Using the formula, 2n2 to find the maximum number of electrons in an orbit (shells), identify the elements that have incomplete shells.
A
Helium (Z = 2)
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B
Zinc (Z = 30)
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C
Argon (Z = 18)
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D
Neon (Z = 10)
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Solution
The correct options are B Zinc (Z = 30) C Argon (Z = 18) The formula to find the maximum number of electrons that can be accommodated in a shell = 2n2 where ‘n’ is the number of the given shell from the nucleus. For example, the maximum number of electrons that can be filled in the: First shell (K - Shell) = 2(1)2 = 2 electrons. Second shell (L - Shell) = 2(2)2 = 8 electrons. Third shell (M - Shell) = 2(3)2 = 18 electrons.
So, Helium has its shell completely filled with 2 electrons. Neon has its shells completely filled as it has 10 electrons in 2 shells K and L, which can accommodate 2 and 8 electrons respectively. Argon has 18 electrons in 3 shells and is a Noble gas. But the maximum number of electrons that can be accommodated in 3 shells is 2 + 8 + 18 = 28. So Argon and Zinc don't have completely filled shells.