Questions: The atomic number of an element is 15. What is the likely arrangement of the valence and core electrons in a neutral atom of this element? A. There are 3 valence electrons and 12 core electrons. B. There are 4 valence electrons and 11 core electrons. C. There are 5 valence electrons and 10 core electrons. D. There are 6 valence electrons and 9 core electrons.

Solution Steps

The atomic number of the element is given as 15. This corresponds to the element phosphorus (P) on the periodic table.

For a neutral atom, the number of electrons equals the atomic number. Therefore, phosphorus has 15 electrons. The electron configuration for phosphorus is: $$1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^3$$

• Core Electrons: These are the electrons in the inner shells. For phosphorus, the core electrons are in the 1s, 2s, and 2p orbitals, totaling $2 + 2 + 6 = 10$ core electrons.
• Valence Electrons: These are the electrons in the outermost shell. For phosphorus, the valence electrons are in the 3s and 3p orbitals, totaling $2 + 3 = 5$ valence electrons.

Final Answer