To determine which atom in the ground state has an outermost electron with the most energy, we need to consider the position of these elements in the periodic table. All the given elements (Cs, K, Li, Na) belong to Group 1, also known as the alkali metals.
1. Group 1 Elements: These elements include lithium (Li), sodium (Na), potassium (K), and cesium (Cs). They all have a single electron in their outermost shell because they are all in Group 1.
2. Trend in Group: As you move down this group in the periodic table, the atomic number of the elements increases, which means the number of electrons and protons also increases with each subsequent element.
3. Electron Shells and Energy Levels: The energy level of the outermost electron increases as you go down the group. This is because each successive element has its outermost electron in a higher principal quantum number (n), indicating a higher energy shell.
4. Energy Comparison:
- Lithium (Li): The outmost electron is in the 2nd energy level (n=2).
- Sodium (Na): The outmost electron is in the 3rd energy level (n=3).
- Potassium (K): The outmost electron is in the 4th energy level (n=4).
- Cesium (Cs): The outmost electron is in the 6th energy level (n=6).
Given this arrangement,
- At n=6, Cesium's (Cs) outermost electron is at a significantly higher energy level compared to the others in the list.
Therefore, among the given options, cesium (Cs) has the outermost electron with the most energy. Thus, the correct answer is:
A) Cs
