Asked by ant
The elements K, Ca, Se+, Kr, and Ga− all appear in Period 4 of the periodic table. Arrange them in order of their atomic size, from largest to smallest.
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K > Ca > Ga− > Se+ > Kr
Reason: Across Period 4 atomic/ionic size generally decreases left → right due to increasing nuclear charge. Adding an electron (Ga−) expands the radius; removing one (Se+) contracts it. Ga− is isoelectronic with Ge and is therefore larger than Se+ (isoelectronic with As) but much smaller than Ca or K. Kr, the noble gas at the right end, is smallest.
Reason: Across Period 4 atomic/ionic size generally decreases left → right due to increasing nuclear charge. Adding an electron (Ga−) expands the radius; removing one (Se+) contracts it. Ga− is isoelectronic with Ge and is therefore larger than Se+ (isoelectronic with As) but much smaller than Ca or K. Kr, the noble gas at the right end, is smallest.
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