Multiple Choice
Which of the following are two characteristics of atomic electron configurations?
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10. Periodic Properties of the Elements
The Electron Configuration
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What is the total number of principal energy levels that are completely filled in an atom of magnesium (atomic number 12)?
A
2
B
4
C
1
D
3
Verified step by step guidance1
Identify the atomic number of magnesium, which is 12. This means a neutral magnesium atom has 12 electrons.
Recall that electrons fill principal energy levels (shells) starting from the lowest energy level (n=1) upwards, following the Aufbau principle.
Determine the maximum number of electrons each principal energy level can hold using the formula $2n^2$, where $n$ is the principal quantum number.
Fill the electrons into the energy levels in order: the first energy level (n=1) can hold up to 2 electrons, the second (n=2) up to 8 electrons, and the third (n=3) up to 18 electrons.
Count how many principal energy levels are completely filled by the 12 electrons. The first level is filled with 2 electrons, the second with 8 electrons, and the remaining electrons go into the third level.
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