# Video: Identifying the Subshell Occupied by the Valence Electrons of a Magnesium Atom in Its Ground State in a Set of Subshells

Which of the following is the subshell occupied by the valence electrons of a magnesium atom in its ground state? [A] 2s [B] 3s [C] 3p [D] 4d [E] 4f

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### Video Transcript

Which of the following is the subshell occupied by the valence electrons of a magnesium atom in its ground state? A) 2s, B) 3s, C) 3p, D) 4d, or E) 4f.

We can determine the subshell that the valence electrons of an atom occupy by using the periodic table. If we move helium over next to hydrogen on the left-hand side of the periodic table, we can divide the periodic table into blocks. Elements within a block will have those kind of electrons within their valence shell. So if an element is located within the d block, for instance, its valence electrons will be in a d orbital.

We are interested in determining the subshell for the valence electrons of a magnesium atom in its ground state. Ground state simply means that the magnesium atom is in its lowest energy. Magnesium is located within the s block of the periodic table. So its valence electrons will occupy an s subshell. We can then use the period number to determine the energy level of the subshell. So magnesiumâ€™s valence electron will be located within the 3s subshell.

We could also answer this question by determining the entire electron configuration for magnesium. We could do this by filling all of the subshells with electrons in order of increasing energy, which is summarized by this diagram. If we did this, we would find that the electron configuration for magnesium is 1s2 2s2 2p6 and 3s2. Its outermost electrons or its valence electrons are in the 3s orbital. So the valence electrons for a magnesium atom in the ground state occupies the 3s subshell.