Chapter 3: Electrons in atoms
number of electrons
Proton number
Symbol
Electronic configuration
1
H
1s1
2
He
1s2
3
Li
1s2 2s1
4
Be
1s2 2s2
5
B
1s2 2s2 2p1
6
C
1s2 2s2 2p2
7
N
1s2 2s2 2p3
8
O
1s2 2s2 2p4
9
F
1s2 2s2 2p5
10
Ne
1s2 2s2 2p6
11
Na
1s2 2s2 2p6 3s1
12
Mg
1s2 2s2 2p6 3s2
13
Al
1s2 2s2 2p6 3s2 3p1
14
Si
1s2 2s2 2p6 3s2 3p2
QUESTION
15
P
1s2 2s2 2p6 3s2 3p3
6 Use 1s2 notation to give the electronic configurations of the atoms with the following atomic numbers:
16
S
1s2 2s2 2p6 3s2 3p4
17
Cl
1s2 2s2 2p6 3s2 3p5
18
Ar
1s2 2s2 2p6 3s2 3p6
principal quantum number
1s1 sub-shell
■
■
Helium has two electrons. Both electrons can go into the 1s orbital, as this can hold a maximum of two electrons. So, the electronic structure of helium is 1s2. Lithium has three electrons. The 1s orbital can only hold a maximum of two electrons so the third electron must go into the next highest subshell, the 2s. So, the electronic structure of lithium is 1s2 2s1.
Electrons are added one by one for successive elements, filling each subshell in order of increasing energy. The electronic configurations of the first 18 elements are shown in Table 3.5. A question about this type of detailed notation will often be stated like this: ‘Use 1s2 notation to give the electronic configuration …’
a 16 b 9 c
20
The electronic configurations of some of the elements after argon are shown in Table 3.6. In this table part of the electronic configuration of each element is represented by [Ar]. This ‘noble gas core’ represents the electronic configuration of argon: 1s2 2s2 2p6 3s2 3p6. This method is a shorthand way of writing electronic structures of atoms with many electrons. However, in an exam you should be prepared to write out the full electronic configuration.
Table 3.5 Electronic configurations for the first 18 elements in the Periodic Table.
subshell. So scandium has the electronic configuration [Ar] 3d1 4s2. This is because electrons occupy the orbitals with the lowest energy – the 3d subshell is just above the 4s subshell but below the 4p subshell. This begins a pattern of filling the 3d subshell ending with zinc. Zinc has the electronic configuration [Ar] 3d10 4s2. ■
You should note the following: ■
Electronic configuration of potassium
Potassium has the electronic structure 1s2 2s2 2p6 3s2 3p6 4s1. The outer electron goes into the 4s subshell rather than the 3d subshell because the 4s is below the 3d in terms of its energy. ■
Filling the 3d subshell
After calcium, a new subshell becomes occupied. The next electron goes into a 3d subshell rather than a 4p
■
Chromium and copper The electronic configurations of chromium and copper do not follow the expected pattern. Chromium has the electronic configuration [Ar] 3d5 4s1 (rather than the expected [Ar] 3d4 4s2). Copper has the electronic configuration [Ar] 3d10 4s1 (rather than the expected [Ar] 3d9 4s2). You will have to learn that these two elements are exceptions to the pattern. Gallium to krypton
The electrons add to the 4p subshell because this is the next highest energy level above the 3d.
39