Problem 125 - Chemistry The Science in Context

Question

The electron affinities of the group 17 elements increase with increasing atomic number. Suggest a reason for this trend.

Step-by-Step Solution

Answer

Answer: Electron affinities increase with the atomic number in group 17 elements due to the increasing atomic size and the shielding effect. As we move down the group, the atomic size increases, and the shielding effect of inner electrons reduces the attraction between outer electrons and the nucleus. This weaker attraction makes it easier to add electrons to these elements, leading to a greater release of energy (higher electron affinity) when an electron is added to form a negative ion.

Step 1: Understand electron affinity and Group 17 elements

Electron affinity is the amount of energy released when an electron is added to a neutral atom to form a negative ion. Group 17 elements, also known as halogens, include Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At). In our analysis, we will consider the first four elements because Astatine is a rare and highly radioactive element.

Step 2: Analyze atomic size

As we move down the group in the periodic table, the atomic size increases because a new energy level (shell) is added for every element. This increase in atomic size means the outermost electrons are further away from the nucleus, leading to a weaker attractive force between the nucleus and these electrons.

Step 3: Analyze effective nuclear charge

Effective nuclear charge refers to the net positive charge experienced by outermost electrons due to the protons in the nucleus. Although the number of protons increases as we move down the group, the increase in the number of electron shells results in more electron shielding. This electron shielding effect reduces the effective nuclear charge experienced by outermost electrons.

Step 4: Understand shielding effect

Electron shielding is the phenomenon in which inner electrons shield outer electrons from the full attractive force of the protons in the nucleus. The more energy levels there are, the greater the shielding effect.

Step 5: Relate the trends to electron affinity

Since atomic size increases as we move down group 17, and because of the shielding effect of the inner electrons, outer electrons experience reduced attraction from the nucleus. This weaker attraction makes it easier to add electrons to these elements, leading to a greater release of energy (higher electron affinity) when an electron is added to form a negative ion. Thus, electron affinities increase with increasing atomic number in group 17 elements due to the increasing atomic size and the shielding effect.

Key Concepts

Group 17 Elements

The Group 17 elements in the periodic table are commonly known as halogens. This group includes the elements: Fluorine (F), Chlorine (Cl), Bromine (Br), and Iodine (I), with Astatine (At) also being part of the group but less commonly studied due to its rarity and radioactivity. These elements are highly reactive and are known for their ability to gain electrons easily.

This is because they have seven electrons in their outermost shell, needing only one more to attain a stable noble gas configuration. The reactivity and properties of these elements are critically affected by electron affinity, which is the energy change that occurs when an electron is added to a neutral atom.

Atomic Size

Atomic size or atomic radius is a pivotal factor when examining the reactivity and trends in group elements. As we move down Group 17 from Fluorine to Iodine, the atomic size increases. This is due to the addition of a new electron shell at each step down the group, resulting in a larger atomic radius.

The outermost electrons are further from the nucleus, which results in a decrease in the nuclear attraction they experience. While the size increases, the ability to add an electron becomes more favorable energetically, impacting the electron affinity. Thus, as the atomic size grows, the electron affinity of the halogens also tends to increase.

Effective Nuclear Charge

Effective nuclear charge ( Z_{eff} ) is the perceived positive charge of the nucleus by the outermost electrons after accounting for electron shielding. Although the number of protons increases down the group, providing a stronger positive charge, this increased charge is largely offset by the added inner electron shells.

With each additional shell, the inner electrons contribute to electron shielding, reducing the effective nuclear charge felt by outer electrons.

• This reduced effective nuclear charge weakens the attraction between the outermost electrons and the nucleus.
• Consequently, this makes it easier for an incoming electron to add to the atom, enhancing its electron affinity.

Shielding Effect

The shielding effect is integral in understanding periodic trends like electron affinity. This effect arises when inner shells of electrons repel outer shell electrons, decreasing the full attractive force of the nucleus. As you descend Group 17, more energy levels are added, which increases this shielding.

The outermost electrons thus feel a reduced effective nuclear charge, making it easier for these elements to attract and add an external electron.

• This increase in the shielding effect down the group is why larger halogens have higher electron affinities.
• The added electron experiences less repulsion from the nucleus due to this shielding, leading to a higher energy release when the electron affinity increases.