Multiple Choice
How many valence electrons does a neutral strontium (Sr) atom have?
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10. Periodic Properties of the Elements
Valence Electrons of Elements
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How many electrons are present in the second principal energy level (n = 2) of a neutral phosphorus atom?
A
5
B
3
C
2
D
8
Verified step by step guidance1
Identify the atomic number of phosphorus, which tells us the total number of electrons in a neutral atom. Phosphorus has an atomic number of 15, so it has 15 electrons.
Recall that electrons are arranged in principal energy levels (shells) denoted by the quantum number $n$. The first principal energy level ($n=1$) can hold up to 2 electrons.
Determine how many electrons fill the first energy level: since $n=1$ can hold 2 electrons, subtract these from the total electrons: $15 - 2 = 13$ electrons remain for higher levels.
Next, consider the second principal energy level ($n=2$), which can hold a maximum of 8 electrons. Since 13 electrons remain, the $n=2$ level will be fully occupied with 8 electrons.
Therefore, the number of electrons in the second principal energy level ($n=2$) of a neutral phosphorus atom is 8.
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