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Which element has an outer shell electron configuration of 2s^2 2p^5?
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10. Periodic Properties of the Elements
Valence Electrons of Elements
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How many valence electrons does a calcium (Ca) atom have?
A
2
B
4
C
8
D
6
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Identify the atomic number of calcium (Ca), which is 20. This means a neutral calcium atom has 20 electrons.
Write the electron configuration of calcium to understand the distribution of electrons in its shells: \$1s^2 2s^2 2p^6 3s^2 3p^6 4s^2$.
Recognize that valence electrons are the electrons in the outermost shell (highest principal quantum number). For calcium, the outermost shell is the 4th shell.
Count the electrons in the 4th shell from the electron configuration. Here, the 4th shell has \$4s^2$ electrons, which means 2 valence electrons.
Conclude that calcium has 2 valence electrons because only the electrons in the outermost shell contribute to valence.
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