Multiple Choice
How many valence electrons does a bromine (Br) atom 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
8
C
5
D
4
Verified step by step guidance1
Identify the atomic number of cobalt (Co), which is 27. This means a neutral cobalt atom has 27 electrons.
Write the electron configuration of cobalt to understand the distribution of electrons in its shells and subshells. The 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 are $2 + 7 = 9$.
Note that in many chemical contexts, only the 4s electrons are considered valence electrons for cobalt, which is why some sources list 2 valence electrons. Understanding the context of valence electrons in transition metals is important.
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