Using the periodic table below,
what is the electronic configuration of element X? (A) 3,15. (B) 2,8,5. (C) 8,8,5. (D) 2,8,8,5. (E) 1,1,2,6,2,3.
There are trends in the electronic
configuration of elements across the rows and columns of the periodic table. The rows, which are also called
periods, tell us the number of energy levels that are occupied by electrons. Element X is in the third row of
the periodic table, so we know electrons occupy three energy levels.
With this information, we can
eliminate some of our answer choices. The electronic configuration of
answer choice (A) has two occupied energy levels. So we can eliminate this answer
choice. The electronic configurations in
answer choices (D) and (E) have four and six occupied energy levels,
respectively. So we can eliminate these answer
choices as well.
The columns in the periodic table,
which are also called groups, tell us the number of outer electrons that atoms of an
element have. These numbers show the number of
outer electrons for the main group elements in the periodic table. The number of outer electrons
increases as we move across the table from left to right. So element X has five outer
electrons. The electronic configurations in
both of the remaining answer choices have five outer electrons. So which one is the right
answer? The only difference between them is
the number of electrons in the first energy level.
To determine which is the right
answer, we need to know that the first energy level can only contain a maximum of
two electrons. That makes answer choice (B) the
correct answer. So, the electronic configuration of
element X is 2,8,5.