What is the electron configuration of an element with atomic number 20? a. 1s2 2s2 2p6 3s2 3p5 b. 1s2 2s2 2p6 3s2 3p6 c. 1s2 2s2 2p6 3s2 d. 1s2 2s2 2p6 3s2 3p6 4s2 e. 1s2 2s2 2p6

D.  The number of electrons equals the atomic number for a neutral element.  Each number after the letter refers to the number of electrons in that shell.  So for D, 2+2+6+2+6+2 = 20 electrons, which is equal to the atomic number.

D. 1s2 2s2 2p6 3s2 3p6 4s2

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