Multiple Choice
How many inner shell electrons does an element in period 3 of the periodic table have?
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2. Atoms & Elements
Periodic Table: Representative Elements & Transition Metals
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Which element has a total of two occupied principal energy levels?
A
Mg
B
Be
C
Na
D
K
Verified step by step guidance1
Recall that the principal energy levels correspond to the main electron shells around the nucleus, labeled by the principal quantum number \(n = 1, 2, 3, \ldots\).
Determine the electron configuration of each element to see how many principal energy levels are occupied. For example, Beryllium (Be) has an atomic number of 4, so its electrons fill the first and second energy levels.
Magnesium (Mg) has an atomic number of 12, so its electrons occupy the first three principal energy levels.
Sodium (Na) has an atomic number of 11, also occupying three principal energy levels.
Potassium (K) has an atomic number of 19, which means electrons occupy four principal energy levels.
Therefore, the element with exactly two occupied principal energy levels is the one whose electrons fill only the first and second shells, which is Beryllium (Be).
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