Page 24 - PRE-U STPM CHEMISTRY TERM 1

Chemistry Term 1 STPM
9 Note that for potassium, the 19th electron, occupies the 4s orbital
and not the 3d because in an empty atom, the 4s is of lower energy
than the 3d.
10 For elements with proton number 21 to 30 (known as the d-block
CHAPTER elements), the energy level of the 4s and 3d are reversed. This is
because once the 3d orbital is/are filled, the 3d electrons repel the 4s
2 electrons to a higher energy level.

Reversal of the 4s and 3d
Energy
orbitals
3d 4s

4s 3d
Empty 3d orbitals Filled 3d orbitals

6
2
10
6
10
5
3
2
s , p , d, p and d are more 11 Orbitals that are fully filled (e.g. s , p or d ) or are half-filled
3
5
stable because of the even (e.g. p or d ) have extra stability due to their symmetrical charge
distribution of charge
distribution. This is evident in the electronic configuration of Cr
24
and Cu.
29
12 The configuration of chromium is [Ar] 3d 4s and not
5
1
4
2
[Ar] 3d 4s .
1
Similarly, the configuration of copper is [Ar] 3d 4s and not
10
9
2
[Ar] 3d 4s .
3d 4s
Cr:
3d 4s
Cu:
Electronic Configuration of Ions
1 In the formation of cations, electrons are lost in the reverse order
from electron filling. That is, last to be in, first to be out.
2 The electronic configuration of aluminium is 2.8.3 or

2
2
6
1
2
1s 2s 2p 3s 3p .
1s 2s 2p 3s 3p

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02 Chapter 2.indd 54 3/26/18 3:14 PM