Final answer:
The transition metals, found in groups 3 to 12 of the periodic table, do not follow the normal electron filling patterns, especially starting in the fourth period due to their d subshell electron configurations.
Step-by-step explanation:
The elements in certain periods do not follow the normal electron filling patterns, specifically the transition metals. Starting in the fourth period, the normal electron filling sequence is disrupted, as evidenced by the filling sequence of 3d, 4d, 4f, and 5d subshells. These deviations occur due to electron interactions within the atom. Transition metals, characterized by the filling of their d (l = 2) subshell, are elements found in groups 3 to 12 of the periodic table. They are known for their properties of forming colored compounds, displaying variable oxidation states, and being good conductors of electricity.
On the other hand, elements like Group 1 (alkali metals) and Group 2 (alkaline earth metals) tend to follow the expected electron configurations and do not exhibit anomalies in their electron filling patterns. The noble gases, in Group 18 of the periodic table, have complete valence shells and are not known for breaking the normal filling order either.