Calcium has an atomic number of 20. A stable calcium atom has an electronic configuration of:

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Multiple Choice

Calcium has an atomic number of 20. A stable calcium atom has an electronic configuration of:

Calcium has 20 electrons in its neutral state, so the ground-state arrangement follows the Aufbau principle: fill the lowest-energy orbitals first, with two electrons occupying the outermost 4s orbital. The full configuration is 1s2 2s2 2p6 3s2 3p6 4s2. This matches the option that shows 1s22s22p63s23p64s2, since the two outer electrons reside in 4s. The 4s shell is the outermost for calcium, and 3d would not be involved in calcium’s ground state until later elements; options that place electrons in 3d or miscount the outer-shell electrons (4s1 or 4s3) do not represent neutral calcium.

Calcium has 20 electrons in its neutral state, so the ground-state arrangement follows the Aufbau principle: fill the lowest-energy orbitals first, with two electrons occupying the outermost 4s orbital. The full configuration is 1s2 2s2 2p6 3s2 3p6 4s2. This matches the option that shows 1s22s22p63s23p64s2, since the two outer electrons reside in 4s. The 4s shell is the outermost for calcium, and 3d would not be involved in calcium’s ground state until later elements; options that place electrons in 3d or miscount the outer-shell electrons (4s1 or 4s3) do not represent neutral calcium.

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