Cambridge International AS Level Chemistry
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Elements in Groups 3 to 18 (apart from He) have outer electrons in a p subshell. Elements that add electrons to the d subshells are called the d-block elements. Most of these are transition elements.
Proton number
Name (Symbol)
Electronic configuration
19
potassium (K)
[Ar] 4s1
20
calcium (Ca)
[Ar] 4s2
21
scandium (Sc)
[Ar] 3d1 4s2
QUESTION
24
chromium (Cr)
[Ar] 3d5 4s1
25
manganese (Mn)
[Ar] 3d5 4s2
8 a An element has the electronic configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p5.
29
copper (Cu)
[Ar] 3d10 4s1
ii Which group does it belong to?
30
zinc (Zn)
[Ar] 3d10 4s2
iii Identify this element.
31
gallium (Ga)
[Ar] 3d10 4s2 4p1
35
bromine (Br)
[Ar] 3d10 4s2 4p5
36
krypton (Kr)
[Ar] 3d10 4s2 4p6
Table 3.6 Electronic configurations for some of the elements 19 to 36, where [Ar] is the electronic structure of argon 1s2 2s2 2p6 3s2 3p6.
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i
b Which block in the Periodic Table does the element with the electronic configuration 1s2 2s2 2p6 3s2 3p6 3d5 4s1 belong to?
Filling the orbitals
A useful way of representing electronic configurations is a diagram that places electrons in boxes (Figure 3.11). ■ ■
QUESTION
40
7 Use 1s2 notation to give the electronic configurations for the following elements:
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a vanadium (Z = 23) b copper (Z = 29) c
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selenium (Z = 34)
Orbitals and the Periodic Table The arrangement of elements in the Periodic Table reflects the electronic structure of the elements. The Periodic Table can be split into blocks of elements (Figure 3.10). ■
Elements in Groups 1 and 2 have outer electrons in an s subshell. p-block
s-block d-block
Figure 3.10 Some of the blocks of elements in the Periodic Table.
Which block in the Periodic Table does this element belong to?
Each box represents an atomic orbital. The boxes (orbitals) can be arranged in order of increasing energy from bottom to top. An electron is represented by an arrow. The direction of the arrow represents the ‘spin’ of the electron. (We imagine an electron rotating around its own axis either in a clockwise or anticlockwise direction.) When there are two electrons in an orbital, the ‘spins’ of the electrons are opposite, so the two arrows in this box point in opposite directions. 2p 2s 1s
Figure 3.11 The electronic configuration of boron in box form.
Electrons in the same region of space repel each other because they have the same charge. So wherever possible, electrons will occupy separate orbitals in the same subshell to minimise this repulsion. These electrons have their ‘spin’ in the same direction. Electrons are only paired when there are no more empty orbitals available within a subshell. The spins are then opposite to minimise repulsion. Figure 3.12 shows the electronic structures of carbon, nitrogen and oxygen to illustrate these points.