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Mendeleev predicted the existence of elements by gaps in his periodic table. Eka-aluminium and Eka-boron are two such predicted elements. If the difference between their atomic numbers is expressed as 2n, what is the value of n?

1. n = 4
2. n = 5
3. n = 7
4. n = 9

Correct answer: n = 5

Solution

Eka-aluminium is Gallium (Z=31) and Eka-boron is Scandium (Z=21). Their atomic number difference is 31-21 = 10 = 2n, giving n = 5.

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