Answer :
Final answer:
For Calcium, only 4s orbital is filled. Nickel and Titanium have electrons in their 3d orbitals too. The arrows signify electron spin and highlight the Pauli Exclusion Principle.
Explanation:
In electron configuration, the 4s orbital is filled before the 3d orbital. In the case of Calcium, only the 4s orbital is filled, illustrated as 4s ↑↓ 3d. Nickel and Titanium have electrons in the 3d orbitals, with Nickel's configuration being 4s ↑↓ 3d ↑ ↑ ↑ ↑ ↓, and Titanium's 4s ↑↓ 3d ↑ ↑. The arrows signify the spin of the electron, either up ↑ (spin +1/2) or down ↓ (spin -1/2), showing the Pauli Exclusion Principle which states that no two electrons in the same atom can have identical values for all four of their quantum numbers.
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