Multiple Choice
What does the number of electrons in an atom's valence shell determine?
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10. Periodic Properties of the Elements
Valence Electrons of Elements
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Join thousands of students who trust us to help them ace their exams!Watch the first videoMultiple Choice
How many unpaired electrons are present in a neutral calcium (Ca) atom?
A
0
B
2
C
1
D
4
Verified step by step guidance1
Identify the atomic number of calcium (Ca), which tells you the number of electrons in a neutral atom. Calcium has an atomic number of 20, so it has 20 electrons.
Write the electron configuration for calcium by filling the orbitals in order of increasing energy: 1s, 2s, 2p, 3s, 3p, and then 4s. The configuration is: $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2$.
Recall that electrons fill orbitals in pairs according to the Pauli exclusion principle, and Hund's rule states that electrons occupy degenerate orbitals singly before pairing up.
Look at the outermost electrons in the 4s orbital, which contains 2 electrons paired together, meaning there are no unpaired electrons in the valence shell.
Conclude that since all electrons in calcium are paired, the number of unpaired electrons in a neutral calcium atom is 0.
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