Which element in the pair of
adjacent elements Mg and Na has a higher first ionization energy?
First ionization energy is the
amount of energy required to remove the most loosely bound electron completely from
an isolated gaseous atom. We need to determine which element,
magnesium or sodium, has a higher first ionization energy. Sodium and magnesium are both found
in period three of the periodic table. An atom of sodium has 11 protons
and 11 electrons. The 11 protons are in the nucleus,
two electrons are in the first electron shell, eight electrons are in the second
electron shell, and one electron is in the third electron shell. An atom of magnesium has 12 protons
and 12 electrons. The 12 protons are found in the
nucleus, two electrons are in the first electron shell, eight electrons are in the
second electron shell, and two electrons are in the third electron shell.
The third electron shell is the
outermost or valence shell in both of these atoms. An electron in the valence shell is
the most loosely bound electron. Each of these electrons is
electrostatically attracted to the positively charged nucleus. But since a magnesium atom has one
more positively charged proton in its nucleus than a sodium atom, its outermost
electron will experience a stronger electrostatic attraction to the nucleus. In order to completely remove this
electron from the atom, the electrostatic attraction between the electron and the
nucleus must be overcome.
Since the electrostatic attraction
between the outermost electron and the nucleus is greater in a magnesium atom than a
sodium atom, more energy will be required to remove the electron and the first
ionization energy will be higher. In general, the first ionization
energy tends to increase as we move from left to right across a period on the
periodic table. This is because the valence shell
of each atom across the period is the same. But the number of protons in the
nucleus is increasing, thereby increasing the electrostatic attraction between the
outermost electrons and the nucleus.
So in comparing magnesium and
sodium, the element which has a higher first ionization energy is magnesium.