Back to: CHEMISTRY SS1

**Welcome to class! **

In today’s class, we will be talking about the structure of the atom. Enjoy the class!

**STRUCTURE OF THE ATOM: ORBITALS AND ELECTRONIC STRUCTURE OF THE ATOM**

**ORBITALS OF THE ATOM**

The region in space within which there is a 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**: **

**EVALUATION**

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

**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 energy levels.

**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 subshells 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—

**EVALUATION**

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

**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.

**EVALUATION**

State the following principle

{a} Aufbau Principle {b} Hund’s rule.

**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.

{4} 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 }

**GENERAL EVALUATION**

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

**READING ASSIGNMENT**

- New School Chemistry By O.Y. Ababio pg 49-54.

**WEEKEND ASSIGNMENT**

- Which of the following orbitals is spherical in shape? (a) s (b) p (c) d (d) f

- Which of the following shells have a maximum of eight electrons? (a) k (b) l (c) m (d) n

- 1s
^{2}2s^{2}2p^{6}3p^{1}is the electronic configuration of (a) potassium (b) calcium (c) sodium (d) aluminum.

- “No two electrons have identical sets of four quantum numbers”. This statement is (a) Aufbau principle (b) Pauli exclusion (c) Hund’s rule (d) Rutherford’s model.

- Which of the quantum number is represented by L? (a) 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+})

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