6
Within Table 1.4, there are various points to note. Firstly, electron filling appears to follow the rules just listed up to element 18, argon, when the 3p orbitals are full. The next electron might be expected to occupy the first 3d orbital; however, it can be seen that the electron in potassium actually occupies the 4s orbital. The reason for this is that the energy of the electrons in a 4s orbital is slightly lower than the energy of the electrons in the 3d orbitals, because the 4s orbital is larger and more diffuse. This means the electron in potassium occupies the 4s orbital preferentially, giving potassium a single outer s electron and placing the element in the s block. The electron configuration of potassium is 1s22s22p63s23p64s1.
Secondly, the next electron at calcium also occupies the 4s orbital, giving an electron configuration of 1s22s22p63s23p64s2. However, once the 4s orbital is filled, the next electron at scandium occupies the first 3d orbital, as this is now lower in energy than the 4p orbitals.
A shorthand way to write electron configurations is to use the symbol for the noble gas element (Group 8) to represent electrons in filled shells. So the electron configuration for potassium, 1s22s22p63s23p64s1 can be written as [Ar]4s1, where [Ar] represents the configuration 1s22s22p63s23p6.
Finally, you may notice another anomaly in filling atomic orbitals at Cr and Cu. These two elements have just a single 4s electron, whereas the preceding elements have a filled 4s shell. Cr has one electron in each of its 3d orbitals, and copper has two electrons in each 3d orbital, leaving just one 4s electron in each case. The usual reason given for this relates to the extra stability associated with having a filled or half‐filled set of d orbitals.
1.2.6 Electronic structures and the periodic table
The original organisation of elements in the periodic table was proposed by Mendeleev in 1869. This was before experiments had been carried out that proved the existence of atomic orbitals and energy levels in atoms. He organised the elements based on their atomic masses and other properties. It is now known that chemical reactivity depends upon electronic structure, so our current periodic table agrees with Mendeleev's. The periodic table consists of four main areas, which are generally shown shaded in different colours.
On the left hand of the periodic table are Groups 1 and 2. The elements in these groups are known as s block elements as their highest energy electrons are in s orbitals.
On the right of the periodic table are the p block elements. These constitute the elements in Groups 3–8 (or Groups 13–18) where the p orbitals are being filled.
In