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)
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 magnesium’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.