Multiple Choice
How many valence electrons does an atom of calcium (Ca) have?
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10. Periodic Properties of the Elements
Valence Electrons of Elements
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How many unpaired electrons are present in the ground state of a group 16 (6A) element, such as oxygen (O)?
A
4
B
2
C
6
D
0
Verified step by step guidance1
Identify the group number of the element. Group 16 (6A) elements have 6 valence electrons.
Write the electron configuration for the valence shell of oxygen, which is 2s and 2p orbitals: $\mathrm{2s^2\ 2p^4}$.
Recall that the 2p subshell has three orbitals, each capable of holding 2 electrons, and electrons fill orbitals following Hund's rule (one electron per orbital before pairing).
Distribute the 4 electrons in the 2p orbitals: place one electron in each of the three 2p orbitals first, then pair the fourth electron with one of the singly occupied orbitals.
Count the number of unpaired electrons remaining after this distribution; this number corresponds to the unpaired electrons in the ground state.
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