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Given the following three electron configurations:

1s2 2s2 2p5

How many core electrons does this element have?

A. 4
B. 5
C. 9
D. 2

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  1. D: 2 core electrons
    The valence level in an electron configuration is the highest principle quantum number. Using the Afbau symbology that would be highest Principle Quantum Number for the element in question => in this case n = 2. The 1s² is the core configuration with 2 electrons, and 2s²2p⁵ is the valence configuration with 5 electrons.

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