Multiple Choice
How many valence electrons does chlorine (Cl) have?
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10. Periodic Properties of the Elements
Valence Electrons of Elements
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How many valence electrons does a neutral cobalt (Co) atom have?
A
2
B
7
C
5
D
8
Verified step by step guidance1
Identify the atomic number of cobalt (Co), which is 27. This tells us cobalt has 27 electrons in a neutral atom.
Write the electron configuration of cobalt to understand the distribution of electrons in its shells and subshells. The full configuration is $1s^2 2s^2 2p^6 3s^2 3p^6 3d^7 4s^2$.
Recall that valence electrons are the electrons in the outermost shell and any electrons in partially filled d or f subshells that can participate in bonding. For transition metals like cobalt, valence electrons include the electrons in the 4s and 3d subshells.
Count the electrons in the 4s and 3d subshells: 4s has 2 electrons and 3d has 7 electrons, so total valence electrons = 2 + 7 = 9.
Note that sometimes only the 4s electrons are considered valence electrons for certain contexts, but generally for transition metals, both 4s and 3d electrons are counted, so cobalt has 9 valence electrons.
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