The correct order of atomic radius of Li, Na, Be and O is
- A.Na > Li > Be > O
- B.Na > Be > Li > O
- C.Be > Li > Na > O
- D.O > Be > Li > Na
Correct answer
Explanation
The correct answer is A, Na > Li > Be > O. Two trends of the periodic table decide the order. Going down a group, atomic radius increases because each new period adds an electron shell; so sodium, in period 3, is larger than lithium, in period 2, of the same group 1. Going across a period from left to right, the radius decreases because the nuclear charge rises while electrons enter the same shell and are pulled in more tightly; so in period 2, lithium is larger than beryllium, which is larger than oxygen. Approximate radii are Na 186 pm, Li 152 pm, Be 112 pm and O about 66–73 pm. Option B is wrong because beryllium, to the right of lithium in the same period, is smaller than lithium. Option C is wrong because it places sodium, the largest, below lithium and beryllium. Option D is wrong because it reverses the true order. Exam tip: atomic size grows down a group and shrinks across a period; caesium and francium are the largest atoms.
Practice Questions
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- A.1,2 and 3
- B.2 and 4
- C.1 and 3 only
- D.1 only
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