Which of the following elements has the highest first ionization energy?
Show the answer and explanation
Correct answer
D. Silicon (Si)
Principle or equation
First ionization energy generally increases across a period from left to right due to increasing nuclear charge and decreasing atomic radius, with some irregularities. Among Na, Mg, Al, Si, all in the third period, ionization energy increases from Na to Si (with a slight drop at Al due to orbital changes, but overall Si is higher than Al).
Why this answer is correct
Step 1: All elements are in the third period. Step 2: Ionization energy generally increases across a period. Step 3: Na (lowest), Mg (higher), Al (slightly lower than Mg due to 3p orbital), Si (higher than Al). Step 4: Therefore Si has the highest first ionization energy among the given options.
Example
The first ionization energies (kJ/mol) are approximately: Na 496, Mg 738, Al 578, Si 786. So Si is highest.
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