Why are sodium (Na) and potassium (K) in the same group on the periodic table?

Sodium (Na) and potassium (K) are in the same group on the periodic table (Group 1, the alkali metals) because they share similar chemical properties and electronic configurations. Here are a few key reasons:

1. **Valence Electrons**: Both sodium and potassium have one electron in their outermost shell (the valence shell). For sodium, the electron configuration is \(1s^2 2s^2 2p^6 3s^1\), and for potassium, it is \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^1\). The presence of a single valence electron makes them highly reactive and gives them similar chemical behavior.

2. **Reactivity**: Being in the same group means that both elements tend to lose their one valence electron easily to form positive ions (cations). This is why they are both highly reactive, especially with water and halogens.

3. **Physical Properties**: Although sodium and potassium can differ in specific physical properties (like melting points), they share many characteristics such as being soft metals that can be cut with a knife, and they have relatively low densities.

4. **Trends in the Periodic Table**: Elements in the same group often exhibit trends in size, reactivity, and other properties. For instance, as you go down Group 1, the atomic radius increases, and the reactivity also increases.

So, while it's true that sodium and potassium are somewhat hard compared to other alkali metals (which tend to be softer), the primary reason for their placement in the same group is their similar electronic structure and chemical behavior.