When we look at how electrons are arranged in different groups of the periodic table, we notice some clear patterns:

1. Valence Electrons: Elements that are in the same group have the same number of valence electrons. This means they often behave similarly in chemical reactions. For example:

   • Group 1 (Alkali Metals): They have one valence electron. Take Sodium for instance: its electron arrangement looks like this: $1s^2 2s^2 2p^6 3s^1$.
   • Group 17 (Halogens): They have seven valence electrons. For example, Chlorine has its electrons arranged like this: $1s^2 2s^2 2p^6 3s^2 3p^5$.

2. Energy Levels: As you go down a group in the periodic table, the energy levels of the electrons increase. For example, Lithium has an energy level of 2, while Cesium has an energy level of 6.

3. Subshell Filling: Transition metals are interesting because they fill their $d$ subshells in a way that isn’t always regular or predictable.

Understanding these patterns can help us predict how different elements will react chemically.