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Calcium (Ca) is a common element in the shells of many marine organisms. Strontium (Sr) atoms can sometimes substitute for calcium atoms in these shells. Which of the following statements best explains this observation?

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Answer is: calcium and strontium have same number of valence electrons.

Beryllium, magnesium,calcium and strontium are alkaline earth metals. The elements have very similar properties. Alkaline earth metals have in common an outer s- electron shell (two electrons).

Electron configuration of calcium atom: ₂₀Ca 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²; calcium has two valence electrons (4s²).

Atomic number of calcium is 20, it means that it has 20 protons and 20 electrons, so atom of calcium is neutral.  

Strontium electron configuration: ₃₈Sr 1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁶5s²; two valence electrons is 5s² orbital.

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