Final answer:
Lithium (Li), being in the same group as calcium (Ca) but higher in the periodic table, will have a smaller ion. Strontium (Sr), barium (Ba), and potassium (K) have larger ions because they are below calcium in the periodic table.
Step-by-step explanation:
The question asks which element has an ion that is smaller than an ion of calcium. To answer this, we need to consider the position of the elements on the periodic table and the general trend of ionic sizes. In the periodic table, as we move from left to right across a period, ions tend to get smaller because the nuclear charge increases, which pulls the electrons closer. However, when we move down a group, ions tend to get larger due to the addition of electron shells.
Considering the options:
- Lithium (Li) is in the same group as calcium (Ca) but it is above Ca in the periodic table. Therefore, an ion of lithium will be smaller than an ion of calcium.
- Strontium (Sr), barium (Ba), and potassium (K) are all below calcium in the periodic table, meaning their ions would typically be larger due to having more electron shells.
Thus, the correct answer is lithium (Li), as it is in the same group but higher in the periodic table than calcium, therefore its ion will be smaller.