Ionization energy is a fundamental concept in chemistry that explains the reactivity of elements. It represents the energy required to overcome the attraction between an atom’s nucleus and its outermost electrons. Comparing the ionization energy of Magnesium (Mg) and Strontium (Sr) allows us to explore how this property changes among elements that share similar chemical characteristics.
Defining Ionization Energy
Ionization energy (IE) is the minimum amount of energy needed to remove the most loosely held electron from a neutral atom in its gaseous state. This process is always endothermic, requiring an input of energy to pull the electron away from the nucleus. The result is the formation of a positively charged ion.
The first ionization energy (\(I_1\)) refers to the energy needed to remove the first electron. If another electron is removed from the unipositive ion, the energy required is the second ionization energy (\(I_2\)). Each successive ionization energy is higher than the last because removing an electron from an already positive ion requires greater energy to overcome the stronger electrostatic attraction.
Periodic Trends and the Group 2 Elements
Magnesium and Strontium are both members of Group 2 on the periodic table, known as the alkaline earth metals. Elements in this group share the characteristic of having two valence electrons in their outermost shell. Their position relative to one another significantly influences their chemical properties, including ionization energy.
The general trend for ionization energy is a decrease in value as one moves down a vertical column, or group, of the periodic table. Elements at the top of a group have a higher ionization energy than those lower down. Magnesium sits above Strontium in Group 2, with Magnesium in Period 3 and Strontium in Period 5.
Following this trend, Magnesium is expected to have a greater first ionization energy than Strontium. Experimental data confirms this, with the first ionization energy for Magnesium being approximately \(738 \text{ kJ/mol}\), while Strontium’s is about \(550 \text{ kJ/mol}\).
Why Magnesium Has Greater Ionization Energy Than Strontium
The reason for Magnesium’s higher ionization energy lies in the distinct atomic structures of the two elements. The electrostatic force of attraction between the nucleus and the valence electrons is the primary factor that dictates the ionization energy value.
Strontium atoms are significantly larger than Magnesium atoms because they possess more electron shells. Magnesium occupies three principal energy levels, while Strontium occupies five. This difference means Strontium’s outermost valence electrons are much farther from the nucleus compared to Magnesium’s.
The increased distance results in a weaker attractive force acting on Strontium’s valence electrons, following Coulomb’s Law. Because the outer electrons in Strontium are held less tightly by the nucleus, they are easier to remove, requiring less energy for ionization.
Another factor contributing to the difference is electron shielding. The inner electrons act like a screen, partially blocking the attractive pull of the nucleus from reaching the valence electrons. Since Strontium has more occupied inner electron shells than Magnesium, its valence electrons experience a greater degree of this shielding effect.
This increased shielding in Strontium reduces the effective nuclear charge—the net positive charge experienced by the valence electrons. Magnesium’s valence electrons are exposed to a stronger effective nuclear charge due to fewer inner shells. The combination of a smaller atomic radius and a stronger effective nuclear charge means Magnesium’s valence electrons are bound more securely.