A light element in period-3 with a neutron /proton ratio around 1:

N a

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M g

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A l

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C l

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Solution

The correct option is A $N a$
As we move across a period, radius decreases and no of protons increases so density increases. So lightest elements are found at the extreme left of the period.
So lightest element in period $3$ is Na which have $11$ proton and $12$ neutron.
Hence, the correct option is $(A)$

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A light element in period-3 with a neutron /proton ratio around 1:

The correct option is A NaAs we move across a period, radius decreases and no of protons increases so density increases. So lightest elements are found at the extreme left of the period.So lightest element in period 3 is Na which have 11 proton and 12 neutron.Hence, the correct option is (A)