Multiple Choice
How many valence electrons does a calcium (Ca) atom have?
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 neutral atom of a group 4A (14) element, such as carbon (C)?
A
2
B
4
C
1
D
0
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Identify the group number of the element. Group 4A (or 14) elements have 4 valence electrons. For carbon (C), the atomic number is 6, and its electron configuration is important to determine unpaired electrons.
Write the ground-state electron configuration of carbon. The full configuration is $1s^2 2s^2 2p^2$. Focus on the valence shell, which is the second shell: $2s^2 2p^2$.
Recall the shape and filling of the p orbitals. There are three degenerate p orbitals ($2p_x$, $2p_y$, $2p_z$), each can hold up to 2 electrons. According to Hund's rule, electrons fill degenerate orbitals singly first with parallel spins before pairing.
Distribute the 2 electrons in the three $2p$ orbitals. Place one electron in $2p_x$ and one in $2p_y$, both unpaired, leaving $2p_z$ empty. This results in 2 unpaired electrons.
Conclude that the number of unpaired electrons in the ground state of carbon is 2, based on the electron configuration and Hund's rule.
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