Which of the following electronic configurations correctly represents an element in period 3 of the periodic table?
1s2 2s2 2p2
1s2 2s2 2p6
1s2 2s2 2p6 3s2 3p4
1s2 2s2 2p6 3s2 3p6 4s1
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Option C represents an element in period 3 of the periodic table, with the electronic configuration of 1s2 2s2 2p6 3s2 3p4.
For those who dont understand, here is a detailed explanation.
The periodic table is arranged in a way that elements in the same period have the same highest principal quantum number (n). In period 3, the highest principal quantum number is 3, so the electronic configuration for any element in that period will have electrons in the 1s, 2s, 2p, 3s, and 3p subshells.
Option A has only 2 electrons in the 2p subshell, which means it represents an element in period 2.
Option B represents the electronic configuration of the element in period 2 up to the 3s subshell, so it does not represent an element in period 3.
Option C has all the expected subshells for an element in period 3, so it is the correct answer.
Option D has the correct subshells for an element in period 4, so it is not the correct answer for an element in period 3.

The "3" at the end of the electronic configuration represent the quantum number and also the period to which the element belong to

