Problem 10 - Chemistry in 30 days

Question

Which of the following arrangements for the three halogens \(\mathrm{Cl}\), Br, I when placed in order of their increasing electron affinity is correct ? (a) \(\mathrm{Cl}, \mathrm{Br}, \mathrm{I}\) (b) \(\mathrm{I}, \mathrm{Br}, \mathrm{Cl}\) (c) \(\mathrm{Br}, \mathrm{Cl}, \mathrm{I}\) (d) \(1, \mathrm{Cl}, \mathrm{Br}\)

Step-by-Step Solution

Answer

Option (b) I, Br, Cl is correct.

Step 1: Understanding Electron Affinity

Electron affinity is the amount of energy released when an electron is added to a neutral atom. Halogens typically have high electron affinities because they are one electron short of having a complete outer shell.

Step 2: Reviewing Trends in the Halogen Group

In the periodic table, electron affinity generally increases across a period and decreases down a group. Therefore, amongst halogens, which are in the same group, electron affinity usually decreases from top to bottom.

Step 3: Arranging Halogens Based on Electron Affinity

Chlorine (Cl), bromine (Br), and iodine (I) are in the same group, with chlorine being at the top followed by bromine and iodine, respectively. Hence, electron affinity decreases in the order: Cl > Br > I.

Step 4: Finding the Correct Arrangement in Options

Based on the decrease in electron affinity down the group, the correct order in terms of increasing electron affinity for the given halogens is option (b) I, Br, Cl.

Key Concepts

Periodic Trends

Periodic trends are a fascinating aspect of the periodic table that refer to predictable patterns observed among chemical elements. These patterns include properties such as atomic radius, ionization energy, and electron affinity. Understanding these trends helps us predict the behavior and reactivity of elements.

Electron affinity is one of these trends. As you move across a period from left to right, electron affinity generally increases. This is because atoms are closer to achieving a full valence shell, leading to a stronger affinity for electrons. However, as you move down a group in the periodic table, this trend often reverses. The added electron is farther from the nucleus due to additional electron shells, causing a decrease in electron affinity.

By recognizing these periodic trends, you can anticipate which elements are most likely to gain electrons and understand their placement on the periodic table. This foundational knowledge is vital for exploring more complex chemical interactions.

Halogen Group

The Halogen group, also known as Group 17 of the periodic table, consists of the elements fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These elements are known for their high reactivity and exist naturally in the form of diatomic molecules (e.g., F₂, Cl₂).

Halogens are highly electronegative, meaning they have a strong tendency to attract electrons. This characteristic makes them excellent at forming salts when combined with alkali metals. Additionally, they each have seven electrons in their outer electron shell, placing them just one electron short of a stable, complete octet configuration.

The chemical nature of halogens makes them important in various applications, from water purification to the production of household products. Their reactivity and distinct characteristics provide a window into understanding basic chemical behaviors and reactions.

Electron Configuration

Electron configuration is a method of representing the arrangement of electrons in an atom. Each element's electrons are situated in "shells" around the nucleus, and the distribution of these electrons influences an element's chemical properties.

For halogens, the electron configuration ends in the p-orbital, with five electrons in this outer shell (denoted as p⁵). This configuration highlights their need to gain one more electron to achieve a full p-orbital, hence their high electron affinity. For example, chlorine's electron configuration is \(3s^2 3p^5\).

Understanding electron configuration is crucial because it helps to predict how atoms will interact and bond with others. Elements with similar configurations typically have similar chemical properties, allowing chemists to make insightful predictions about unknown reactions.

Chemical Properties of Halogens

Halogens are known for their distinctive and varied chemical properties. One of the key characteristics is their reactivity, which decreases as you move down the group. This is due to the increasing size of the atoms and the shielding effect, which makes it harder for the nucleus to attract additional electrons.

Halogens readily form compounds with metals, known as halides, by gaining one electron to complete their octet. For instance, sodium chloride (NaCl) is produced when chlorine gains an electron from sodium. They also have the ability to form interhalogen compounds by reacting with each other, such as iodine chloride (ICl).

Notably, halogens have significant uses in everyday life, from disinfecting agents to components in medications and industrial chemicals. Their versatile chemical properties make them invaluable in various scientific and practical applications.