Question

The second ionization energy of magnesium is only about twice as great as the first, but the third ionization energy is 10 times as great. Why does it take so much more energy to remove the third electron?

Short answer

The third ionization energy is much higher because it disrupts a stable, filled shell.

Step-by-step solution

Understanding Ionization Energy

Ionization energy is the energy required to remove an electron from an atom or ion. In the case of magnesium, it's important to understand the electronic configuration and the process of removing successive electrons.

Electronic Configuration of Magnesium

Magnesium (Mg) has the electronic configuration of \ \(1s^2 2s^2 2p^6 3s^2\). It has two electrons in the 3s orbital, which are its valence electrons.

First and Second Ionization Energies

The first ionization energy removes one 3s electron, and the second ionization energy removes the second 3s electron. These require similar amounts of energy because they are both removing electrons from the same energy level (3s).

Understanding the Increase to the Third Ionization Energy

Once both 3s electrons are removed, the ion now has the configuration of \((1s^2 2s^2 2p^6)\), which is the same as the noble gas neon (Ne). This makes the ion highly stable. To remove the third electron, energy must be used to disrupt this stable, full-shell configuration.

Reason for the Large Increase

The dramatic increase in the third ionization energy is because removing one more electron involves taking it from a stable, completed octet from the lower 2p shell. This requires overcoming a large stability, hence the need for much more energy.

Key concepts

Electronic Configuration

Every atom has a unique electronic configuration, which describes the arrangement of electrons in its orbitals. For magnesium, this configuration is written as \(1s^2 2s^2 2p^6 3s^2\). This notation tells us several things:

• The numbers (1, 2, 3) represent energy levels or shells, where 1 is closest to the nucleus.
• The letters (s and p) refer to different types of orbitals within each energy level. 's' orbitals can hold up to 2 electrons, while 'p' orbitals can hold up to 6 electrons.
• The superscripts (such as 2 and 6) tell us how many electrons are in each type of orbital.

This configuration is important because it determines how an atom interacts with others, its chemical properties, and how it fulfills the octet rule. Removing electrons during ionization changes this configuration, affecting the atom's behavior.

Magnesium Ionization Energies

Ionization energy is critical in understanding how easily an atom can lose an electron to form an ion. For magnesium, we examine the first, second, and third ionization energies:

• The first ionization energy is the energy required to remove one of the 3s electrons from magnesium, resulting in a \(Mg^+\) ion.
• The second ionization energy removes the remaining 3s electron, forming a \(Mg^{2+}\) ion.
• The third ionization energy involves removing an electron from a full, stable 2p shell, which requires much more energy.

The third ionization energy of magnesium is significantly higher because it disrupts a stable, noble gas-like configuration. This stability makes removing one of these inner electrons much harder, reflecting in the energy needed for ionization.

Valence Electrons

Valence electrons are the outermost electrons in an atom and determine its chemical properties and reactivity. Magnesium has two valence electrons in its 3s orbital:

• These electrons are involved in bonding and determining how the atom will interact with others.
• When magnesium undergoes ionization, these are the first electrons to be removed.

Understanding valence electrons helps explain why certain elements are more reactive or why they form specific types of ions. In magnesium's case, losing these valence electrons leads to a more stable electron configuration.

Noble Gas Configuration

The term "noble gas configuration" refers to an atom having the same electron configuration as one of the noble gases in the periodic table, which are known for their stability and low reactivity. For magnesium, achieving a noble gas configuration involves the removal of its two valence 3s electrons, reaching the same electronic setup as neon (Ne), \((1s^2 2s^2 2p^6)\).

• Once magnesium loses these two electrons, it achieves a stable, full outer shell.
• This stability is part of why the third ionization energy is so high; removing an electron from a noble gas configuration demands disrupting a highly stable, full orbital shell.

Studying these configurations helps us predict chemical reactions and stability of ions, explaining patterns and trends across the periodic table.