Cambridge International AS Level Chemistry
Shapes of the orbitals
Each orbital has a three-dimensional shape. Within this shape there is a high probability of finding the electron or electrons in the orbital. Figure 3.7 shows how we represent the s and p orbitals. Figure 3.8 The shape of a dz2 orbital. normally drawn as
Filling the shells and orbitals
a s orbitals
normally drawn as
z
y
The most stable electronic configuration (electronic structure) of an atom is the one that has the lowest amount of energy. The order in which the subshells are filled depends on their relative energy. The subshell with the lowest energy, the 1s, is therefore filled first, followed by those that are successively higher in energy. As we noted in Figure 3.6, the order of the subshells in terms of increasing energy does not follow a regular pattern of s then p then d after argon, where the 3p subshell is full. Figure 3.9 shows the order of filling the subshells.
x b p orbitals 38
4f
Figure 3.7 Representations of orbitals (the position of the nucleus is shown by the black dot): a s orbitals are spherical; b p orbitals, px, py and pz, have ‘lobes’ along the x, y and z axes.
An s orbital has a spherical shape. The 2s orbital in the second principal quantum shell has the same shape as the 1s orbital in the first quantum shell. They are both spherical, but electrons in the 2s orbital have more energy than electrons in the 1s orbital. There are three 2p orbitals in the second quantum shell. Each of these has the same shape. The shape is like an hourglass with two ‘lobes’. The three sets of ‘lobes’ are arranged at right angles to each other along the x, y and z axes. Hence the three 2p orbitals are named 2px, 2py and 2pz. The three 2p orbitals have the same energy as each other. There are also three 3p orbitals in the third quantum shell. Their shapes are similar to the shapes of the 2p orbitals. The d orbitals are more complex in shape and arrangement in space. In 1925 Louis de Broglie suggested that electrons behaved like waves. This led to the idea of electron probability clouds. The electron probability cloud for one type of d orbital is very strange – it is like a modified p orbital with a ring around the middle (Figure 3.8). You will not need to know the d-orbital shapes at AS level, but you will for A level when studying the transition elements (see Chapter 24).
4d
4p
4s
n=4
3d
3p
3s
n=3
2p
2s
n=2
1s
n=1
Figure 3.9 Diagram to show the order in which orbitals are filled up to shell n = 4.
QUESTION 5 a Name the three types of orbital present in the third principal quantum shell. b State the maximum number of electrons that can be found in each subshell of the third quantum shell.
Electronic configurations Representing electronic configurations
A detailed way of writing the electronic configuration of an atom that includes information about the number of electrons in each subshell is shown below for hydrogen.