Potassium (K) is larger than Calcium (Ca) primarily because atomic radius decreases across a period in the periodic table, and K comes before Ca in the same period.
Explanation
Both Potassium (K) and Calcium (Ca) reside in the same period (Period 4) of the periodic table. Atomic size is influenced by two primary factors:
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Nuclear Charge (Number of Protons): As you move from left to right across a period, the number of protons in the nucleus increases. This increased positive charge pulls the electrons closer to the nucleus, effectively shrinking the atomic radius.
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Number of Electron Shells: Elements in the same period have the same number of electron shells. Therefore, the number of shells doesn't play a significant role in the size difference between K and Ca.
Because Calcium (Ca) has more protons (20) than Potassium (K) (19), its nucleus exerts a stronger pull on its electrons. This stronger attraction causes the electron cloud of Calcium to contract more than that of Potassium, resulting in a smaller atomic radius for Calcium compared to Potassium. Therefore, even though they are in the same period, the effective nuclear charge is greater for Ca, leading to a smaller size.
In summary:
- K and Ca are in the same period (4).
- Ca has more protons than K.
- Greater nuclear charge in Ca pulls electrons in more strongly.
- Therefore, K is larger than Ca.
