To determine which element has the largest size among rubidium, magnesium, lithium, and sodium, we can follow these steps:
### Step 1: Identify the Group and Period of Each Element
- **Rubidium (Rb)**: Alkali metal, Group 1, Period 5
- **Sodium (Na)**: Alkali metal, Group 1, Period 3
- **Lithium (Li)**: Alkali metal, Group 1, Period 2
- **Magnesium (Mg)**: Alkaline earth metal, Group 2, Period 3
### Step 2: Understand the Trend in Atomic Size
- Atomic size increases as you move down a group in the periodic table. This is because additional electron shells are added, which increases the distance between the nucleus and the outermost electrons.
- Atomic size decreases as you move from left to right across a period. This is due to the increase in nuclear charge (Z effective), which pulls the electrons closer to the nucleus.
### Step 3: Compare the Elements
- **Rubidium** is in Period 5 and is the largest among the alkali metals listed.
- **Sodium** is in Period 3, which is smaller than rubidium.
- **Lithium** is in Period 2 and is smaller than both sodium and rubidium.
- **Magnesium**, although in the same period as sodium, is an alkaline earth metal and is smaller than sodium due to its higher nuclear charge.
### Step 4: Conclusion
Based on the trends:
- Rubidium (Rb) is the largest because it is the lowest in the group of alkali metals listed, which means it has the most electron shells and therefore the largest atomic size.
Thus, the answer is **Rubidium (Rb)**.
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