atomic orbital the spectra of more complex atoms than hydrogen atom can be explained as follows:

- a) that within a given principal quantum number or energy level, there are sub energy levels, i.e. energy levels otherwise called K,L,M,N,O,P AND Q shells have sub-energy levels otherwise called s,p,d and f orbitals

(b) The total number of sub-shells within a shell is given by n^{2}

while the total maximum number of electrons is given by 2n^{2} where n is the number of enregy level.

Energy Level Number of orbitals Maximum No of electrons

n = 1(K- shell) 1^{2} =1 2´1^{2 =2}

^{ }n =2 (L- shell) 2^{2 }= 4 2´2^{2 = 8}

n = 3 (M- shell) 3^{2} = 9 2´3^{2} = 18

n = 4 (N- shell) 4^{2} =16 2´ 4^{2} = 32

n = 5 (O- shell) 5^{2} = 25 2´ 5^{2} =50

n = 6 (P-shell) 6^{2} = 36 2´6^{2 }=72

n = 7 (Q – shell) 7^{2 }= 49 2´7^{2} = 98

(c) In a given orbital there could be a maximum of only two electrons and electron in all orbitals of the same type within a principal quantum number possess equal energies.

(d) The electrons in the different sub- shells or orbitals within a principal quantum number do not all have equal energies.

The gradation of energies of orbitals is as follows:

1s< 2s<2p<3s<3p<3d<4s<4p<4d—

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**EVALUATION**

- State the first five energy levels.
- Calculate the total no of electrons when n is equal to (i) 2 (ii) 4.

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**FILLING OF ELECTRONS IN ORBITALS**

In filling electrons into the atoms of elements, considerations are given to the conditions laid down by Aufbau Principle, Pauli exclusion principle and Hund’s rule of maximum multiplicity.

AUFBAU PRINCIPLE states that electrons go in to fill orbitals of lower energy first before filling orbitals of higher energy and each orbital may hold up to two electrons.

PAULI EXCLUSION PRINCIPLE states that no two electrons have identical sets of the four quantum numbers {n, l, m and s in an atom}.

HUND’S RULE OF MAXIMUM MULTIPLICITY state that in filling degenerate orbitals with electrons, electrons go in singly first before pairing up occurs.

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**EVALUATION**

State the following principle

{a} Aufbau Principle {b} Hund’s rule.

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**QUANTUM NUMBERS**

Studies show that the energy of an electron may be characterized by four quantum numbers. These are

{1}The principal quantum number represented by n with integral values of 1,2,3,4 e.t.c.

{2}The subsidiary or Azimuthal quantum number represented by l with integral values

ranging from O to (n-1).

(3) The magnetic quantum number represented by m with integral values ranging

from –l to +l.

- The spin quantum number represented by s with integral values –
^{1}/_{2}and =^{1}/_{2}.

**Element Atomic Number Electronic configuration.**

H 1 1S^{1}

He 2 1S^{2}

Li 3 1S^{2 }2S^{1}

Be 4 1S^{2} 2S^{2}

B 5 1S^{2} 2S^{2 }2P^{1}

C 6 1S^{2} 2S^{2 }2P^{2}

N 7 1S^{2} 2S^{2 }2P^{3}

O 8 1S^{2} 2S^{2 }2P^{4 }

F 9 1S^{2} 2S^{2 }2P^{5 }

Ne 10 1S^{2} 2S^{2 }2P^{6 }

Na 11 1S^{2} 2S^{2 }2P^{6 }3S^{1}

Mg 12 1S^{2} 2S^{2 }2P^{6 }3S^{2}

Al 13 1S^{2} 2S^{2 }2P^{6 }3S^{2 }3P^{1}

Si 14 1S^{2} 2S^{2 }2P^{6 }3S^{2 }3P^{2}

P 15 1S^{2} 2S^{2 }2P^{6}3S^{23}3P^{3}

S 16 1S^{2} 2S^{2 }2P^{6}3S^{2 }3P^{4}

Cl 17 1S^{2} 2S^{2 }2P^{6 }3S^{2 }3P^{5}

Ar 18 1S^{2} 2S^{2 }2P^{6 }3S^{2}3P^{6}

K 19 1S^{2} 2S^{2 }2P^{6 }3S^{2}3P^{6 }4S^{1}

Ca 20 1S^{2} 2S^{2 }2P^{6 }3S^{2}3P^{6 }4S^{2 }

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**GENERAL EVALUATION**

- State the four quantum numbers.
- Write the electronic configuration of the following a) Mg b) S
^{2-}c) Ca^{2+}d) Si.

**WEEKEND ASSIGNMENT**

- Which of the following orbitals is spherical in shape?

- s (b) p (c) d (d) f

- Which of the following shells have a maximum of eight electrons ?

- k (b) l (c) m (d) n

- 1s
^{2}2s^{2}2p^{6}3p^{1}is the electronic configuration of

- potassium (b) calcium (c) sodium (d) aluminum.

- “No two electrons have identical sets of four quantum numbers”. This statement is

- Aufbau principle (b) Pauli exclusion (c) Hund’s rule (d) Rutherford’s model.

- Which of the quantum number is represented by L?

- principal quantum no (b) subsidiary quantum no (c) magnetic quantum (d) spin quantum.

**THEORY**

- State the following principle (a) Pauli exclusion principle. (b) Aufbau principle.
- Write the electronic configuration of

(a) Oxygen

(b) Calcium (c) Chloride ion (Cl^{–}) (d) Aluminum ion (Al^{3+})

**See also**

NOMENCLATURE OF CHEMICAL COMPOUNDS