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Which of the following best describes the core electrons of a strontium (Sr) atom?
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10. Periodic Properties of the Elements
Valence Electrons of Elements
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An atom of phosphorus (atomic number 15) is in its ground state. How many valence electrons does this atom have?
A
5
B
3
C
7
D
8
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Identify the atomic number of phosphorus, which is 15. This means phosphorus has 15 electrons in its neutral ground state.
Write the electron configuration for phosphorus by filling the orbitals in order of increasing energy: 1s, 2s, 2p, 3s, and 3p.
The electron configuration for phosphorus is \$1s^2 2s^2 2p^6 3s^2 3p^3$.
Determine the valence electrons by looking at the electrons in the outermost shell (the highest principal quantum number, which is n=3 for phosphorus).
Count the electrons in the 3s and 3p orbitals: \$3s^2$ and \$3p^3$, which sum to 5 valence electrons.
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