Video Transcript
Which of the following is the
correct order from greatest to least according to the first ionization energy? (A) Na, K, Mg, Ca; (B) K, Ca, Na,
Mg; (C) Mg, Na, Ca, K; (D) Mg, Ca, Na, K; or (E) Mg, Na, K, Ca.
In this question, we want to know
which order of elements best fits the trend of decreasing first ionization
energy.
The first ionization energy is the
amount of energy required to remove the most loosely bound electron completely from
an isolated gaseous atom. The first ionization energy depends
on different physical properties, such as atomic radius. The electrostatic interactions
between the outermost electrons and the nucleus of an atom or ion depend on the
distance between these oppositely charged particles. Larger atoms have a greater
distance between their protons and outermost electrons. Therefore, they have a weaker force
of attraction between the protons in the nucleus and the outermost electrons.
Often, larger atoms also have an
increased shielding effect, which is due to the repulsion between core
electrons. Because of this, the outermost
electrons are often easier to remove completely from larger atoms. And so they have lower first
ionization energies. This also explains why as we go
down a group on the periodic table, the first ionization energy decreases.
If we look at a period on the
periodic table, we find that first ionization energy typically increases as we move
from left to right across the period. This is due to the increasing
number of protons in the nucleus. Within a period, there is no change
in shielding from the core electrons. There is an increase in the
electrostatic attraction between the increasingly positive nuclei and the outermost
electrons. This also causes a decrease in
atomic radius across a period.
Let’s apply this information to the
four elements in each of our answer choices. We must compare the metals sodium,
magnesium, potassium, and calcium. We can compare each atom by
following the trend of decreasing first ionization energy down a group and from
right to left on the periodic table.
Of the four metals, potassium is
the furthest to the left and furthest down on the periodic table. Therefore, as the largest atom of
the four, it is likely that potassium would have the lowest first ionization
energy. Magnesium is the metal both
farthest to the right in group two and highest up in period two. Magnesium is the smallest of the
given four elements. It would likely have the highest
first ionization energy. Since we know that magnesium will
have the greatest first ionization energy and potassium will have the least, we can
eliminate answer choices (A), (B), and (E).
This just leaves sodium and
calcium. We cannot rely on the trends alone
to compare the first ionization energies of sodium and calcium. As it turns out, calcium has a
higher first ionization energy than sodium. This is actually related to the
electronic configurations of these atoms. Removing the outermost electron of
sodium is energetically favorable. Removing one outer electron from
sodium leads to a full valence shell. The same is not true for
calcium. This is a less favorable
configuration and requires more energy.
Therefore, the correct order from
greatest to least according to the first ionization energy is answer choice (D) Mg,
Ca, Na, K.
