Question

Elements with very large ionization energies also tend to have highly negative (favorable) electron affinities. Explain. Which group of elements would you expect to be an exception to this statement?

Short answer

Elements with large ionization energies have strong attractions to their electrons due to high effective nuclear charges, making it difficult to remove an electron. This strong attraction also makes it favorable for these atoms to gain electrons, resulting in highly negative electron affinities. The noble gases (Group 18) are an exception to this statement. They have high ionization energies due to their stable electron configuration, but their electron affinities are positive, indicating they don't readily accept electrons, as it would disrupt their stable electron configuration.

Step-by-step solution

Define Ionization Energy and Electron Affinity

Ionization energy is the energy required to remove an electron from a neutral atom in its gaseous state. It indicates how strongly an atom holds on to its electrons. A high ionization energy means it is difficult to remove an electron from the atom. Electron affinity is the energy change that occurs when an electron is added to a neutral atom in its gaseous state to form a negative ion. A highly negative electron affinity means that the atom releases a significant amount of energy when it gains an electron, making it favorable for the atom to gain electrons.

Explain the Correlation between Ionization Energy and Electron Affinity

Elements with large ionization energies have strong attraction to their electrons, making it difficult for an electron to be removed. This strong attraction is due to a high effective nuclear charge experienced by the outermost electrons. When an electron is added to such an atom, the effective nuclear charge will also act upon the additional electron, making it favorable for the atom to gain electrons. As a result, these atoms will have highly negative electron affinities.

Identify the Exception Group

The group of elements that are an exception to the statement are the noble gases (Group 18). These elements have a complete set of electrons in their outer shell, which makes them very stable and unreactive. Their ionization energies are very high because of this stability, but their electron affinities are not highly negative. In fact, they have positive electron affinities, which means that they don't readily accept electrons. The reason behind this is that gaining an electron would disrupt their stable electron configuration rather than making a more stable negative ion.

Key concepts

Ionization Energy

Ionization energy is a critical concept in chemistry that refers to the amount of energy needed to remove an electron from an atom in its gaseous state. It's a key indicator of an atom's reluctance to give up its electrons. The higher the ionization energy, the stronger the hold an atom has on its electrons.

For example, in the periodic table, ionization energy generally increases as you move from left to right across a period. This is because the atoms have more protons, which translate to a greater effective nuclear charge attracting the electrons more tightly. Additionally, as you move up a group, ionization energy also increases for a similar reason – atoms are smaller, so electrons are closer to the nucleus and more tightly bound.

Ionization energies are not constant; they change as consecutive electrons are removed, which is referred to as the first, second, third, etc., ionization energies. The first ionization energy is always the lowest because the electron is being removed from a neutral atom, whereas subsequent electrons are removed from an increasingly positive ion, which is a more difficult task due to the increased electrostatic attraction.

Electron Affinity

Electron affinity deals with an atom's ability to accept an electron. It's defined by the energy change that happens when an electron is added to a neutral atom in its gaseous state and forms a negative ion. If the electron affinity is highly negative, it means that the atom releases a substantial amount of energy in the process of gaining an electron.

Atoms with a high effective nuclear charge often have a significant electron affinity, as the added electron enters an environment where it's subject to a strong electrostatic pull from the nucleus. This process is exothermic for many atoms, indicating that they release energy and thus have a 'favorable' electron affinity. This phenomenon usually parallels the trends of ionization energy, wherein elements on the right side of the periodic table, except the noble gases, have more negative electron affinities.

As we come closer to completing an outer shell, electron affinity increases. However, once we reach a full outer shell, as with the noble gases, the trend breaks, and these elements do not exhibit a 'favorable' electron affinity.

Effective Nuclear Charge

The effective nuclear charge is a term that describes the net positive charge experienced by an electron in a multi-electron atom. It accounts for the fact that the presence of other electrons in the atom can shield the outer electrons from the full positive charge of the nucleus. The effective nuclear charge can be thought of as the 'felt' charge by an electron, reduced by the shielding effects of other electrons.

Mathematically, the effective nuclear charge (\(Z_{eff}\)) is given by the formula \(Z_{eff} = Z - S\)), where \(Z\) is the atomic number and \(S\) is the number of shielding electrons. This concept is essential for understanding periodic trends, such as why ionization energy increases across a period. As the number of protons (atomic number) increases without a corresponding increase in the shielding effect, the electrons are held more tightly by the nucleus, resulting in a larger effective nuclear charge.

Noble Gases

Noble gases, the elements found in Group 18 of the periodic table, are fascinating due to their unique chemical properties. These gases, which include helium, neon, and argon, are characterized by having complete outer electron shells. This electron configuration imparts a remarkable level of stability and non-reactivity, making it rare for noble gases to participate in chemical reactions.

The noble gases have exceptionally high ionization energies because removing an electron would mean disrupting a full and stable electron shell. However, their electron affinities present an unusual case; noble gases have generally very low or positive electron affinities, indicating they are not predisposed to gain electrons. In essence, noble gases break the trend that atoms with high ionization energy also have a strongly negative (favorable) electron affinity. Instead, noble gases are content with their electron arrangement and have little to gain energetically from accepting extra electrons.